First slide

Electrical power

Question

If the d.c. voltage applied across a bulb increases by 2% and the resistance of its filament increases by 1% what is the maximum change in power dissipated in the bulb?

Easy

A

3%
B

5%
C

7%
D

9%

Solution

$\begin{array}{l} P = \frac{V^{2}}{R} \Rightarrow {(\frac{Δ P}{P})}_{\max} \times 100 % = 2 (\frac{Δ V}{V}) \times 100 % + (\frac{Δ R}{R}) \times 100 % \\ ∴ {(\frac{Δ P}{P})}_{\max} \times 100 % = 2 \times 2 % + 1 % = 5 % \end{array}$

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