Two lamps are connected in series to a 250 V power supply. One lamp is rated 240 V, 60 W and the other is rated 10 V, 2.5 W.

Question 11

Two lamps are connected in series to a 250 V power supply. One lamp is rated 240 V, 60 W and the other is rated 10 V, 2.5 W.

Which statement most accurately describes what happens?

A Both lamps light at less than their normal brightness.

B Both lamps light at their normal brightness.

C Only the 240 V lamp lights.

D The 10 V lamp blows.

Reference: Past Exam Paper – June 2016 Paper 12 Q33

Solution:

Answer: B.

One lamp is rated 240 V, 60 W.

This means that for the lamp to operate normally, there should be a p.d. across it.

P = VI

Current flowing through the lamp = P / V = 60 / 240 = 0.25 A

The other lamp is rated 10 V, 2.5 W.

This lamp operates normally when there is a p.d. of 10 V across it.

Current flowing through the lamp = P / V = 2.5 / 10 = 0.25 A

So, for normal operation, the same current of 0.25 A is required for both lamps.

Since the lamps are connected in series, the same current would flow. So, both lamps light at their normal brightness.

 

In the same way, the sum of p.d. across the 2 lamps is equal to 240 + 10 = 250 V.