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.
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