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JAMB Physics 2018 Paper

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Question : 61

Total: 87

A 40W instrument has a resistance 90 Ohms. On what voltage should it be operated normally

60V
150V
225V
360V

Solution:

Concept:

The power rating of an electrical device is related to the voltage and resistance by

P = \frac{V^2}{R}

.

Explanation:

Given,

P = 40\, \text{W}

and

R = 90\, \Omega

.

Using

P = \frac{V^2}{R}

, rearrange to find voltage:

V^2 = P \times R

V^2 = 40 \times 90 = 3600

V = \sqrt{3600} = 60\, \text{V}

Answer:

The instrument should be operated normally on 60V.

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