Question

The variation of induced emf (E) with time (t) in a coil if a short bar magnet is moved along its axis with a constant velocity is best represented as \(\{\)North pole is situated at left end of the bar magnet and South pole is situated at right end of the bar magnet\(\}\)

• A. C
• B. D
• C. A
• D. B

Hint:

Flux through the coil rises, stays nearly constant, then falls, so the emf is a pulse, a zero stretch, and an opposite pulse.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
The induced emf is \(\varepsilon=-\dfrac{d\phi}{dt}\), so it follows the rate of change of flux.

Step 2: Approach
While the magnet approaches and enters, the flux increases, so an emf of one sign appears (Lenz's law).

Step 3: Inside the coil
While the short magnet is well inside, the flux hardly changes, so the emf is nearly zero.

Step 4: Leaving
As it leaves, the flux decreases and the emf reverses sign. At constant speed the two pulses are of similar size. This is graph B, option (D).

Final Answer:
Graph B, with a pulse of one sign, a zero stretch and an opposite pulse, is correct, option (D). \[ \boxed{\text{(D) }\text{Graph B}} \]