Physics

Article Outline

Introduction; Scope of Physics; Early History of Physics; Newton and Mechanics; Modern Physics; Developments in Physics Since 1930

Electricity and Magnetism

Although the ancient Greeks were aware of the electrostatic properties of amber, and the Chinese as early as 2700 bc made crude magnets from lodestone, experimentation with and the understanding and use of electric and magnetic phenomena did not occur until the end of the 18th century. In 1785 the French physicist Charles Augustin de Coulomb first confirmed experimentally that electrical charges attract or repel one another according to an inverse square law, similar to that of gravitation. A powerful theory to calculate the effect of any number of static electric charges arbitrarily distributed was subsequently developed by the French mathematician Siméon-Denis Poisson and the German mathematician Carl Friedrich Gauss.

A positively charged particle attracts a negatively charged particle, tending to accelerate one toward the other. If the medium through which the particle moves offers resistance to that motion, this may be reduced to a constant-velocity (rather than accelerated) motion, and the medium will be heated up and may also be otherwise affected. The ability to maintain an electromotive force that could continue to drive electrically charged particles had to await the development of the chemical battery by the Italian physicist Alessandro Volta in 1800. The classical theory of a simple electric circuit assumes that the two terminals of a battery are maintained positively and negatively charged as a result of its internal properties. When the terminals are connected by a wire, negatively charged particles will be simultaneously pushed away from the negative terminal and attracted to the positive one, and in the process heat up the wire that offers resistance to the motion. Upon their arrival at the positive terminal, the battery will force the particles toward the negative terminal, overcoming the opposing forces of Coulomb's law. The German physicist Georg Simon Ohm first discovered the existence of a simple proportionality constant between the current flowing and the electromotive force supplied by a battery, known as the resistance of the circuit. Ohm's law, which states that the resistance is equal to the electromotive force, or voltage, divided by the current, is not a fundamental and universally applicable law of physics, but rather describes the behavior of a limited class of solid materials. See Electric Circuit.

The historical concepts of magnetism, based on the existence of pairs of oppositely charged poles, had started in the 17th century and owe much to the work of Coulomb. The first connection between magnetism and electricity, however, was made through the pioneering experiments of the Danish physicist and chemist Hans Christian Oersted, who in 1819 discovered that a magnetic needle could be deflected by a wire nearby carrying an electric current. Within one week after learning of Oersted's discovery, the French scientist André Marie Ampère showed experimentally that two current-carrying wires would affect each other like poles of magnets. In 1831 the British physicist and chemist Michael Faraday discovered that an electric current could be induced (made to flow) in a wire without connection to a battery, either by moving a magnet or by placing another current-carrying wire with an unsteady—that is, rising and falling—current nearby. The intimate connection between electricity and magnetism, now established, can best be stated in terms of electric or magnetic fields, or forces that will act at a particular point on a unit charge or unit current, respectively, placed at that point. Stationary electric charges produce electric fields; currents—that is, moving electric charges—produce magnetic fields. Electric fields are also produced by changing magnetic fields, and vice versa. Electric fields exert forces on charged particles as a function of their charge alone; magnetic fields will exert an additional force only if the charges are in motion.

These qualitative findings were finally put into a precise mathematical form by the British physicist James Clerk Maxwell who, in developing the partial differential equations that bear his name, related the space and time changes of electric and magnetic fields at a point with the charge and current densities at that point. In principle, they permit the calculation of the fields everywhere and any time from a knowledge of the charges and currents. An unexpected result arising from the solution of these equations was the prediction of a new kind of electromagnetic field, one that was produced by accelerating charges, that was propagated through space with the speed of light in the form of an electromagnetic wave, and that decreased with the inverse square of the distance from the source. In 1887 the German physicist Heinrich Rudolf Hertz succeeded in actually generating such waves by electrical means, thereby laying the foundations for radio, radar, television, and other forms of telecommunications. See Electromagnetic Radiation.

