that reversing the change will produce an induced voltage of opposite polarity. Some authors use the term induced e.m.f. ( electromotive force ) rather than induced voltage . In FLAP we refrain from using that term to avoid the misconception that the quantity involved is a force . The quantity is a potential difference , not a force.
FLAP P4.4 Electromagnetic induction COPYRIGHT © 1998 THE OPEN UNIVERSITY S570 V1.1 bar magnet motion of magnet wire ring Figure 5 4 Inducing a current in a wire ring. In Figures 2 and 3 you can think of the coil in the meter circuit as acting like a voltage generator, whilst the field is changing , with its ends as the generator terminals and the induced voltage as the potential difference between them, but this picture can sometimes be unhelpful. For example, in Figure 5, moving the magnet induces a current in the wire, but now the ‘coil’ consists of a single ring of wire and there is no obvious location for the generator. ☞ It is often better to think of electromagnetic induction as something that takes place throughout a circuit, and to think in terms of induced potential differences or voltages being produced between any two points in the circuit. The induced current and voltage are related via the circuit resistance 1 — 1 just as current and voltage are related by Ohm’s law in a d.c. circuit. Thus, in a circuit of total resistance R , an induced voltage V ind will produce an induced current I ind where I ind = V ind R (1)
FLAP P4.4 Electromagnetic induction COPYRIGHT © 1998 THE OPEN UNIVERSITY S570 V1.1 Electromagnetic induction is an important phenomenon with many useful applications. However, there is also a situation where an induced voltage in a circuit is particularly troublesome. This is where fluctuating magnetic fields (e.g. from a.c. mains currents nearby) produce unwanted voltages in a circuit. This phenomenon is called electromagnetic pick-up . Often a great deal of trouble is taken to avoid this problem, for example by screening out the fields or by careful electrical grounding of the circuits and the electrical equipment. We can summarize this introductory subsection as follows: Electromagnetic induction is a process by which a changing magnetic field causes an induced voltage. The more rapid the change, the greater the voltage. The direction of the change determines the polarity of the voltage.
FLAP P4.4 Electromagnetic induction COPYRIGHT © 1998 THE OPEN UNIVERSITY S570 V1.1 S S N flexible suspension diaphragm output Figure 6 4 See Question T1. Question T1 A microphone is a device used to produce an electrical signal from a sound signal. Figure 6 shows one type of microphone (a moving-coil microphone) which uses electromagnetic induction. ☞ Write one or two sentences to describe what will happen when sound waves make the diaphragm (and the attached coil) vibrate; N and S are the north and south poles of a magnet. 4 ❏
FLAP P4.4 Electromagnetic induction COPYRIGHT © 1998 THE OPEN UNIVERSITY S570 V1.1 2.2 The polarity of an induced voltage and Lenz’s law We can predict the polarity of an induced voltage by considering the principle of energy conservation, applied to electromagnetic induction.
- Fall '19
- Magnetic Field, Lenz