A conducting square loop initially lies in the XZ plane with its lower edge hinged along the X-axis. Only in the region y 0, there is a time dependent magnetic field pointing along the Z-direction, B(t)=B0( t) k, where B0 is a constant. The magnetic field is zero everywhere else. At time t=0, the loop starts rotating with constant angular speed about the X axis in the clockwise direction as viewed from the +X axis (as shown in the figure). Ignoring self-inductance of the loop and gravity, which of the following plots correctly represents the induced e.m.f. (V) in the loop as a function of time: [figure]
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- [figure]
- [figure]
- [figure]
Answer
(1) [figure]
Solution
At time t the loop (side L) makes angle t with the Z-axis; its projection on the XY plane has area L2 t, lying in y 0 for 0< t< .
For 0<t< : =B0 t L2 t= B0L22 2 t
V=- d dt=-B0L2 2 t: starts at -B0L2 , rises to +B0L2 at t= 2 , back to -B0L2 at t= .
For <t< 2 the loop is in y<0 where B=0: =0, V=0 (sudden drop to zero at ).
From 2 the pattern repeats — this is plot (A).
Answer: (A)