Question
A copper bar of length L is moving to the right with a uniform speed v in a region of uniform magnetic field of flux density B, directly perpendicularly downwards into the paper in the figure below.
The ends of the rods are rigidly connected to a voltmeter which moves with the rod. What is the reading on the voltmeter?
A) zero
B) non-zero reading less than Blv
C) BLv
D) more than BLv
Answer
Method 1:
Consider wires PQ and RS. Since both wires are moving through the magnetic field, both of them will have an induced e.m.f. of BLv.
Based on Fleming’s Right Hand rule, the induced current of PQ and RS will both be upwards.
This means that the resultant current will be zero and the e.m.f. measured by the voltmeter will be zero.
Answer is A.
Method 2:
Consider the closed loop PQRS. As the loop moves through the magnetic field, the magnetic flux linkage remains the same since the area of the loop does not change.
Since there is no change in the magnetic flux linkage, by Faraday’s law there is no induced e.m.f. and the voltmeter will read 0 V.
Answer is A.