Light

The apparent linear propagation of light was known since antiquity, and the ancient Greeks believed that light consisted of a stream of corpuscles. They were, however, quite confused as to whether these corpuscles originated in the eye or in the object viewed. Any satisfactory theory of light must explain its origin and disappearance and its changes in speed and direction while it passes through various media. Partial answers to these questions were proposed in the 17th century by Newton, who based them on the assumptions of a corpuscular theory, and by the English scientist Robert Hooke and the Dutch astronomer, mathematician, and physicist Christiaan Huygens, who proposed a wave theory. No experiment could be performed that distinguished between the two theories until the demonstration of interference in the early 19th century by the British physicist and physician Thomas Young. The French physicist Augustin Jean Fresnel decisively favored the wave theory.

Interference can be demonstrated by placing a thin slit in front of a light source, stationing a double slit farther away, and looking at a screen spaced some distance behind the double slit. Instead of showing a uniformly illuminated image of the slits, the screen will show equispaced light and dark bands. Particles coming from the same source and arriving at the screen via the two slits could not produce different light intensities at different points and could certainly not cancel each other to yield dark spots. Light waves, however, can produce such an effect. Assuming, as did Huygens, that each of the double slits acts as a new source, emitting light in all directions, the two wave trains arriving at the screen at the same point will not generally arrive in phase, though they will have left the two slits in phase. Depending on the difference in their paths, “positive” displacements arriving at the same time as “negative” displacements of the other will tend to cancel out and produce darkness, while the simultaneous arrival of either positive or negative displacements from both sources will lead to reinforcement or brightness. Each apparent bright spot undergoes a timewise variation as successive in-phase waves go from maximum positive through zero to maximum negative displacement and back. Neither the eye nor any classical instrument, however, can determine this rapid “flicker,” which in the visible-light range has a frequency from 4 × 10¹⁴ to 7.5 × 10¹⁴ Hz, or cycles per second. Although it cannot be measured directly, the frequency can be inferred from wavelength and velocity measurements. The wavelength can be determined from a simple measurement of the distance between the two slits, and the distance between adjacent bright bands on the screen; it ranges from 4 × 10^-5 cm (1.6 × 10^-5 in) for violet light to 7.5 × 10^-5 cm (3 × 10^-5 in) for red light with intermediate wavelengths for the other colors.

The first measurement of the velocity of light was carried out by the Danish astronomer Olaus Roemer in 1676. He noted an apparent time variation between successive eclipses of Jupiter's moons, which he ascribed to the intervening change in the distance between Earth and Jupiter, and to the corresponding difference in the time required for the light to reach the earth. His measurement was in fair agreement with the improved 19th-century observations of the French physicist Armand Hippolyte Louis Fizeau, and with the work of the American physicist Albert Abraham Michelson and his coworkers, which extended into the 20th century. Today the velocity of light is known very accurately as 299,292.6 km (185,971.8 mi sec) in vacuum. In matter, the velocity is less and varies with frequency, giving rise to a phenomenon known as dispersion. See also Optics; Spectrum; Vacuum.

Maxwell's work contributed several important results to the understanding of light by showing that it was electromagnetic in origin and that electric and magnetic fields oscillated in a light wave. His work predicted the existence of nonvisible light, and today electromagnetic waves or radiations are known to cover the spectrum from gamma rays (see Radioactivity), with wavelengths of 10^-12 cm (4 × 10^-11 in), through X rays, visible light, microwaves, and radio waves, to long waves of hundreds of kilometers in length (see X Ray). It also related the velocity of light in vacuum and through media to other observed properties of space and matter on which electrical and magnetic effects depend. Maxwell's discoveries, however, did not provide any insight into the mysterious medium, corresponding to the string, through which light and electromagnetic waves had to travel (see the Electricity and Magnetism section above). Based on the experience with water, sound, and elastic waves, scientists assumed a similar medium to exist, a “luminiferous ether” without mass, which was all-pervasive (because light could obviously travel through a massless vacuum), and had to act like a solid (because electromagnetic waves were known to be transverse and the oscillations took place in a plane perpendicular to the direction of propagation, and gases and liquids could only sustain longitudinal waves, such as sound waves). The search for this mysterious ether occupied physicists' attention for much of the last part of the 19th century.

