A circular loop has its radius increasing at a rate of 2 m/s. The loop is placed perpendicular to a constant magnetic field of 0.1 Wb/m². When the radius of the loop is 2 m, the emf induced in the loop will be

• A. 0.2π V
• B. 0.4π V
• C. 0.8π V
• D. zero

Answer: (C)

emf = dΨ/dt = change of flux linkage
emf = dф/dt
emf = d(B*Area)/dt
emf = B*d(area)/dt
emf = B*d(πr²)/dt
emf = Bπ*2r*dr/dt
emf = Bπ*2r*change of radius per unit time
emf = 0.1*π*2*2
emf = 4π