A 60 W light bulb is connected to a 120 V mains supply. What is the resistance of the bulb?

hard 5 marks Class 6 · Science · Electricity and Circuits

Answer

240, O m e g a

The correct answer is $240, O m e g a$ — Substitute $V = 120$  V and $P = 60$  W: $R = \frac{12 0 ^{2}}{60} = \frac{14400}{60} = 240$  Ω.

Step-by-step solution

Use the formula for power: $P = \frac{V ^{2}}{R}$ , which rearranges to $R = \frac{V ^{2}}{P}$ .
Substitute $V = 120$  V and $P = 60$  W: $R = \frac{12 0 ^{2}}{60} = \frac{14400}{60} = 240$  Ω.

Common mistake: using

R = P / V^{2}

instead of

R = V^{2} / P

Concept tested: Power–Resistance relation