Physics | 🍎 | Reference Guide and Teacher

Physics is the science of the most general laws of nature, and of matter—its structure, motion, and transformations. Simply put, it is an attempt to understand how the world around us works: why objects fall, how electricity functions, and what light and heat are. What exactly does physics study? Physics investigates a wide range of phenomena—from the motion of the tiniest particles to the evolution of galaxies. Its primary focus areas include: Matter. In physics, matter is defined as anything in nature that exists independently of our consciousness—this includes substances (such as water or metal) and fields (such as electromagnetic or gravitational fields).

Forms of motion and interaction. Physics seeks the general rules governing how matter moves, interacts, and changes state. Specific patterns. Scientists aim to identify fundamental laws (such as the law of conservation of energy or Newton's laws) that describe a broad spectrum of natural phenomena. How physics works Physics is an experimental science. Its methodology relies on a specific sequence: observing phenomena → gathering facts → formulating hypotheses → planning and conducting experiments to test those hypotheses → analyzing results and drawing conclusions (establishing laws or theories). Physical quantities (such as mass, velocity, and temperature) are used to describe phenomena, allowing for the quantitative assessment and analysis of changes. Why is this important? The laws of physics form the foundation of all natural sciences. They help explain processes in other fields: chemistry (atomic and molecular structure), biology (physical processes in living organisms), geology (tectonic plate movement), and astronomy (stellar evolution).

The practical significance is also immense: discoveries in physics have laid the groundwork for the technologies we use every day—from smartphones and computers to automobiles and space rockets. Physics not only explains the world but also helps create new things and solve global challenges (for instance, in the fields of energy or medicine).

Text: “What is physics”?

Physics is one of the most ancient sciences about nature. The word "physics" takes its origin from the Greek word "phewsis" meaning nature.

The development of other sciences depends in many respects on the knowledge of physical phenomena.

Physics studies various phenomena in nature: mechanical motion, heat, sound, electricity, magnetism and light.

Physics divides itself very naturally into two great bran­ches, experimental physics and theoretical physics. The former is the science of making observations and devising experiments which give us accurate knowledge of the actual behaviour of natural phenomena. On the basis of experimental facts theo­retical physics formulates laws and predicts the behaviour of natural phenomena. Every physical law is based on expe­riments and is devised to correlate and to describe accurately these experiments. The wider the range of experience covered by such a law, the more important it is. Physics is divided into half a dozen or more different fields — mechanics, sound, heat, electricity and magnetism, light, molecular, atomic and nuclear physics. These different fields are not distinct but merge into each other.

In all cases physics deals with phenomena that can be accurately described in terms of matter and energy. Hence, the basic concepts in all physical phenomena are the concepts of matter and energy. And it is important to determine accurately the characteristics of both matter and energy, the laws that govern their transformations, and the fundamental relations that exist between them.

Matter. Every substance саn be divided into particles known as molecules. Chemical reactions indicate that the molecules are composed of smaller units, or atoms. Modern physical methods of investigation have shown that the atom consists of a centrally situated nucleus with a total positive charge surrounded by a number of electrons which revolve about the nucleus. In a stable atom, the total positive charge of the nu­cleus is equal to the total negative charge of the electrons which surround the nucleus. The total electrical charge is zero and this is the conventional state of most atoms.

Matter can exist in four states: solid, gas, liquid and plasma. Text: “Units of Measurement”.

Real science has various recognised steps. It always begins with ob­servation followed by classification and measurement. Classification has become the first step towards understanding of a new phenomenon. Phe­nomena have to be put in some order before any­thing can be done with them. Measurement is one further step in the process of putting them in order. It is only by measurement that new knowledge enters science.

Much of physics deals with measurements of physical quantities such as length, time, velocity, area, volume, mass, density, temperature and energy. Many of these quantities are interrelated. For example, density is mass divided by volume. Most of the physical quantities are related to length, time and mass. Therefore all the systems of physical units are derived from these three fundamental units.

Practically there are three main systems of measurement in use today: the British system of units, the metric system of units and the (International) System of Units (SI). With a few exceptions nearly all the nations of the world use the metric system. The value of the MKS (metre-kilogram-second) system is that its various units possess simple and logical relationships among themselves, while the British system (the f. p. s. — foot-pound-second) is a very complicated one. For example, in the British system 1 mile is equal to 1,760 yards; 1 yard is equal to 3 feet, and 1 foot is equal to 12 inches. In the English system converting one unit into another is a hard and monotonous job, while in the MKS system conversions of one unit to another can be carried out by shifts of a decimal point (comma in Russian writing).

The standard metre of the world was originally defined in terms of the distance from the north pole to the equator. This distance is close to 10,000 kilometres or 107 metres.

In fact, the SI Units is an internationally agreed coherent system of units derived from the MKS system. The seven basic units in it are: the metre (m), kilogram (kg), second (s), ampere (a), Kelvin (K), mole (mol), and candle (свеча) (cd).

Note. 107 metres – ten to the seventh power metresText: “The Nature of Matter.

The Basic Constituents of Matter.”

Physics is concerned with studying the properties of matter, forces, and energy and their various interactions. In trying to understand the behaviour of solids, liquids, and gases physicists regard substances in terms of their basic constituents.

Experimental evidence supports the idea that matter in all three possible phases is composed of tiny particles, called molecules, which are continuously in motion. For any given single substance, the molecules are identical in mass, structure, and other properties. All molecules in a substance exert a force on each other.

Molecules consist of groups of atoms, which themselves consist of electrons and nuclei. In solids and liquids, the molecules move relatively slowly and they therefore interact fairly strongly. But in the study of gases, the molecular force can often be neglected because the molecules are, on average, widely separated and interact relatively briefly. Despite the minuteness of molecules, evidence of their existence is provided by various phenomena, such as Brownian motion.

Just as substances can be broken down into molecules, molecules can be broken down into atoms. An atom is the smallest particle that can represent a particular chemical element.

There are three types of particles that can be considered as making up a typical atom. Central nucleus is comprised of neutrons and protons. The neutron is a particle with no electrical charge, whereas the proton has a single positive charge. Both have roughly the same mass. Circulating about this central region, held in orbit under the influence of the proton’s positive charge, are the electrons. These are subatomic particles, each with a single negative charge and an extremely small mass.

Electrons play a major role in determining the properties of the various elements. Text: “States of Matter”.

All substances can, under the right conditions, exist as a solid, liquid, or gas. These are the three basic physical states (or phases) of matter.

Solids possess the property called cohesion; that is, their component particles (atoms or molecules) are held together by attractive forces. As a result, solid substances are rigid and retain their shape unless deformed by external forces.

There are two principal types of solids: crystalline and amorphous. Crystalline solids have definite crystalline structures. Most also melt at specific temperatures to become liquids. Examples include metals, ice, and many plastics, in addition to obviously crystalline substances such as common salt and diamond. In contrast to crystalline solids, amorphous solids have neither crystalline structures nor specific melting points. Glass is an example of amorphous solids.

Liquids represent the intermediate stage between solid and gas. A liquid’s atoms or molecules have some degree of cohesion and so tend to remain together. But they are not rigidly linked and can therefore move in relation to each other – which is not possible in solids. For this reason liquids flow and in a gravitational field take on the shape of the vessels in which they are contained.

In a gas the constituent particles have negligible cohesion and can therefore move almost completely independently of each other. Like liquids, gasses flow and assume the shape of their containers. Unlike liquids, however, gasses always fill the entire space in their containers – and the container needs to be closed if the gas is not to escape.

The attractive forces between atoms or molecules in a solid are balanced by repulsive forces. As two of these particles approach each other, the outer electrons of one repel the outer electrons of the other, and the atoms or molecules move apart. But because of the attractive forces between them, they move back toward each other again. The overall result of the attractive and repulsive forces is to cause each of the atoms or molecules in a solid to vibrate continually about the same position in a lattice.

The atoms or molecules of a liquid are also affected by attractive and repulsive forces. But a liquid is hotter than the same substance in solid form and its vibrating particles therefore have greater kinetic energy – that is, they vibrate more violently. As a result, the attractive forces cannot hold them in a lattice, and they are relatively free to move.

In a gas the atoms or molecules have so much energy that they have largely broken free of the influences of the attractive and repulsive forces and, therefore, have almost complete freedom of movement. Text: “Properties of Solids and Liquids”.

