An ideal transformer with turns ratio N1:N2 = 2:1 has secondary current 3 A. What is the primary current?

An ideal transformer with turns ratio N1:N2 = 2:1 has secondary current 3 A. What is the primary current?

• A. 6 A
• B. 3 A
• C. 1.5 A
• D. 0.5 A

Answer: (C)

The key idea is how currents relate to turns in an ideal transformer. Power is conserved, so V1 I1 = V2 I2, and voltages scale with turns: V1/V2 = N1/N2. With a turns ratio of 2:1, the primary has twice as many turns as the secondary, which means V1 is twice V2. To keep power equal, the primary current must be half of the secondary current: I1 = I2 × (N2/N1) = 3 A × (1/2) = 1.5 A. So the primary current is 1.5 A.

The key idea is how currents relate to turns in an ideal transformer. Power is conserved, so V1 I1 = V2 I2, and voltages scale with turns: V1/V2 = N1/N2. With a turns ratio of 2:1, the primary has twice as many turns as the secondary, which means V1 is twice V2. To keep power equal, the primary current must be half of the secondary current: I1 = I2 × (N2/N1) = 3 A × (1/2) = 1.5 A. So the primary current is 1.5 A.