NEIEP Magnetism And Electromagnetism (355)

In an ideal transformer, how are primary and secondary currents related to the turns ratio?

Is/Ip = Np/Ns Is/Ip = Ns/Np Is/Ip = (Np/Ns)^2 Is/Ip independent of turns

Correct Answer:

Is/Ip = Np/Ns

Explanation:

In an ideal transformer, power must be conserved: Vp Ip = Vs Is, and the voltages relate to turns as Vp/Vs = Np/Ns. Combine these two relations: Is/Ip = Vp/Vs = Np/Ns. So the secondary-to-primary current ratio equals the turns ratio, meaning Is/Ip = Np/Ns. This also matches the intuitive idea that increasing voltage on the secondary (more turns) comes with a proportional decrease in current, and vice versa, to keep power the same. For example, if Np is twice Ns, the secondary current is twice the primary current, since Is/Ip = Np/Ns = 2.

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