Discussion
Calculate the current passing through a resistor of 500 Ω if the power consumed in it is 0.8 W.
Answer: C
The current is 40 mA. The power consumed in a resistor is P = I²R, so the current is found from the power and the resistance.
\[ I = \sqrt{\frac{P}{R}} = \sqrt{\frac{0.8}{500}} \]
\[ = \sqrt{0.0016} = 0.04\ \text{A} \]
That is 40 mA. The values 10 mA, 20 mA and 80 mA do not give 0.8 W in 500 Ω: for example 20 mA gives only 0.2 W, and 80 mA gives 3.2 W. So the third option is correct.
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