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

A. 60V
B. 150V
C. 225V
D. 360V

Correct Answer:

Option A – 60V

Explanation

P = 40W

R = 90W

V = ?

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

V\(^2\) = PR = 40 x 90

V\(^2\) = 3000

V = √3600

V = 60N