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Anyone who has ever grabbed an automobile spark-pluq wire at the wrong place, with the engine running, has an appreciation of the ability of a changinq current in (part of) a coil of wire to induce an emf in the coil. What happens is that the breaker contacts open, suddenly interrupting the current, and causing a sudden large change in the magnetic field through the coil; according to Faraday's law, this results in a (large) induced emf. In general, the production of an emf in a coil by a changing magnetic field due to a current in that same coil is called self-induction; and the ability of a coil to produce an emf in this way is commonly measured by its self-inductance L, usually called more briefly its inductance. A coil used in this way is more formally called an inductor.
The transmission of electric signals by television, radio, and telephone depends on time-varying currents and fields to represent the appearance of pictures and the sound of voices; and so, as you can well imaqine, capacitors and inductors play an important role in the circuits of such devices. You already know that a capacitor can store energy; so can an inductor. If an inductor carrying a current is connected to a resistor, its energy is dissipated as heat in the resistor, much as for a charged capacitor. But now suppose you connect it instead to a capacitor; the inductor will try to give its energy to the capacitor - and vice versa - but the initial energy is not quickly dissipated from the electrical circuit. What do you suppose happens? If you do not already know, can you guess, before studyinq this module?