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.
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.
The behavior of electric and magnetic fields in these waves is quite similar to that of a very long taut string, one end of which is rapidly moved up and down in a periodic fashion. Any point along the string will be observed to move up and down, or oscillate, with the same period or with the same frequency as the source. Points along the string at different distances from the source will reach the maximum vertical displacements at different times, or at a different phase. Each point along the string will do what its neighbor did, but a little later, if it is further removed from the vibrating source (Oscillation). The speed with which the disturbance, or the message to oscillate, is transmitted along the string is called the wave velocity (Wave Motion). This is a function of the medium, its mass, and the tension in the case of a string. An instantaneous snapshot of the string (after it has been in motion for a while) would show equispaced points having the same displacement and motion, separated by a distance known as the wavelength, which is equal to the wave velocity divided by the frequency. In the case of the electromagnetic field one can think of the electric-field strength as taking the place of the up-and-down motion of each piece of the string, with the magnetic field acting similarly at a direction at right angles to that of the electric field. The electromagnetic-wave velocity away from the source is the speed of light.
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.
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.
The behavior of electric and magnetic fields in these waves is quite similar to that of a very long taut string, one end of which is rapidly moved up and down in a periodic fashion. Any point along the string will be observed to move up and down, or oscillate, with the same period or with the same frequency as the source. Points along the string at different distances from the source will reach the maximum vertical displacements at different times, or at a different phase. Each point along the string will do what its neighbor did, but a little later, if it is further removed from the vibrating source (Oscillation). The speed with which the disturbance, or the message to oscillate, is transmitted along the string is called the wave velocity (Wave Motion). This is a function of the medium, its mass, and the tension in the case of a string. An instantaneous snapshot of the string (after it has been in motion for a while) would show equispaced points having the same displacement and motion, separated by a distance known as the wavelength, which is equal to the wave velocity divided by the frequency. In the case of the electromagnetic field one can think of the electric-field strength as taking the place of the up-and-down motion of each piece of the string, with the magnetic field acting similarly at a direction at right angles to that of the electric field. The electromagnetic-wave velocity away from the source is the speed of light.
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