The forces that hold atoms and molecules in place give solids their strength.

The strength of a material is determined by measuring its elasticity – that is how stiff it is and how it behaves when it is stretched or compressed. Elasticity is measured as the ratio of stress to strain.

Stress is defined as the force acting on a material divided by the area over which the force is applied. Stress may be applied in three different ways: tensile, shear and compressive.

Tensile stress causes a material to become elongated (stretched), and shear stress causes it to be twisted out of shape. In both cases the component atoms or molecules are pulled apart as they move from their original positions. When the stress is removed, the attractive forces between the atoms or molecules pull them back together again, and the material is restored to its original shape. Compressive stress forces the atoms or molecules together, and it is the repulsive forces that restore the shape of the material when the stress is removed. If stress is applied to a material and slowly increased, the material accordingly changes shape. But at a certain point it reaches its elastic limit.

Another property of solid materials that depends on the strength of the bonds between atoms or molecules is hardness.

As in a solid, the atoms or molecules of a liquid are held together by attractive forces. But these forces are not great enough to hold the atoms or molecules in a fixed pattern, they move about on random. As a result, a liquid can flow and it cannot be stretched or distored. Like a solid, it can be compressed slightly and shows the same sort of elasticity when subjected to compressive stress. Unlike a solid, but like a gas, a liquid exerts pressure, which at any point depends on the depth and the density of the liquid.

A liquid does not expand to fill the whole of the volume available to it. Instead, the cohesion between its molecules forces it to maintain a fixed volume (at a given temperature). It takes on the shape of all or part of its container – but this is only because the forces of gravity makes it do so. In zero gravity conditions a liquid takes on the shape with the minimum possible surface area – that is, a sphere.

The fact that a liquid tends to take on a spherical shape is due to a phenomenon known as surface tension. Inside a volume of liquid all the atoms or molecules attract each other equally. Each molecule experiences attractive forces in all directions and they cancel out each other. But the atoms or molecules on the surface experience few, if any, attractive forces from the outside. As a result, the forces between them and inner molecules tend to pull them inwards and towards each other.Text: “Unusual States of Matter”.

Some substances exist in states that do not comply with the normal definitions of a gas, a liquid, or a solid. For example, jelly is neither a true solid nor a liquid, and smoke is neither a pure gas nor a solid. Matter in stars and in the tails of comets exists as a plasma, a mixture of charged particles that is outside the normal definition of a gas. In general, a plasma can exist only at extremely high temperatures. At extremely low temperatures, approaching absolute zero, some materials take on remarkable properties. Although they are not strictly different states of matter, their behaviour is exceptional.

Colloids. In 1861, the physical chemist Thomas Graham discovered that some substances in solution, such as salt, sugar, and copper sulfate, diffuse through parchment, whereas others, such as glue and gelatine do not. He therefore divided substances into two groups: crystalloids (that diffuse through parchment) and colloids (that do not diffuse). And he believed that the difference between a crystalloid and a colloid depended largely upon particle size. We now know that Graham was broadly correct. But we also know that most crystalloids can be brought into the colloidal state. A colloid is a solution in which the component particles are large molecules or clumps of small molecules.

Liquid crystals. The liquid state is intermediate between the solid state and the gaseous state. Some substances, however, are intermediate between solids and liquids. In a liquid crystal the atoms or molecules are arranged in a pattern, like those of a solid crystal. But the pattern is not completely fixed; it can be altered by heat or an electric field. Some liquid crystals change colour at certain temperatures and can, therefore, be used in liquid crystal thermometers. In others an electrical voltage causes a change in patterns that alters the plane of polarized light. Such types are used to make liquid crystal displays for watches and calculators.

Plasmas. A plasma is sometimes described as being the fourth state of matter – that is, one phase farther on from a gas. In fact a plasma is created by heating a gas to such a high temperature that its atoms or molecules lose electrons and become ions. The gas is almost fully ionized and becomes a very good electrical conductor. The gases that are involved in the thermonuclear reactions of the sun and other stars are in the form of a plasma. Scientists are trying to recreate such a plasma on earth by heating the gases deuterium and tritium (isotopes of hydrogen) to tens of millions of degrees in special installations. In this way, they hope to produce a controlled thermonuclear fusion reaction as a source of power. Text: “Statics”.

Statics is the branch of physics that deals with the behavior of bodies that are held stationary under the influence of a system of forces. The study of statics depends crucially on an understanding of the concept of force, which can be defined as an agent that is capable of altering the state of rest or motion of an object.

The most common force is that of gravity which on earth attracts everything downwards to its surface. Other forces include friction, which in some cases can act against another force to prevent movement, and the viscous force that acts against the force of gravity when a small object is dropped into, for example, a container of syrup.

The action of a force depends on the mass of the object on which it is exerted. Mass is defined as the resistance of an object to any change of its state of motion or rest by the action of a force. Mass is, therefore, a measure of the inertia of an object. The greater an object’s mass, the smaller is the acceleration, that a given force produces. The weight of an object is not, however, an unchanging intrinsic property. It is the downward force of an object resulting from the action of the force of gravity. So, although an object’s mass is the same everywhere, its weight depends on the strength of the gravitational field acting on it.

One of the major areas of study in statics is the behavior of objects in fluids (that is, in liquids and gases). Much of this work revolves around the concept of density, which is the mass of an object per unit volume. Another useful concept is that of specific gravity, or relative density. This property is given by the density of an object divided by the density of water.

An important law in the statics of fluids is Archimedes’ principle. It states that the upward force acting on an object immersed in a fluid is equal to the weight of fluid that is displaced. The value of the principle lies in providing a way of finding the specific gravity, and therefore the density of an object without measuring both its mass and volume. Legend tells of Archimedes applying the principle after being asked by the King of Syracuse to prove that a gold crown was indeed made of solid gold. The crown was intricately worked and finding its volume would have been extremely difficult. So Archimedes found its density by putting the crown and then an equal weight of pure gold in a bowl of water and measuring the amount of water displaced by each. He found that the crown displaced more water than the gold weight did. It meant that the crown had a greater volume and therefore consisted of a material of lower density than pure gold. Text: “Dynamics. Laws of Motion”.

Dynamics is the study of the ways in which objects behave when they are acted on by forces. Such forces are all around us – for example, the gravitational force acting on a falling object, the air resistance that offsets the full effect of gravity on it, and the frictional force that makes it difficult to drag an object along the ground.

Just as there are many different types of forces, there are also different types of motion produced by those forces. In linear motion, an object moves in a straight line and a falling body is an example. Circular motion is produced when an object is acted on by a force that originates from a central point. If an object is held in equilibrium by two forces, and the extra force resulting from slightly moving the object from its equilibrium position is directly proportional to the distance moved, then the object oscillates regularly in simple harmonic motion.

For an understanding of dynamics, it is necessary to have a knowledge of kinematics, which is the study of motion alone, without taking into account what causes the motion. The most basic concept in kinematics is that of the displacement of a particle; this is the length and direction of the line along which the particle moves from some fixed point, the origin. A quantity, in which both magnitude and direction are specified, is called a vector. And vectors are of great importance in physics.

The next concept is that of the velocity of the particle. It is defined as the rate of change of the displacement with time. Velocity is itself a vector. The commonly-used word speed is reserved by physicists to denote only the magnitude of an object’s velocity, but not its direction. Any quantity that measures only the magnitude of some phenomenon and not its direction as well is known as a scalar quantity. Multiplying the velocity of an object by its mass gives us another vector quantity – momentum.

When the velocity of an object changes, it either accelerates or decelerates. The rate of change of velocity with time is measured in meters per second per second (ms-2) or an equivalent unit such as kilometers per hour per hour (km hr-2). Acceleration itself can vary with time.

The fundamental problem of dynamics is determining the motion of an object acted on by forces. The three basic laws governing this behaviour are called Newton’s laws of motion, after the English mathematician and physicist Isaac Newton (1642-1727), who formulated them in the seventeenth century, along with the law of universal gravitation.

Newton’s first law of motion states that an object will remain at rest or will continue to move uniformly in a straight line at constant velocity unless acted on by a force.

Newton’s second law goes a stage further to describe how an object behaves when a force does act on it. This law states that the rate of change of momentum of an object is proportional to the force acting on it.

