Question

Two bulbs rated at 240 V-60 W and 240 V-100 W are linked in series to a 240 V mains supply. Why does the 60 W lamp use more electric power? Explain.

Answer 1

The two bulbs are connected in series, so the same current flows through both bulbs.

The resistance of a bulb is given by:

R = `V^2/P`

Hence,

Resistance of the 60 W bulb:

R = `240^2/60`

= 960 Ω

Resistance of the 100 W bulb:

R = `240^2/100`

= 576 Ω

Since the 60 W bulb has a higher resistance, the power consumed by it in the series circuit is greater because

P = I²R

As the current is the same through both bulbs, the bulb with the higher resistance (60 W bulb) dissipates more electrical power. Therefore, the 60 W lamp uses more electric power and glows brighter than the 100 W lamp when connected in series.