A long solenoid (cross-sectional area = 1.2 x 10-6 m2, number of turns per unit

Question

A long solenoid (cross-sectional area = 1.2 x 10-6 m2, number of turns per unit length = 1973 turns/m) is bent into a circular shape so it looks like a doughnut. This wire-wound doughnut is called a toroid. Assume that the diameter of the solenoid is small compared to the radius of the toroid, which is 0.059 m. Find the emf induced in the toroid when the current decreases from 2.5 A to 1.1 A in a time of 0.15 s.

I tried following another problem that was the same with different numbers and got 7.49E-009 V which isn't right. Please help!

Explanation / Answer

induced emf = -NA dB/dt

for a toroid B = 0Ni/2r

dB/dt = 4*10^-7*1973*0.059*-1.4/0.15/2*0.059

dB/dt = 3.68 *10^-3 T

= -1973*1.2*10^-6 *3.68*10^-3

= 5140 *10^-9 VOLTS

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