Q: A coil is placed in a transverse magnetic field of 0.02 T. If this coil starts shrinking at a rate of 1 mm/s, while its radius remains 4 cm, then what is the value of induced e.m.f.?

(a) 2 μV

(b) 5 μV

(c) 8 μV

(d) 50 μV

Ans: (b)

Sol: Here, B = 0.02T , dr/dt = 1 mm/s =10-3 m/s

r= 4 cm =4 × 10-2 m , e = ?

φ = BA = B (π r2 )

e= dφ/dt= d/dt (B π r2 )= B π 2 r (dr/dt)

= 0.02 × 22/7 ×2 × 4 ×10-2 × 10-3

= 5× 10-6 V = 5 μV