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The mutual inductance between a primary and a secondary circuit is 0.5 H. The primary and secondary resistances are 20 ohm and 5 ohm respectively. To produce a current of 0.4 A in the secondary, at what rate must the current in the primary be changed?

1. 4.0 A/s
2. 16.0 A/s
3. 1.6 A/s
4. 8.0 A/s

Correct answer: 4.0 A/s

Solution

The EMF induced in the secondary is M*(dI/dt) and it drives the secondary current through R_secondary: M*(dI/dt) = Iₛ * Rₛ. So dI/dt = Iₛ * Rₛ / M = (0.4 * 5)/0.5 = 2/0.5 = 4.0 A/s.

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