In an ideal transformer, what is conserved across windings?

• A. Power
• B. Resistance
• C. Capacitance
• D. Inductance

Answer: (A)

Power is conserved across windings in an ideal transformer because there are no losses. The primary side supplies power Vp Ip, and the secondary delivers power Vs Is, and with an ideal transformer these are equal: Vp Ip = Vs Is. The turns ratio fixes the voltage relationship, Vs = Vp × (Ns/Np). To keep power the same, the current on the secondary must adjust inversely, Is = Ip × (Np/Ns). So when voltage is stepped up, current steps down proportionally, keeping the total power constant. The other quantities—resistance, capacitance, and inductance—are properties of the windings and aren’t what remains invariant across windings in the ideal model (they’re neglected to focus on lossless power transfer).