Electromagnetic Induction MCQs for NEET — Physics Questions with Answers

The NCERT states, 'Magnetic flux through a plane of area A placed in a uniform magnetic field B (Fig. 6.4) can be written as $\Phi_B = B \cdot A = BA \cos \theta$' and also, 'Magnetic flux is a scalar quantity.' This quantity quantifies the total 'flow' or number of magnetic field lines through an area.

Experiment 6.3 description states, 'It is observed that the galvanometer shows a momentary deflection when the tapping key K is pressed. The pointer in the galvanometer returns to zero immediately. If the key is held pressed continuously, there is no deflection in the galvanometer.' This supports Faraday's law that emf is induced only by a change in magnetic flux. When the key is pressed, current changes from zero to maximum, causing a change in flux. When held pressed, current and thus flux are constant, so no emf.

The definition of magnetic flux is given as $\Phi_B = B \cdot A = BA \cos \theta$, where $\theta$ is the angle between the magnetic field vector B and the area vector A.

Magnetic flux is given by $\Phi_B = BA \cos \theta$. Even if B and A are constant, changing the angle $\theta$ (orientation of the coil) will change $\cos \theta$ and thus change the magnetic flux, inducing an emf. The NCERT states, 'one method to induce an emf or current in a loop is through a change in the loop’s orientation or a change in its effective area.'

Faraday's experiments (6.1, 6.2, 6.3) consistently showed that induced current/emf arises from 'relative motion between a magnet and a coil' (or two coils) or from a 'change in the current (and resulting magnetic field) in a nearby coil'. The core principle is that the magnetic flux 'associated with coil C1' must change. Hence, relative motion creating changing flux or the field itself changing is necessary.

Faraday’s laws of induction imply that the emf induced in a coil of N turns is directly related to the rate of change of flux through it. ($\epsilon = -N \frac{d\Phi_B}{dt}$).

Lenz’s law states that the polarity of the induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produces it. This opposition ensures that the induced current does work against the change in flux, which is consistent with the conservation of energy.

When a metal rod of length $l$ is placed normal to a uniform magnetic field $B$ and moved with a velocity $v$ perpendicular to the field, the induced emf (called motional emf) across its ends is $\epsilon = Blv$.

Inductance is the ratio of the flux-linkage to current. It is equal to $N\Phi/I$.

A changing current in a coil (coil 2) can induce an emf in a nearby coil (coil 1). This phenomenon is called mutual induction.