Page 61 - Basic Electrical Engineering

The minus sign is introduced in accordance with Lenz’s law which is stated
below.

Lenz’s law: This law states that the induced EMF due to change of flux

linkage by a coil will produce a current in the coil in such a direction that it
will produce a magnetic field which will oppose the cause, that is the change

in flux linkage.
The students may conduct an experiment in the laboratory, similar to that

done by Faraday, which is explained below.
If the magnet shown in Fig. 1.10 (a) is quickly brought near the coil, there
will be deflection in the galvanometer indicating EMF induced in the coil and

current flow in the circuit. If the magnet is held stationary near the coil,
although there is flux linking the coil, there will be no induced EMF since

there is no change in the flux linkage. The induced EMF will be there only if
there is increase or decrease in flux linkage by the coil. It will be observed

that when the magnet is taken away quickly there will be deflection in the
galvanometer. It may also be noted that EMF will also be induced in the coil

when the coil is moved keeping the magnet stationary.

Figure 1.10 Faraday’s experiment on electromagnetic induction. (a) A magnet is suddenly brought
near a coil; (b) determination of the direction of current produced in the coil