Newton’s third law of motion concerns the interaction of forces produced by objects. It states that if one object exerts a force on another, that second object exerts an equal and opposite force on the first. This is the principle of action and reaction. A common example of this principle in operation occurs when someone tries to step onto land from a boat. In getting out of the boat, the person exerts a force on it. So as he goes forward onto land, the boat is pushed in the other direction.Text: “Energy”.

Energy can be defined as the ability to do work. Physicists classify energy into several types: kinetic, potential, heat, sound, radiant energy (light, for example), and electrical, chemical, and nuclear energy.

Kinetic energy is possessed by a moving object by virtue of its motion. It equals the work done to accelerate the object to a particular velocity; it also equals the work done to bring a moving object to rest. The two principal forms of kinetic energy are known as translational and rotational. The first is possessed by an object moving from one position to another. The second is possessed by rotating objects, which revolve about an axis and therefore periodically return to the same position.

An object has potential energy by virtue of its position. Two common types are gravitational and elastic potential energy.

An object possesses heat, or thermal, energy by virtue of its temperature. It is, in fact, merely a form of kinetic energy, because the temperature of a substance depends on the motion of its component atoms or molecules; the higher its temperature, the faster the molecules move.

Radiant energy consists of electromagnetic radiation and includes radio waves, visible light, ultraviolet and infrared radiation, and X rays. The only form of energy that can exist in the absence of matter, it consists of a wave motion in electric and magnetic fields. Radiant energy is emitted when electrons within atoms fall from a higher to a lower energy level and release the “excess” energy as radiation.

Sound energy consists of moving waves of pressure in a medium such as air, water, or metal. They consist of vibrations in the molecules of the medium.

Matter that has gained or lost some electric charge has electrical energy. The movement of charges constitutes an electric current, which flows between two objects at different potentials when they are joined by a conductor.

Chemical energy is possessed by substances that undergo a chemical reaction, such as combustion. It is stored in the chemical bonds between the atoms that make up the molecules of a substance.

Nuclear energy is produced when the nuclei of atoms change, either by splitting apart or joining together. The splitting process is known as nuclear fission, the joining together as nuclear fusion. Such changes can be accompanied by the release of enormous amounts of energy in the form of heat, light, and radioactivity (the emission of atomic particles or gamma radiation, or both).

When an object loses or gains one type of energy, another kind is correspondingly gained or lost. The total amount of energy, possessed by an object, remains the same. This phenomenon is the principle of conservation of energy, which states, that energy can neither be created nor destroyed, but only converted into other forms.

If mass and energy are considered together, the total amount of mass and energy remains the same. Consequently the principle of mass conservation has been modified into what is called the principle of conservation of mass-energy. The Theory of Relativity shows that mass and energy can be considered to be totally interconvertible, and the amount of energy produced, when matter is destroyed, is given by the well-known equation E = mc2 (E is the energy released, m is the mass destroyed, and c is the velocity of light).

The transmission of energy. Energy is often transmitted by wave motions, and for this reason the study of waves is of crucial importance in physics – from the wave mechanics of the atom to the study of gravitational waves produced by black holes. In general, a travelling wave is the movement of a disturbance from a source, and energy is transported as the disturbance moves outwards.

If the disturbance produced is parallel to the direction of energy travel, the wave is said to be longitudinal; sound waves are of this type. If the disturbance is perpendicular to the direction of energy travel – as in electromagnetic radiation and waves on the surface of water – then the wave is transverse.

Four properties of a wave can be distinguished and described mathematically: wavelength, frequency, velocity, and amplitude. Text: “Radiant Energy”.

Energy may take various forms – sound, chemical, and electrical energy, for example. One whole range of related forms is known as radiant energy or electromagnetic radiation. Together they make up the electromagnetic spectrum, whose various radiations are characterized by particular ranges of wavelengths and frequencies. At the long-wavelength, low-frequency end of the spectrum are radio waves, followed by microwaves and infrared radiation. Then comes visible light and ultraviolet radiation, and the shortest wavelengths and highest frequencies comprise X rays and gamma rays.

Apart from light, all forms are invisible to the human eye, although many can be detected by electronic devices and special types of photographic film. Almost all have found practical applications: in radio communications and radar, infrared and microwave heating, photography and spectrographic analysis, and medical and industrial radiology. The most important forms of electromagnetic radiation are visible light and infrared radiation (both of which are emitted by the sun, our principal natural source of radiant energy). Without them, life would be impossible.

Nature and properties of radiant energy. The various forms of radiant energy have many common characteristics, the most fundamental of which concerns their nature. They are all wave motion, consisting of varying electric and magnetic fields, but they can also be considered as being made up of "particles," or quanta, of energy called photons. Radiant energy therefore has a dual wave/particle nature.

In theory it is possible to explain all the effects and properties exhibited by radiant energy in terms of both wave motion and photons. But in practice it is usually more convenient to employ whichever of the two concepts explains a particular effect most easily.

Another basic characteristic of electromagnetic radiation is the fact that it does not need a medium in which to travel, and so it can traverse the vast emptiness of space. But the various forms of radiation can also travel through other mediums: light travels through air, water, and glass, for example, and X rays and gamma rays can penetrate body tissues and even pass through metal; that is to say, even a metal is “transparent” to X rays and gamma rays.

In a vacuum, all forms of electromagnetic radiation travel at the same constant velocity – called the speed of light. In other transparent mediums, they travel more slowly, the velocity depending on the wavelength of the radiation and, generally, on the density of the medium. In other mediums, different wavelengths travel at slightly different velocities; the longer the wavelength, the faster the radiation travels. So in the case of visible light, red light travels faster than blue light in any particular transparent medium does.

The various types of radiant energy have different wavelengths and frequencies. These two characteristics multiplied by the frequency are equal to the velocity. Because the velocity is constant in any particular medium, wavelength and frequency are inversely proportional to each other; the longer the wavelength, the lower the frequency is, and vice versa.

Effects of radiant energy. Radiant energy can be detected only when it is absorbed and brings about an observable effect—usually by causing a physical or chemical change. Gamma rays ionize certain gases when absorbed by their molecules. X rays and visible light cause chemical changes in a photographic emulsion that darken it when it is developed. Ultraviolet radiation causes substances such as fluorite to fluoresce (emit light). Visible light stimulates the nerve cells in the retina of the eye, thereby making vision possible. Text: “Electromagnetic Radiation”.

Electromagnetic radiation results whenever electrons oscillate, decelerate, or change energy levels in an atom. X rays, for example, are emitted when fast-moving electrons decelerate rapidly, and radio waves (at the opposite end of the electromagnetic spectrum) are produced by oscillating electrons in the metal of transmitting aerials.

Waves and particles.Electromagnetic radiation consists of a wave motion in both an electric and magnetic field, which oscillate with the same frequency but at right angles to the direction of travel. Electromagnetic waves are therefore examples of transverse waves. But electromagnetic radiation also has properties associated with particles, and its precise nature is something of an enigma. In some circumstances – such as the diffraction of light by a grating – it acts like a wave, whereas in others – for example, the generation of an electric charge when ultraviolet radiation strikes certain metals (a phenomenon called the photoelectric effect) – it acts like a stream of particles. This apparent wave-particle duality has not been fully resolved. For practical purposes electromagnetic radiation can be considered to behave as a wave motion or as a stream of energy particles.

Wave characteristics.Electromagnetic waves are emitted in all directions away from their source. So if the source is a single point, each wave front can be regarded as an expanding sphere. With increasing distance from the source, the intensity of radiation diminishes as the surface area of the sphere increases. The intensity of radiation is thus inversely proportional to the square of the distance travelled.

Another important wave characteristic of electromagnetic radiation is phase. In most sources of radiant energy the atoms emit waves at random intervals. As a result, the vibrations of waves of the same frequency are out of phase with each other – that is, the maximum and minimum strengths of the electric and magnetic fields of one wave do not coincide with those of other waves. Furthermore, although in each single wave the electric and magnetic fields are at right angles to each other, in a collection of waves not all the electric fields (or magnetic ones) lie in the same plane, unless the waves are polarized. Randomly-emitted electromagnetic radiation is described as incoherent, and this is the usual way in which radiant energy is emitted. In lasers, on the other hand, the atoms do not emit randomly and the light produced is coherent – that is, all the waves are in phase.

