Tricky physics a level circuits question

For AS OCR physics A.
http://tinypic.com/r/2s85v75/9

The answer is 'C'. How?

This question is as much about reading the question carefully as well as the calculation.

The question asks for the ratio

\mathrm {\frac{Power \ dissipated \ in \ R}{Power \ supplied \ by \ the \ cell}}

You could first find the total current (you should get 0.125A but we don't even need to calculate this in the final answer).

Then use (or simply go straight to)

P = I^2R

Total power supplied by the cell uses the sum of all the resistances in the circuit (3 + 7 + 2 Ohms = 12 Ohms)

The power dissipated in the resistance uses 7 Ohms.

Putting it all together you should get

\mathrm {Ratio} = \frac{I^2R_{variable}}{I^2R_{total}}

The

I^2

terms cancel leaving a ratio of:

\frac{7}{12} = 0.58

(edited 6 years ago)

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