AQA · GCSE · Physics · Higher

Coin Resistance from Power

Q05.2 4 marks

The vending machine identifies the value of the coin by measuring the resistance of the coin.

power = (current)² × resistance,

P = I^2R

The power dissipated by the coin is 340 mW when the current in the coin is 0.75 A.

Calculate the resistance of the coin.

Use the Physics Equations Sheet.

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Marking points

1 Converts 340 mW to 0.34 W.
2 Uses P = I²R with I = 0.75 A.
3 Rearranges correctly for R.
4 Obtains approximately 0.60 Ω, subject to official error-carried-forward guidance.

Full-mark answer

$340\text{ mW}=0.340\text{ W}$ . Using $P=I^2R$ , $R=P/I^2=0.340/0.75^2=0.604\ldots\,\Omega$ , so $R=0.60\,\Omega$ .

Why this answer loses marks

I substituted the power in milliwatts directly and divided by the current.

Convert power to watts and remember that current is squared before rearranging for resistance.