Particle nature. The particle nature of electromagnetic radiation is explained by the quantum theory, according to which radiant energy is emitted in discrete, individual “packets” of energy called quanta or photons. But even the quantum theory cannot explain all the properties of electromagnetic radiation. As a result, wave-particle duality remains fundamental to understanding the nature of radiant energy.

Basic Properties. All forms of electromagnetic radiation exhibit several basic properties. One of the most fundamental is velocity: all electromagnetic radiations travel at the same constant velocity through a vacuum. They may also pass through other mediums, although not all mediums are transparent to all types of electromagnetic radiation. X rays, for instance, can pass through metal but light cannot. In transparent mediums the velocity of the radiation is slower than in a vacuum; the exact speed depends on the frequency of radiation and the medium’s density. If electromagnetic radiation is not transmitted through a medium, it may be reflected or absorbed.

The other principal properties of electromagnetic radiation are scattering, diffraction, interference, and polarization. Text: “Niels Bohr and Quantum Mechanics”.

Niels Bohr (1885-1962), a Danish physicist, was one of the greatest and most influential scientists of the twentieth century. Bohr developed basic ideas about the structure of an atom. These ideas have been widely accepted by scientists ever since. For his important work, Bohr was awarded the Nobel Prize in physics in 1922.

Bohr, along with other distinguished physicists of the early 1900’s, developed a new branch of physics known as quantum mechanics. These physicists recognized that the laws of motion and gravity formulated by Sir Isaac Newton in the 1600’s (called the Newtonian laws) could not properly explain motion at the atomic level. Newton had developed his physics with the movements of planets in mind. Early atomic researchers – scientists such as Ernest Rutherford and J.J.Thomson – had thought of atoms as tiny models of the solar system. As physicists probed deeper into the atom, however, they gradually realized that the solar system was not an accurate model for atomic motion.

Albert Einstein, Max Planck, Niels Bohr, and others developed the theories of quantum mechanics in the early 1900’s to better describe the atom and its motion. Several important ideas emerged. One idea is that radiation – such as light – is emitted in a stream of separate bundles of energy called quanta. This principle can also be applied to atomic radiation, such as that of electrons. These speculations led physicists to conclude that atomic particles, such as electrons, have properties of both particles and waves. This idea came to be known as the “wave-particle duality” principle. It revolutionized physics. No longer could scientists calculate with certainty the position of an electron in its orbit at a particular moment. Such movements, according to quantum mechanics, can be mathematically described only as probabilities. In other words, since a particle, such as an electron, has qualities of both a particle and a wave, the traditional Newtonian laws of bodies in motion do not apply to it.

Niels Bohr developed these ideas further. He claimed that an atom exists in a series of stable states. Between such states, a jump in energy level – a “quantumjump” – occurs in the atom. The jump can be to a higher energy level (increase) or to a lower energy level (decrease). Such a change in energy happens in a fraction of time. With every quantum jump, light photons are given off. Photons are quanta – tiny bundles – of light energy. Bohr based this theory on observations of thespectrum of light that the element hydrogen gives off. Every chemical element gives off a unique spectrum – that is, band of light frequencies. Bohr thought that the band of light frequencies given off by a chemical element could only be explained by the theory of “quantum jumps” of energy in the atom. Scientific observations made during and after his lifetime have proved his theory true.

Niels Bohr strongly influenced the physicists and scientists of his time. The reasons for this influence went beyond Bohr’s own contributions to physics. In 1921 the University of Copenhagen opened a new Institute of Theoretic Physics with Niels Bohr as its director. This institute became the world center for exchange of ideas among physicists. The institute owed much of its success and influence to its director. Bohr was a scientist who welcomed the exchange of ideas. He also formed close friendships with his co-workers and encouraged them in their work. In his own country as well as abroad, Niels Bohr earned the respect of fellow scientists. He served as president of the RoyalDanishAcademy from 1939 until his death in 1962. Text: “Atomic Structure and Quantum Theory”.

At the turn of the 20th century, physicists spent much effort in trying to derive all the observed phenomena relating to elements from a mathematical model of the atom. One key phenomenon was the spectral lines produced by atoms on being heated: each element has its own unique atomic spectrum. In trying to explain the generation and appearance of atomic spectra, physicists found that they had to introduce the concept of quantum.

Close scrutiny of a black-and-white newspaper photograph reveals that the apparent differences in light and dark tones are not the result of gradual shading but rather are created by subtly different sizes of dots, or “packets” of ink on the plain paper. Physicists have discovered that, in dealing with matter on the smallest scale, even energy must be treated as existing in “packets” or quanta, and not as being continuous. The quantum concept provides the key to understanding how atoms can absorb and emit energy, and how the various atomic spectra are produced.

In a simple quantum-based visualization of the atom, the electrons orbit the central nucleus like planets in a miniature solar system. The electrons can move only in certain orbits about the nucleus. By giving up that energy, they drop back into the original lower orbits (called the ground state). With each energy change, radiation of a particular frequency is absorbed or emitted.

From this analysis, the origin of atomic spectra can be explained. The individual spectral lines (of certain frequencies) represent electrons falling from high-energy excited states back to their low-energy ground states. As they do so, they emit quanta of radiation (photons).Text: “The Nature of the Electron and Wave Mechanics”.

According to the theory of wave mechanics, electrons orbiting a nucleus are not particles moving in orbits. There are standing waves that can be represented mathematically by what is called a wave function (which measures the probability of an electron being at a particular point in space). Peak values of this function can be taken to represent the orbits of the electrons. There is only a high probability – not a certainty – that the electrons will be found in the orbits. The certainty of the old theory has been replaced by a statistical probability measured by the wave function.

According to Heisenberg’s uncertainty principle, which arises from wave-particle duality, it is impossible to measure simultaneously both the position and momentum of a particle within certain limits. It can also be shown that it is similarly impossible to measure the total energy and lifetime of a particle simultaneously and with limitless accuracy.

Although the seemingly totally accurate and certain theories of classical physics have been replaced by the probability arguments and uncertainties of the quantum theory, it turns out that such concepts as Heisenberg’s uncertainty principle predict new, hitherto unexpected phenomena. For example, the law of conservation of energy has been interpreted in a new light; that is, energy cannot be created or destroyed but it may be developed from matter and turned into matter. The presence of this “created” energy – so-called vacuum particles – has been demonstrated experimentally. Text: “Light and Reflection”.

Visible light constitutes only a very small part of the electromagnetic spectrum of wavelengths, just over a hundred-thousandth of an inch long. The longest visible wavelengths (lowest frequencies) are seen as red light, the shortest (highest frequencies) as violet. Various other colors can be identified between these two extremes. Conventionally, the spectral colors are red, orange, yellow, green, blue, indigo, and violet.

Being a form of electromagnetic radiation, visible light exhibits all the properties characteristic of such radiation – for example, wave-particle (photon) duality, reflection, refraction, diffraction, dispersion, interference, and polarization.

Light waves travel outwards in all directions from their source, thereby forming an expanding spherical wavefront. Each individual wave travels in a straight line in a vacuum, or in any isotropic medium (one with uniform properties throughout).

As light travels in straight lines, shadows are formed behind opaque objects. If the light source is very small, effectively a point source, the entire shadow is equally dark and has well-defined edges. If, on the other hand, the source is relatively large – which is usually the case – the shadow is blurred at the edges and has a dark central region of complete shadow (called the umbra) surrounded by a region of partial shadow (the penumbra), which is illuminated by light from part of the source.

Reflection. All objects and surfaces encountered in everyday life reflect light. That is why they are visible. Only a perfectly matte, black surface absorbs all light, and such a surface is extremely difficult to obtain. Many surfaces reflect only light of certain wavelengths, and so they appear colored when illuminated with light, which contains all visible wavelengths.

The nature of the surface also affects the type of reflection. Irregular surfaces and smooth, matte ones reflect light randomly and so cannot form images. For example, the pages of the book reflect light randomly, so enabling the type to be read rather than reflecting an image of the reader. Very smooth, shiny surfaces on the other hand, reflect light in a regular way and these types of “mirror” surfaces can therefore form well-defined images.

