22.1

Induced Emf and Induced Current

There are a number of ways a magnetic field can be used to generate an electric current, and Figure 22-1 illustrates one of them. This drawing shows a bar magnet and a helical coil of wire to which an ammeter is connected. When there is no relative motion between the magnet and the coil, as in part a of the drawing, the ammeter reads zero, indicating that no current exists. However, when the magnet moves toward the coil, as in part b, a current I appears. As the magnet approaches, the magnetic field that it creates at the location of the coil becomes stronger and stronger, and it is this changing field that produces the current. When the magnet moves away from the coil, as in part c, a current is also produced, but with a reversed direction. Now the magnetic field at the coil becomes weaker as the magnet moves away, and once again it is the changing field that generates the current.

Figure 22-1 (a) When there is no relative motion between the coil of wire and the bar magnet, there is no current in the coil. (b) A current is created in the coil when the magnet moves toward the coil. (c) A current also exists when the magnet moves away from the coil, but the direction of the current is opposite to that in (b).

A current would also be created in Figure 22-1 if the magnet were held stationary and the coil were moved, because the magnetic field at the coil would be changing as the coil approached or receded from the magnet. Only relative motion between the magnet and the coil is needed to generate a current; it does not matter which one moves.
The current in the coil is called an induced current because it is brought about (or “induced”) by a changing magnetic field. Since a source of emf (electromotive force) is always needed to produce a current, the coil itself behaves as if it were a source of emf. This emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current.
Induced emf and induced current are frequently used in the cruise control devices found in many cars. Figure 22-2 illustrates how a cruise control device operates. Usually two magnets are mounted on opposite sides of the vehicle’s drive shaft, with a stationary sensing coil positioned nearby. As the shaft turns, the magnets pass by the coil and cause an induced emf and current to appear in it. A microprocessor (the “brain” of a computer) counts the pulses of current and, with the aid of its internal clock and a knowledge of the shaft’s radius, determines the rotational speed of the drive shaft. The rotational speed, in turn, is related to the car’s speed. Thus, once the driver sets the desired cruising speed with the speed control switch (mounted near the steering wheel), the microprocessor can compare it with the measured speed. To the extent that the selected cruising speed and the measured speed differ, a signal is sent to a servo, or control, mechanism, which causes the throttle/fuel injector to send more or less fuel to the engine. The car speeds up or slows down accordingly, until the desired cruising speed is reached.

Figure 22-2 Induced emf lies at the heart of an automobile cruise control device. The emf is induced in a sensing coil by magnets attached to the rotating drive shaft.

The physics of an automobile cruise control device.

Figure 22-3 shows another way to induce an emf and a current in a coil. An emf can be induced by changing the area of a coil in a constant magnetic field. Here the shape of the coil is being distorted so as to reduce the area. As long as the area is changing, an induced emf and current exist; they vanish when the area is no longer changing. If the distorted coil is returned to its original shape, thereby increasing the area, an oppositely directed current is generated while the area is changing.

Figure 22-3 While the area of the coil is changing, an induced emf and current are generated.

In each of the previous examples, both an emf and a current are induced in the coil because the coil is part of a complete, or closed, circuit. If the circuit were open—perhaps because of an open switch—there would be no induced current. However, an emf would still be induced in the coil, whether the current exists or not.
Changing a magnetic field and changing the area of a coil are methods that can be used to create an induced emf. The phenomenon of producing an induced emf with the aid of a magnetic field is called electromagnetic induction. The next section discusses yet another method by which an induced emf can be created.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.