Question

The voltage across a bulb is decreased by $2\%.$ Assuming that the resistance of the filament remains unchanged the power of the bulb will

• A. decrease by 2%
• B. increase by 2%
• C. decrease by 4%
• D. increase by 4%

Answer 1

The correct option is C decrease by 4%

Power $P=\frac{v^2}{R}$
Therefore $△P=\frac{2V△V}{R}$
Hence $\frac{△P}{P}=\frac{2△V}{V}$
Now $\frac{△V}{V}=-2\%$
Therefore $\frac{△P}{P}=-2\times 2=-4\%$ i.e. the power decreases by $4\%$