ENERGY IN TRANSFORMERS

3 marks · Normal

A step-down transformer has a primary input voltage of 120 V and a primary input current of 0.25 A. The transformer is 100% efficient, and the output voltage is 24 V. Calculate the output current of the transformer.

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1.25 A (±0.01)

Common mistakes: Using input voltage/output voltage directly rather than input power = output power; Not multiplying voltage and current

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Marking points

M1 · 1 mark · States or uses input power = output power (VpIp = VsIs)

M2 · 1 mark · Correct substitution of values (120 x 0.25 = 24 x Is) (follow-through)

A1 · 1 mark · Correct calculation and final answer (1.25 A) (follow-through)

Worked solution

For a 100% efficient transformer:

Substitute values:

Calculate input power:

Now solve for Is: