In the coils of a transformer, current I is ______ proportional to the voltage V on the coils.

• A. Directly proportional to V
• B. Inversely proportional to V
• C. Independent of V
• D. Exponentially related to V

Answer: (B)

In a transformer, the current in a winding is set by the load connected to the other winding and the turns ratio, through the reflected impedance. For an ideal transformer with turn ratio a = N_p/N_s, the impedance seen from the primary is Z_in = a^2 R_L, where R_L is the secondary load. The primary current is then I_p = V_p / Z_in. Since Z_in is fixed for a given load and turns ratio, the primary current changes linearly with the applied primary voltage V_p. In other words, the current is directly proportional to the voltage, not inversely proportional. The power on the primary, P_in = V_p I_p, then grows roughly with V_p^2 for a fixed load, reinforcing that the relationship is not inverse. Real transformers have additional factors like magnetizing current and losses, but the fundamental behavior under load is a direct proportionality between primary current and primary voltage.