![]() ![]() Initially, seven magnetic field lines are going through the loop (see left-hand image). To understand why an emf is generated in a coil due to a moving magnet, consider Figure 20.34, which shows a bar magnet moving downward with respect to a wire loop. Thus, emf is the energy per unit charge added by a source, which contrasts with voltage, which is the energy per unit charge released as the charges flow through a circuit. The emf may be defined as the rate at which energy is drawn from a source per unit current flowing through a circuit. For this reason, we avoid the term electromotive force and just use the abbreviation emf, which has the mathematical symbol ε. Unfortunately, the name electromotive force stuck and with it the potential for confusing it with a real force. The energy added per unit charge has units of volts, so the electromotive force is actually a potential. This interpretation turned out to be incorrect instead, the external source doing the work of moving the magnet adds energy to the charges in the coil. If a current is induced in the coil, Faraday reasoned that there must be what he called an electromotive force pushing the charges through the coil. The sign of voltage will remain same because the magnitude of current flow will not change by moving south pole of the magnet to the left. ![]() In addition, moving the magnet in the opposite direction (compare Figure 20.33 with Figure 20.33) or reversing the poles of the magnet (compare Figure 20.33 with Figure 20.33) results in a current in the opposite direction. When the magnet is motionless with respect to the coil, no current is induced in the coil, as in Figure 20.33. He found that current is induced only when the magnet moves with respect to the coil. ![]() A schematic of this experiment is shown in Figure 20.33. One experiment Faraday did to demonstrate magnetic induction was to move a bar magnet through a wire coil and measure the resulting electric current through the wire. Other devices that use magnetism to induce currents include pickup coils in electric guitars, transformers of every size, certain microphones, airport security gates, and damping mechanisms on sensitive chemical balances. The electric generator-found in everything from automobiles to bicycles to nuclear power plants-uses magnetism to generate electric current. “Madam,” he replied, “What good is a baby?” Today, currents induced by magnetic fields are essential to our technological society. When Faraday discovered what is now called Faraday’s law of induction, Queen Victoria asked him what possible use was electricity. The basic process of generating currents with magnetic fields is called induction this process is also called magnetic induction to distinguish it from charging by induction, which uses the electrostatic Coulomb force. In 1831, some 12 years after the discovery that an electric current generates a magnetic field, English scientist Michael Faraday (1791–1862) and American scientist Joseph Henry (1797–1878) independently demonstrated that magnetic fields can produce currents. If nature is symmetrical, then perhaps a magnetic field can create a current.
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