Question

A wire loop is rotated in a magnetic field. The frequency of change of direction of the induced emf is

• A. Twice per revolution
• B. Four times per revolution
• C. Six times per revolution
• D. Once per revolution

Answer 1

The correct option is A Twice per revolution

Flux through the loop $\varphi =BA\cos \theta =BA\cos \omega t$
where $\theta =\omega t$ is the angle made by area vector with magnetic field at any instant.

emf induced $=-\frac{d\varphi }{dt}=BA\omega \sin \omega t$


From the above graph, we have 4 points of change of direction in emf induced, in two time periods, $T$ each. Thus there two changes in one time period (i.e twice per revolution).