Mirrors and images. There are three main types of mirrors: plane, concave, and convex. Plane mirrors are flat and are the most familiar type. Concave mirrors are hollowed – shaving mirrors are an example. And convex mirrors bulge outwards – they are used for some driving mirrors, for instance, because they give a wide angle of view.

Reflection in all three types of mirrors is governed by two principal laws. The first law of reflection states that the incident ray (or beam) striking the surface and the reflected ray (or beam) leaving it are in the same plane as the normal (an imaginary line perpendicular to the surface at the point where the incident ray hits it). The second law of reflection states that the angle of incidence (between the incident ray, or beam, and the normal) is equal to the angle of reflection (between the reflected ray, or beam, and the normal).

Using these two laws it is possible to determine the types of images formed by the different sorts of mirrors.Text: “Luminescence”.

Nearly all light-emitters are extremely hot; examples include the sun and other stars, the filament of an electric lamp, and a candle flame. In physical terms they are all incandescent light sources. But there is another smaller group of “cold” sources that emit light by the phenomenon of luminescence. Fluorescent lamps and the luminescent numerals on a clock, which glow in the dark are examples. And a third type of light source – also not incandescent – is the laser.

Both incandescence and luminescence have the same fundamental origin, which results from energy changes within the atoms of the radiating object. Electrons within the atoms acquire energy, are “excited” to higher energy levels, and in decaying again to their ground states emit electromagnetic radiation in the visible light range. In an incandescent object, the exciting energy is externally applied heat; in a luminescent object, it derives from within the object itself or from an external source other than heat. There are two kinds of luminescence. The type that ceases when external excitation stops is known as fluorescence; the type, that persists even in the absence of external excitation, is called phosphorescence.

Certain bacteria and some minerals and organic chemical compounds that readily glow when irradiated by ultraviolet light are examples of fluorescence. Some washing powders contain fluorescent compounds called optical brighteners that make fabrics appear brighter and cleaner.

The cold, steady light given off by certain fungi is an example of phosphorescence: it persists after the removal of the initial stimulus. An inorganic example is the glow of one type of luminous paint, for which the energy source is daylight, which is re-emitted as green light, the glow fading as the “stored” energy diminishes.

Incoherent and coherent light. In a fluorescent lamp, atoms of mercury vapor are bombarded by electrons and emit ultraviolet light. This radiation in turn excites a phosphor coating inside the tube that emits visible light. But not all the mercury atoms are hit by electrons at the same instant. Thus, some are in an excited state while others are in the ground state. The overall light output is therefore a randomly discontinuous series of extremely brief pulses. This kind of light, which is also produced by all normal incandescent sources, is said to be incoherent.

Waves of light from two such sources do not stay in phase (in step) with each other for any length of time. Where they meet, they cannot, therefore, produce the regular pattern of intensification and extinction that is typical of what is called the interference of light waves, even though the wavelengths may be more or less identical and the light thus monochromatic.

An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent. It is produced, by a process similar to fluorescence, in a laser – a name that denotes Light Amplification by Stimulated Emission of Radiation. Stimulated emission can be best understood by considering light as a stream of electromagnetic radiation particles (photons).Text: “Luminescence”.

Nearly all light-emitters are extremely hot; examples include the sun and other stars, the filament of an electric lamp, and a candle flame. In physical terms they are all incandescent light sources. But there is another smaller group of “cold” sources that emit light by the phenomenon of luminescence. Fluorescent lamps and the luminescent numerals on a clock, which glow in the dark are examples. And a third type of light source – also not incandescent – is the laser.

Both incandescence and luminescence have the same fundamental origin, which results from energy changes within the atoms of the radiating object. Electrons within the atoms acquire energy, are “excited” to higher energy levels, and in decaying again to their ground states emit electromagnetic radiation in the visible light range. In an incandescent object, the exciting energy is externally applied heat; in a luminescent object, it derives from within the object itself or from an external source other than heat. There are two kinds of luminescence. The type that ceases when external excitation stops is known as fluorescence; the type, that persists even in the absence of external excitation, is called phosphorescence.

Certain bacteria and some minerals and organic chemical compounds that readily glow when irradiated by ultraviolet light are examples of fluorescence. Some washing powders contain fluorescent compounds called optical brighteners that make fabrics appear brighter and cleaner.

The cold, steady light given off by certain fungi is an example of phosphorescence: it persists after the removal of the initial stimulus. An inorganic example is the glow of one type of luminous paint, for which the energy source is daylight, which is re-emitted as green light, the glow fading as the “stored” energy diminishes.

Incoherent and coherent light. In a fluorescent lamp, atoms of mercury vapor are bombarded by electrons and emit ultraviolet light. This radiation in turn excites a phosphor coating inside the tube that emits visible light. But not all the mercury atoms are hit by electrons at the same instant. Thus, some are in an excited state while others are in the ground state. The overall light output is therefore a randomly discontinuous series of extremely brief pulses. This kind of light, which is also produced by all normal incandescent sources, is said to be incoherent.

Waves of light from two such sources do not stay in phase (in step) with each other for any length of time. Where they meet, they cannot, therefore, produce the regular pattern of intensification and extinction that is typical of what is called the interference of light waves, even though the wavelengths may be more or less identical and the light thus monochromatic.

An interference pattern results when two sources emit light of the same wavelength and in phase; such light is called coherent. It is produced, by a process similar to fluorescence, in a laser – a name that denotes Light Amplification by Stimulated Emission of Radiation. Stimulated emission can be best understood by considering light as a stream of electromagnetic radiation particles (photons).

Text: “X-Rays and Gamma-Rays”.

The part of the electromagnetic spectrum with very short wavelengths (between about 10-9 m and 10-15 m) and very high frequencies corresponds to X-rays and gamma rays. Both are high-energy, penetrating types of radiation that can ionize gasesand cause other chemical and physical changes, such as fluorescence.

X-rays. X-rays are one of the types of radiation emitted by some stars and galaxies. Studies of heavenly bodies from artificial satellites have revealed various X-ray sources, among which are possible locations of black holes. The observations have to be made from space because X-rays do not pass through the atmosphere (although highly energetic, X-rays do not penetrate the atmosphere because they interact with molecules in the air). Here on earth, X-rays are generated artificially.

X-rays are produced using thermionic vacuum tubes that operate at potentials of up to 2 million volts. A stream of electrons, fired from the cathode of the tube, strikes a metal anode (the target), which emits X-rays. The X-ray beam is emitted at right angles to the electron beam. Because of the high voltage necessary and the penetrating power of the radiation itself, commercial X-ray machines are heavily insulated and shielded with lead, which absorbs X-ray.

The best-known uses of X-rays are in medicine: in radiology to “photograph” the internal structures of the body and in radiotherapy for destroying cancerous tumors. X-rays are also used extensively for internal examination of industrial materials, for example in the detection of flaws and other weaknesses in metal castings and welds.

Gamma rays. During the radioactive decay of unstable isotopes of elements, such as uranium, thorium, and radium, gamma rays are emitted. These constitute electromagnetic radiation of extremely short wavelength, from about 10-10 m down to 10-15 m or less, overlapping to some extent with those of “hard” X-rays. The main difference between the twotypes of radiation is in their origins. X-rays are produced by bombardment of atoms with high-energy electrons, which cause excitation of the atomic electrons. Gamma rays derive from the excitation of the atomic nucleus itself. In both cases, characteristic radiation is emitted when the excited electrons or nucleons return to their original energy levels.

The properties of gamma rays resemble those of X-rays. Like X-rays, gamma rays can be used to reveal flows in solid materials; a suitable source for such applications is cobalt-60 or some other long-lived radioisotope. In research into particle physics, gamma rays often occur as product of collisions between high-energy subatomic particles. They are, therefore, important in the study of the ultimate structure of matter.Text: “Sound”.

Sound consists of waves of alternate compression and rarefaction that transmit kinetic energy through a medium. If there is no medium, there is no sound; sound waves cannot pass through a vacuum. All sounds originate from vibrating objects, such as the vocal cords in the human throat, the skin of a drum, or the strings of a violin. The vibrations are then passed on through a medium – most commonly air – until they strike the ear drum; the ear converts the vibrations into nerve impulses, which pass to the brain where they are interpreted as sounds. Microphones are the other most common instruments of sound waves into electrical signals.

