Question
A thin semicircular conducting ring (PQR) of radius r is falling with its plane vertical in a horizontal magnetic field B, as shown in figure. The potential difference developed across the ring when its speed is v, is
-
zero
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is at higher potential
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and R is at higher potential
-
is at higher potential
Solution
D.

For motional emf,e = Bv x (2r)
= 2rBv
R will be at higher potential.
The electrons of the wire will move towards end P due to due to electric force and at end R the excess positive charge will be left.