Inductance is a phenomenon important in alternating-current (ac) circuits or in any circuit in which current is changing, whether it be transitory change or regularly repeating change. It is a sort of
pas de trois around the
Q–
ℰ–
ℬ triangle (described in
Essay E4):
Q→
ℬ→
ℰ→
Q. A current, which is charge in motion (
Q), generates a magnetic field (
ℬ). When the current changes, the magnetic field changes, and a changing magnetic field generates an electric field (
ℰ). This field exerts a force on electric charge (
Q), specifically on the charge whose motion constitutes the original current. Does this newly generated electric field work to increase or to decrease the original current? If you guess that it works against the original current, you will be right. This could be worked out with the contortions of right-hand rules, or it can be convincingly argued using a conservation-of-energy argument. If, after one trip around the
Q–
ℰ–
ℬ triangle, the newly generated electric field were to increase the current, that increased current would, after another trip around the
Q–
ℰ–
ℬ triangle, augment the current even more—a runaway situation, violating energy conservation. If the newly generated field opposes the original change of current, the current would be damped. The energy released could be absorbed by a resistor in the circuit or be stored in a capacitor. Overall, the effect of inductance is to slow down change, preventing a too-sudden increase or too-sudden decrease of current. This general rule that a changing effect produces a result that opposes the change goes by the name “Lenz’s law.”
In some circuits, one wants more inductance. Just as a circuit element designed to add resistance to a circuit is called a resistor, a circuit element designed to add inductance is called an inductor. An ordinary straight wire has some inductance, but far more inductance can be achieved by winding a wire into a coil. Then the changing magnetic field created by one part of the wire has a chance to influence many other parts of the wire. In some circuits, on the other hand, one wants to minimize the so-called transient effects that arise from the time lag in current changes. Then the circuit must be designed with as little inductance as possible. Equally or more often, however, the inertial effect of an inductor serves some beneficial purpose in a circuit. Probably the most important role of inductance is in a circuit designed to produce electric oscillations. That requires that the inductor be joined by a capacitor.
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