Question

A lamp rated 120 watts and 240 volts what is its resistance

Answer 1

We can find the resistance of the lamp from the definition of the electric power:

$$P = IV,$$

here, $P$ is the power, $I$ is the current flowing through the lamp and $V$ is the voltage across the lamp.

We can find the current flowing through the lamp from the Ohm's law:

$$I = \dfrac{V}{R},$$

here, $R$ is the resistance of the lamp.

Substituting $I$ into the formula for the electric power, we get:

$$P = IV = \dfrac{V^2}{R}.$$

From this formula we can find the resistance of the lamp:

$$R = \dfrac{V^2}{P} = \dfrac{(240V)^2}{120W} = 480 \Omega.$$

Answer:

$R = 480 \Omega.$