Waves and sound.There are two types of waves in physics: longitudinal and transverse. Sound waves are of the longitudinal type – that is, their vibrations take place in the same direction as they travel. The phenomenon of longitudinal wave propagation can be demonstrated with a long, large-coiled spring. When the string is fixed at one end and the other end is moved backwards and forwards, alternating regions of compression (where the coils are bunched together) and rarefaction (where the coils are farther apart than normal) move along the spring.

Transverse waves are those in which the vibrations occur at right angles to the direction, in which the waves travel – as happens, for example, when a length of rope is moved regularly up and down to give it a wavelike appearance. Ocean waves and light waves also travel by transverse propagation.

Frequency and intensity. Two obvious ways of distinguishing one “pure” sound (that is, a “pure” tone) from another are by their pitch and loudness. Neither property is a scientifically accurate notion. Pitch is a word used by musicians and is closely related to frequency; but it is a subjective term, whereas frequency can be measured physically. Similarly loudness, closely related to a sound’s intensity, differs according to the sensitivity of the hearer’s ears, whereas intensity can be measured in exact physical quantities. (Real sounds are mixtures of many different pure sounds and also have a characteristic “quality”, which can be assessed by detailed examination of the shape of a real sound’s waveform.)

The frequency of a sound wave is the number of wavelengths that are completed in a given period of time. The universal unit of measurement is the hertz (Hz), which represents one complete wavelength, or cycle, per second. The human ear is sensitive to sound in the frequency range from about 20 Hz. As people grow older, their ability to perceive high frequencies diminishes.

The intensity of sound is measured in terms of the amount of power that passes each second through a given area perpendicular to the direction of the sound wave. The intensity unit is the decibel (db). There is no absolute decibel scale; it is a relative scale.

Velocity. Sound can be transmitted by any medium – gas, liquid, or solid – and all sound waves travel at a constant velocity through any given medium at a constant temperature. But depending on their elasticity and density, some mediums transmit waves faster than others. It is extremely difficult for sound waves to pass from a medium of one density to another of a much higher or lower density.

Anything that changes the density of a medium also alters the speed at which sound travels through it. Hence the velocity of sound is affected by temperature; as the temperature increases, the medium expands. Its density decreases, with the result that the velocity of sound.

Text: “Electricity”.

Electricity and all its observable effects result from properties of stationary or moving elec­tric charges. There are two types, known as positive charges and negative charges, which exist in all substances and in all states of matter—solid, liquid, and gas.

All substances are made up of atoms, which consist of a nucleus (containing protons and neutrons) and orbiting electrons. Protons are positively charged, and electrons are neg­atively charged. Electrons can be removed from atoms and made to take part in various electrical phenomena. Protons, on the other hand, are usually found as part of a cluster of protons, neutrons, and electrons known as an ion. Ions may be positively or negatively charged, depending on whether they have an excess of protons or electrons.

In an ordinary piece of material the num­bers of protons and electrons within each atom, and therefore within the material as a whole, are equal. The positive and negative charges cancel each other to a large extent, and no electrical effects are observed. But if extra charges of one type are added, the mate­rial is left with a net charge and electrical ef­fects result.

Charges can be transferred from one mate­rial to another by rubbing them together. The study of these net stationary charges and their effects on each other constitutes the subject of static electricity or electrostatics.

In certain solid materials—metals in particular—some electrons are sufficiently "free" to be able to move through the material in a stream, forming an electric current. This is the basis of current electricity.

Solids, liquids, and gases can all conduct electric current if there are enough free elec­trons or ions. Electrons and positively charged ions move in opposite directions through the material, but in general both contribute to the current in a conducting medium. In an insula­tor, the electrons are too firmly fixed by chemical bonding to be able to move, and the mate­rial does not normally conduct electricity.

Static electricity. Static electricity can be produced by rubbing an insulating material such as a polyethylene rod with dry cloth. Charges are transferred so that the polyethylene is left with a net charge of one type and the cloth with a net charge of the other type. The rod and cloth then attract each other.

On the other hand, two rubbed rods of the same material repel, and the two cloths repel. Unlike charges thus experience a force of attraction, whereas like charges repel each other.

It is impossible to find a material that is repelled by both polyethylene and cloth.

Electrostatic induction. Although nothing is repelled by both polyethylene and cloth, uncharged pieces of material (for example, small scraps of paper) are attracted to both. This results from a phenomenon called electrostatic induction. The paper is neutral overall, but the influence of the charged polyethylene redistributes the charges in the paper. The polyethylene is positively charged and exerts an attractive force on the paper's negative charges The net result is mutual attraction.

Coulomb's law and electric field. The forces of electrostatic attraction and repulsion between charged objects are caused by the electric fields associated with them. The magnitude of the force between two charges depends on their size, distance apart, and the substance they are in.The force is proportional to the product of the charges, and inversely proportional to the square of the distance between them – a relationship known as Coulomb's law.

The force also decreases if the charges are placed in a material that undergoes electrical polarization, which separates the charges and has the effect of partly shielding them from each other. The amount of shielding is quantified by a property called the permittivity of a substance. A vacuum, which cannot undergo polarization, has the lowest permittivity, called the permittivity of free space. The permittivity of other materials depends on their structure; ionic substances such as water, for example, generally have higher permittivities than non-ionic substances.Text: “Electronics”.

Electronics is a general term for the production and processing of electric signals (consisting of moving electrons, hence the name electronics) that convey information – for example, the sound reproduction by a radio or record player and the data output of a computer. Electronics also includes the production and processing of beams of electrons, used in such devices as cathode-ray oscilloscopes, television sets, and electron microscopes.

Electric signals. Electric signals are produced by devices that convert the primary information source (which is in the form of another type of energy – sound or light, for instance) into small electric currents. For example, a microphone converts sound into an electric current, and in input unit of a computer converts data. These currents, which constitute a signal, are then transmitted along wires or, after conversion to radio waves, via radio links. On reaching a suitable receiver, they may be electronically manipulated – to reduce distortion in the signal, for instance – and are than amplified so that they can drive an output device, such as a loudspeaker, television set, or computer printer.

There are two types of electric signal: analog and digital. Analog signals vary continuously in voltage or current, corresponding to variations in the primary information source; thus the electric signals produced by a microphone are electrical “copies” of the original sound waves. Digital signals are not continuous but consist of numerous bursts of electric current between two voltage levels (one of which may be zero).

A computer uses digital signals. They consist of binary codes that represent the numbers, letters, and symbols in the input data and in the various program instructions. In the computer’s central processor, the data codes are subjected to processes of binary arithmetic or are compared with each other to obtain the results required by the program.

In modern electronics, electric signals may be processed in two main ways – by passing them trough semiconductor devices (such as transistors) or by converting them into a beam of electrons, as in the formation of a picture by a television set (which also uses semiconductors).

Semiconductors. Semiconductors are substances (such as silicon and germanium) whose electrical resistance lies between that of conductors and insulators. They have this intermediate resistance because they have a few free electrons that can drift from atom to atom – unlike conductors, which have many free electrons, and unlike insulators, which have none.

The semiconductors used in electronic devices are “doped” to change their electrical properties. Doping involves introducing minute traces of other elements into the semiconductor’s crystal lattice. Silicon and germanium each have four outer electrons per atom; doping them with an element with five outer electrons, such as phosphorus, frees the fifth electron so that the semiconductor has an electron excess. It is then known as an n-type (negative-type), because electrons have a negative charge.

Doping with an element that has only three outer electrons, such as boron, produces a crystal lattice with spaces, known as holes, which free electrons readily fill. This type of semiconductor has a lack of free electrons, which is equivalent to an excess of positive charges, and it is therefore known as p-type.

If a piece of n-type semiconductor is joined to a piece of p-type, the resulting device is called a p/n junction diode. If the diode is then connected to a battery so that the negative terminal is joined to the n-type semiconductor and the positive terminal to the p-type, current flows through the junction.

Transistors. Transistors are arrangements of semiconductor diodes that act as amplifiers. There are two main types: junction and field-effect transistors.Text: “Magnetism”.

Magnetism is one of the fundamental properties of matter. All substances – even those such as glass, plastics, and wood, for example – exhibit magnetic properties to some extent, although most materials do so to such a minute degree that they are generally regarded as nonmagnetic. Only the metallic elements iron, nickel, and cobalt, and certain of their alloys (such as steel) are strongly magnetic; such substances are described as being ferromagnetic.

