# Revision problems

### Explanation of question 4

A battery has an e.m.f. of 12V and an internal resistance of 4. The power dissipated in a 20 resistor connected across its terminals will be

1. 36W
2. 7.2W
3. 6W
4. 5W

The answer is :- 4

Total circuit resistance is 24. Therefore, by Ohm's law, the circuit current will be. I= V / R = 12/24 = 0.5A. The voltage on the 24 resistor will be V = I x R = 0.5 x 20 = 10V. The power dissipated by the resistor, P = V x I = 10 x 0.5 = 5W. This can also be calculated using the equation P = I2 x R = 0.52 x 20 = 5W

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