A 60 W light bulb is connected to a 120 V supply. Calculate (a) the current drawn by the bulb, and (b) its resistance.

easy 2 marks Class 7 · Science · Electric Current and its Effects

Answer

(a) 0.5 A (b) 240 Ω

The correct answer is (a) 0.5 A (b) 240 Ω — Use Ohm’s law $R = \frac{V}{I}$ to find resistance: $R = \frac{120}{0.5} = 240$  Ω.

Step-by-step solution

Use the relation $P = I V$ to find the current: $I = \frac{P}{V} = \frac{60}{120} = 0.5$  A.
Use Ohm’s law $R = \frac{V}{I}$ to find resistance: $R = \frac{120}{0.5} = 240$  Ω.

Common mistake: Using

P = V^{2} / R

instead of

P = I V

to find current.

Concept tested: Electric current and Ohm’s law