In general all materials can be classed as diamagnetic, paramagnetic, or ferromagnetic. Each behaves in a characteristic way when placed inside a current-carrying solenoid (that is, in a uniform magnetic field), depending on the electronic structure of the atoms and the arrangement of the atoms themselves within the material.

In a piece of diamagnetic material (such as bismuth or copper), the electrons in each atom are arranged so that their magnetic effects cancel out each other; hence a diamagnetic substance has no overall magnetic field associated with it. If a bar of the material is placed in an inducing field, however, the electron paths are slightly distorted. As a result, the substance becomes very weakly magnetized, but in the opposite direction to that of the inducing field. Hence the susceptibility (the ratio of the induced to the inducing magnetic field) of diamagnetic materials is very small and negative (typically about – 10-5). If a bar of diamagnetic material is freely suspended in a strong uniform magnetic field, it aligns itself across the field.

In a paramagnetic material (such as platinum or aluminum) the magnetic effects of the electrons do not cancel completely and each atom acts like a minute, weak bar magnet. Under normal circumstances, these atomic dipoles are oriented at random and there is no overall magnetic effect. Under the influence of an inducing field, however, the dipoles tend to align with the field. As a result, the magnetic field induced in the substance is in the same direction as that of the inducing field, and a freely-suspended bar of paramagnetic material therefore aligns itself with the inducing field. But the dipole alignment is not complete and so the induced magnetism tends to be weak, although it is still strong enough to overwhelm the diamagnetic effect. The susceptibility of paramagnetic substances is small and positive (in the order of +10-3).

Many substances are paramagnetic, but very few exhibit the third and most important form of magnetism – ferromagnetism. Ferromagnetic materials, such as iron and cobalt, display paramagnetic behavior, but much more strongly than most paramagnetic substances because many more atomic dipoles align when in an inducing field. Thus ferromagnetic materials have large, positive susceptibilities (often greater that +103).Text: “Gravity”.

Gravity is a force of the mutual attraction between objects that have mass. Its influence in everyday life is so pervasive that it is taken for granted. The seventeenth-century mathematician and physicist Isaac Newton was the first to formulate the law of gravitation. He expressed it in mathematical terms that could be used in calculations. It was called Newton’s law of universal gravitation. It states that the gravitational force between two objects is directly proportional to the product of the masses of the two objects, and inversely proportional to the square of the distance between them.

To make calculations possible, it is necessary to include a constant, G, which is known as the universal constant of gravitation. Experimental measurements have shown that the gravitational constant G has a value of about 6.67 x 10-11 m-3 / (kg · s2).

Physicists characterize the strength of a gravitational field in terms of the acceleration that a body of a certain mass and radius produces on a falling object. It can be found by equating the force of gravity between the body and a mass of 1kg with Newton’s second law of motion, which gives the acceleration produced by a force acting on a particular mass. The result (the acceleration due to gravity, or the gravitational field strength) is denoted by g, which for the Earth has a value of about 9.8 ms-2.

Newton’s mathematical description of gravity is adequate for weak gravitational fields. But it fails when applied to large masses confined to small regions in which the force of gravity is very strong. In this case Einstein’s theory of gravitation (the General Theory of Relativity) has to be used.

According to Einstein’s General Theory of Relativity, the mutual attraction between objects is the result of the curving of space and time around them. The curvature has observable effects on rays of light that pass close to massive objects, such as the sun or other stars. Studies of these effects indicate that the idea of space-time curvature is valid. Einstein suggested that gravitation is a curved field in the space-time continuum, rather than a force as Sir Isaak Newton had thought.

One prediction of the General Theory is the existence of gravitational waves, a phenomenon produced whenever an object is accelerated. But gravitational waves are so weak that they have not yet been detected directly; at present there is only indirect evidence for their existence. In most situations, however, Newton's law suffices for calculating gravitational forces.

A commonplace manifestation of the gravitational attraction between massive bodies is the phenomenon of the tides, which results from the unequal attractions of the moon and sun on the water in the oceans of the earth. Text: “Albert Einstein”.

Albert Einstein (1879-1955) was one of the greatest scientists and most creative minds of all time. Before he was 30 years old, Einstein had published a group of brilliant theories that completely changed modern science. Einstein revealed several of his earth-shaking theories in the single year of 1905. He presented his ideas in three scientific papers.

One of the papers of 1905 was entitled, On a Heuristic Viewpoint Concerning the Production and Transformation of Light. Einstein suggested that light could be thought of as a stream of tiny particles, called quanta. For this reason, light demonstrates qualities of both waves and particles. Einstein’s ideas about light laid important groundwork for the quantum theory of physics. The quantum theory would become, in a short time, a revolutionary way of thinking about motion at the atomic level. The 1921 Nobel Prize in physics was awarded to Einstein for this work.

Another paper of 1905, The Electrodynamics of Moving Bodies, presented Einstein’s special theory of relativity. This theory introduced the following idea: given that the speed of light is constant and all natural laws are the same, then both time and motion are relative to the observer. This was a radical change in scientific thinking, because no one had yet thought that time or motion might be subjective, that is changeable depending upon the observer’s perspective.

Still another Einstein’s paper of 1905 was On the Motion of Small Particles Suspended in a Stationary Liquid. In this paper, Einstein explained the apparently random motion of tiny particles in a liquid. This type of motion is known in science as Brownian motion. Some years later, Einstein developed his general theory of relativity. In 1916 he published an article entitled The Foundation of the General Theory of Relativity. Einstein explained that gravitation is a curved field in the space-time continuum, rather than a force as Sir Isaac Newton had thought. Theoretically, this meant that starlight should be bent, or deflected, as it passes by the sun. But there was no immediate way to test Einstein’s theory.

From the 1920s on, Einstein directed his scientific efforts toward developing unified field theory that would unify all kinds of forces, such as gravity and electromagnetism. Einstein’s efforts in this field drew him into controversy. Many, if not most, physicists believed that the quest for a unified field theory was an impossible one. They reasoned that the movement of a single particle cannot be predicted, according to the quantum theory. For the most part, they also believed that future theoretical advances in physics would never remove this basic uncertainty principle. For these reasons, Einstein became rather isolated from the scientific community toward the end of his life. Nevertheless, Einstein continued his work in physics with unified field theory. He published a version of the theory in 1950.Text: “The Nature of the Universe”.

When Pluto was discovered in 1930; scientists already knew a lot about the stars beyond our own solar system. But now the frontiers of our knowledge have moved into deepest space. As this has happened, we have had to develop new ways of seeing and thinking.

Here are two examples of what we think we understand.

The size of the universe. All space with everything in it, including all stars, is called the universe. If we measure space distances by light years instead of earth years, the universe still seems too large to understand. A light year is the distance light travels in a year. Light travels at 186,000 miles per second (300,000 kilometres per second).

By this measure we know that:

Earth to Sun is

Our solar system is

The nearest star (Proxima Centauri) is

Our galaxy (the Milky Way) is

The galaxy Andromeda (visible to the naked eye) is

We can now detect other galaxies

81/2 light minutes.

11 light hours across.

light years away.

100,000 light years across and

contains about 100,000 million stars.

2 million light years away.

15,000 million light years away.

What does the last enormous number in the table really mean to us?

a) The light we now see, through a twentieth century telescope, started its journey toward us 15,000 million years ago.

b) The light started toward us billions of earth years before the sun or the earth had even started to form.

c) If the light has taken so long to reach us, then the stars that gave out that light have had 15,000 million years to change. They may have moved elsewhere, died, or given birth to new kinds of stars. What we now see has not been there for billions of earth years.

d) We are looking back in time to the very beginning of the universe itself.

The expanding universe. All objects, including stars that give out light reveal facts about themselves in the kind of light they give out. The whole range of energy that can be given out is shown in the diagram below. Only a small part of it is visible light.

In the 1920s, the American astronomer Edwin Hubble began to measure the spectrum of distant galaxies. He discovered that the farther away the galaxy was, the longer were the wavelengths of red light. He realized he was experiencing something common to everyday life. When we hear a siren or horn, its noise changes as it moves toward or away from us. This is called the Doppler effect. It is caused by the wavelengths of sound being shortened as the horn moves nearer and stretched out as it moves away.

