In a transformer, what quantity is stepped down when voltage is stepped up?

• A. Current
• B. Voltage
• C. Power
• D. Impedance

Answer: (A)

In a transformer, voltage and current trade off to conserve power. In an ideal transformer, the input power equals the output power, so V1 I1 ≈ V2 I2. If voltage is stepped up from the primary to the secondary (V2 > V1), the current on the secondary must be smaller than the primary current to keep the product V·I the same. Put simply, increasing voltage means a decrease in current for the same load power. This is why high-voltage transmission uses higher voltage and lower current—to minimize I²R losses in the conductors. The same principle helps explain how impedance appears differently on each side due to the turns ratio, but the direct effect of stepping up voltage is a stepped-down current.