VCE · Topic explainer

How do AC and DC differ?

Alternating current reverses direction periodically. Direct current flows in one direction. For a fixed resistor, an alternating voltage with a given RMS value produces the same average heating power as a steady DC voltage of that value. For a sinusoidal AC voltage, divide the peak voltage by √2 to find its RMS value.

Worked example

A resistor of 8 Ω is connected to a sinusoidal supply with RMS voltage 12 V. Its average power is 12² ÷ 8 = 18 W. A steady 12 V DC supply gives the same power in that resistor.

The sinusoidal AC voltage has a peak of 12√2 V, about 17.0 V. Its average over a complete cycle is zero, while its RMS value is 12 V. RMS captures the heating effect because instantaneous power depends on the square of the voltage.

AC: 12 V RMS across 8 Ω. DC: constant 12 V across 8 Ω. Each gives 18 W average power.

Official exam practice

2025 VCE Physics, Section A, Question 11

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