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In practice, for a step-up transformer the current in the primary is I_P and the current in the secondary is I_S; their respective voltages are V_P and V_S. Then,

Asked in GSEB Board March 2020 (old course) · Transformers

• (1) I_SV_S>I_PV_P
• (2) I_SV_S=I_PV_P
• (3) I_SV_S<I_PV_P
• (4) (I_S)/(V_S)=(I_P)/(V_P)

Step-by-step solution

Why the other options are wrong

• (1) Output power greater than input would mean the transformer creates energy. Stepping the voltage up steps the current down, so the output power can never exceed the input.
• (2) I_SV_S=I_PV_P describes an ideal, loss-free transformer. The question says in practice: real windings and cores always waste some power, so a little less comes out than goes in.
• (4) This says the current changes in the same ratio as the voltage, as in a resistor obeying Ohm's law. In a transformer the current changes in the inverse ratio: stepping the voltage up steps the current down.

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