The problem was further compounded by an extension of a simple problem. A person walking forward with a speed of 3.2 km/h (2 mph) in a train traveling at 64.4 km/h (40 mph) appears to move at 67.6 km/h (42 mph), to an observer on the ground. In terms of the velocity of light the question that now arose was: If light travels at about 300,000 km/sec (about 186,000 mi/sec) through the ether, at what velocity should it travel relative to an observer on earth while the earth also moves through the ether? Or, alternately, what is the earth's velocity through the ether? The famous Michelson-Morley experiment, first performed in 1887 by Michelson and the American chemist Edward Williams Morley using an interferometer, was an attempt to measure this velocity; if the earth were traveling through a stationary ether, a difference should be apparent in the time taken by light to traverse a given distance, depending on whether it travels in the direction of or perpendicular to the earth's motion. The experiment was sensitive enough to detect even a very slight difference by interference; the results were negative. Physics was now in a profound quandary from which it was not rescued until Einstein formulated his theory of relativity in 1905.

Thermodynamics

A branch of physics that assumed major stature during the 19th century was thermodynamics. It began by disentangling the previously confused concepts of heat and temperature, by arriving at meaningful definitions, and by showing how they could be related to the heretofore purely mechanical concepts of work and energy. See also Heat Transfer.

Heat and Temperature

A different sensation is experienced when a hot or a cold body is touched, leading to the qualitative and subjective concept of temperature. The addition of heat to a body leads to an increase in temperature (as long as no melting or boiling occurs), and in the case of two bodies at different temperatures brought into contact, heat flows from one to the other until their temperatures become the same and thermal equilibrium is reached. To arrive at a scientific measure of temperature, scientists used the observation that the addition or subtraction of heat produced a change in at least one well-defined property of a body. The addition of heat, for example, to a column of liquid maintained at constant pressure increased the length of the column, while the heating of a gas confined in a container raised its pressure. Temperature, therefore, can invariably be measured by one other physical property, as in the length of the mercury column in an ordinary thermometer, provided the other relevant properties remain unchanged. The mathematical relationship between the relevant physical properties of a body or system and its temperature is known as the equation of state. Thus, for an ideal gas, a simple relationship exists between the pressure, p, volume V, number of moles n, and the absolute temperature T, given by pV = nRT, where R is the same constant for all ideal gases. Boyle's law, named after the British physicist and chemist Robert Boyle, and Gay-Lussac's law or Charles's law, named after the French physicists and chemists Joseph Louis Gay-Lussac and Jacques Alexandre César Charles, are both contained in this equation of state (see Gases).

Until well into the 19th century, heat was considered a massless fluid called caloric, contained in matter and capable of being squeezed out of or into it. Although the so-called caloric theory answered most early questions on thermometry and calorimetry, it failed to provide a sound explanation of many early 19th-century observations. The first true connection between heat and other forms of energy was observed in 1798 by the Anglo-American physicist and statesman Benjamin Thompson who noted that the heat produced in the boring of cannon was roughly proportional to the amount of work done. In mechanics, work is the product of a force on a body and the distance through which the body moves during its application.

The First Law of Thermodynamics

The equivalence of heat and work was explained by the German physicist Hermann Ludwig Ferdinand von Helmholtz and the British mathematician and physicist William Thomson, 1st Baron Kelvin, by the middle of the 19th century. Equivalence means that doing work on a system can produce exactly the same effect as adding heat; thus the same temperature rise can be achieved in a gas contained in a vessel by adding heat or by doing an appropriate amount of work through a paddle wheel sticking into the container where the paddle is actuated by falling weights. The numerical value of this equivalent was first demonstrated by the British physicist James Prescott Joule in several heating and paddle-wheel experiments between 1840 and 1849.

That performing work or adding heat to a system were both means of transferring energy to it was thus recognized. Therefore, the amount of energy added by heat or work had to increase the internal energy of the system, which in turn determined the temperature. If the internal energy remains unchanged, the amount of work done on a system must equal the heat given up by it. This is the first law of thermodynamics, a statement of the conservation of energy. Not until the action of molecules in a system was better understood by the development of the kinetic theory could this internal energy be related to the sum of the kinetic energies of all the molecules making up the system.

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Electricity and Magnetism

Light

Thermodynamics

Heat and Temperature

The First Law of Thermodynamics