A semicircular wire of radius R is rotated with constant angular velocity $\mathrm{\omega }$ about an axis passing through one end and perpendicular to the plane of the wire. There is a uniform magnetic field of strength B. The induced e.m.f. between the ends is:

• A

  $\mathrm{B} {\mathrm{\omega R}}^2/2$

• B

  $2 \mathrm{B} {\mathrm{\omega R}}^2$

• C

  is variable

• D

  None of these