Hubble realized this was exactly what was happening with the light from the galaxies. But all the red light showed was lengthening; none showed shortening. Therefore, all the galaxies are moving away from us. From the amount of red shift, he could also tell how quickly they were moving away. His conclusion was that the farther away the galaxy was, the faster it was moving away.

This led to two important theories becoming widely accepted.

The universe is expanding, and space is being stretched apart between the galaxies. The Big Bang Theory. As the theory of the expanding universe has been accepted, two important questions have been asked: a) How long has the universe been expanding?

b) What was it like when it started to expand?

These questions led to the Big Bang Theory, which states that all the universe we can see had a single point of origin about 15,000 million years ago. It suddenly came into existence in a gigantic explosion. This was when everything began, and the “fragments” are still moving apart. It is only a theory, but there is much evidence to support it.Text: “Stephen Hawking”.

Stephen Hawking was born on the 300th anniversary of Galileo’s death. He has come to be thought of as the greatest mind in physics since Albert Einstein. Hawking grew up outside London in an intellectual family. His father was a physician and specialist in tropical diseases; his mother was active in the Liberal Party. He was an awkward schoolboy, but knew from early on that he wanted to study science. He became increasingly skilled in mathematics and in 1958 he and some friends built a primitive computer that actually worked. In 1959 he won a scholarship to OxfordUniversity, where his intellectual capabilities became more noticeable.

In 1962 he got his degree with honors and went to CambridgeUniversity to pursue a PhD in cosmology. There he became intrigued with black holes (first proposed by Robert Oppenheimer) and “space-time singularities,” or events in which the laws of physics seem to break down. After receiving his PhD, he stayed at Cambridge, becoming known even in his 20s for his pioneering ideas and use of Einstein’s formulas.

In 1968 he joined the staff of the Institute of Astronomy in Cambridge and began to apply the laws of thermodynamics to black holes by means of very complicated mathematics. Hawking’s best-known work is on the nature of black holes, a phenomenon suggested by Einstein’s equations describing what might happen should a huge star die. He proposes it would crush into its own center with such strong gravitational force, that nothing could escape, not even light. Introducing his own theoretical modifications, Hawking posits the existence of mini-black holes. Created by the tremendous force of the Big Bang, these mini-black holes would not only emit sub-atomic particles and radiation (now called Hawking radiation), but would gradually evaporate over the space of 1066 years, only to explode with the energy of millions of hydrogen bombs. At the same time, Hawking is the first to admit that there is as yet no physical evidence that black holes actually exist.

He continued working on the theory of the origin of the universe, and in doing so found ways to link relativity (gravity) with quantum mechanics (the inner workings of atoms). This contributed enormously to what physicists call Grand Unified Theory, a way of explaining, in one equation, all physical matter in the universe.

At the remarkably young age of 32, he was named a fellow of the Royal Society. He received the Albert Einstein Award, the most prestigious in theoretical physics. And in 1979, he was appointed Lucasian Professor of Mathematics at Cambridge, a post once occupied by Sir Isaac Newton. There he began to question the big theory, which by then most had accepted. Perhaps, he suggested, there was never a start and would be no end, but just change – constant transition of one “universe” giving way to another through glitches in space-time. All the while, he was digging into exploring black holes, string theory, and the birth of black holes in our own galaxy.

In 1988 Hawking published A Brief History of Time: From the Big Bang to Black Holes. He hoped the book would help non-scientists understand theories of nature, from the big bang to black holes. It became a best-seller of long standing and established his reputation as an accessible genius. He wrote other popular articles and appeared in movies and television. He remains extremely busy, his work hardly slowed by Lou Gehrig’s disease (amyotriphic lateral sclerosis, an incurable disease of the nervous system that affects muscle control) for which he uses a wheelchair and speaks through a computer and voice synthesizer. His movement is limited to a barely perceptible lifting of one finger and very little facial expression. “My goal is simple. It is complete understanding of the universe, why it is as it is and why it exists at all”.

Fundamental physical constants These constants characterize the properties of space, time, and matter at the deepest level. Their values ​​do not depend on the choice of the system of units.

Speed ​​of light in a vacuum (c): 299,792,458 m/s (exact). The limiting speed of the propagation of interactions.

Gravitational constant (G): 6.674 30(15)·10−11 m3·kg−1·s−2. The coefficient in Newton's law of universal gravitation.

Planck constant (h): 6.626 070 15·10−34 J·s (Planck action). Describes the quantization of energy. Reduced Planck constant (ħ = h/2π): 1.054 571 817...·10−34 J·s.

Elementary charge (e): 1.602 1892 · 10−19 C. The magnitude of the electron charge. Boltzmann constant (k): 1.380 662(20)·10−23 J/K. Relates temperature to energy.

Avogadro constant (Na): 6.022 141 29·1023 mol−1. The number of particles in one mole of substance. Faraday constant (F = Na·e): 9.648 539 12·104 C/mol. The charge of one mole of electrons. Electric constant (ε0): 8.854 187 8128 · 10−12 F/m. The permittivity of a vacuum. Magnetic constant (μ0): 4π·10−7 H/m. Or 1.256 637 061 43 × 10−6 H/m. Stefan-Boltzmann constant (σ): 5.670 × 10−8 W/(m2·K4). Total radiated power of a black body. Wien's displacement constant (b): 2.897 82 × 10−3 m·K. Relationship between temperature and the wavelength of peak emission. Rydberg constant (R∞): 1.097 373 15 × 107 m−1. Determines atomic spectral series. Fine-structure constant (α): ≈1/137. Characterizes the strength of electromagnetic interaction.

Other important constants Acceleration due to gravity (g): 9.80665 m/s2 (at Earth's surface). Atomic mass unit (amu): 1.660 539 066 × 10−27 kg. Bohr radius (a0): 0.529 177 210 × 10−10 m. Radius of the electron orbit closest to the nucleus in a hydrogen atom. Bohr magneton (μp): 9.274 010 × 10−24 J/T. Unit of magnetic moment.

Mechanics

Distance. Equal to the product of velocity and time.

Speed. Equal to the ratio of distance to time.

Density. Equal to the ratio of mass to volume.

Elastic force (Hooke's law). Equal to the product of the spring stiffness and its elongation.

Gravity. Equal to the product of mass and the acceleration due to gravity.

Pressure. Equal to the ratio of force to the area over which it acts.

Liquid column pressure. Equal to the product of the liquid density, the acceleration due to gravity, and the height of the liquid column.

Archimedes' force. Equal to the product of the liquid density, the acceleration due to gravity, and the volume of the submerged body.

Mechanical work. Equal to the product of force and the distance traveled in the direction of the force.

Power. Equal to the ratio of work to the time it takes to complete that work.

Kinetic energy. Equal to half the product of the mass of an object and the square of its velocity.

Potential energy. Equal to the product of the mass, the acceleration due to gravity, and the height above ground level.

Molecular Physics and Thermal Phenomena

The amount of heat required for heating or cooling. Equal to the product of the specific heat capacity, the mass, and the temperature difference (final and initial).

The amount of heat released during fuel combustion. Equal to the product of the specific heat of combustion and the mass of the fuel.

The amount of heat required for melting. Equal to the product of the mass of an object and the specific heat of fusion.

Electricity and Magnetism

Ampere's force. Equal to the product of the magnitude of the magnetic induction vector, the current, the length of the circuit section, and the sine of the angle between the direction of the current and the magnetic induction vector.

Electric voltage. Equal to the work done by the electric field to move the charge.

Resistance. Equal to the ratio of the voltage across a circuit section to the current. Ohm's Law for a circuit section. Voltage is equal to the product of current and resistance.

EMF (electromotive force). Equal to the work done by external forces to move charge along a circuit.

Joule-Lenz Law. The amount of heat released by a current-carrying conductor is equal to the product of the square of the current, resistance, and time.

Optics

Thin lens formula. Determines the position of the image relative to the object, focal length, and distance to the focal point.

Lens optical power. Equal to the reciprocal of the focal length.

Diffraction grating formula. Determines the angular position of the maxima.

Thin lens formula (second condition). Links focal length, optical power, and the position of the foci.

Atomic and Nuclear Physics

Law of conservation of electric charge. The algebraic sum of charges before and after interaction remains unchanged.

The law of radioactive decay. The number of decays is directly proportional to the number of existing nuclei.