Question 4
The electromotive
force of a power supply is 120 V. It delivers a current of 1.2 A to a resistor
of resistance 80 Ω and
a current of 0.40 A to another resistor, as shown.
What is the internal
resistance of the power supply?
A 15 Ω B 20 Ω C 60 Ω D 75 Ω
Reference: Past Exam Paper – November 2015 Paper 12 Q37
Solution:
Answer: A.
For a parallel connection,
the p.d. across each branch is the same.
p.d. across middle branch
= IR = 1.2 × 80 = 96 V
So, even if the e.m.f. of
the supply is 120 V, only 96 V is found across each branch. There is some ‘lost
volts’ due to the internal resistance of the supply.
Lost volt = 120 – 96 = 24
V
The total current in the
supply is
I = 1.2 + 0.4 = 1.6 A
For the internal
resistance,
R = V / I = 24 / 1.6 = 15 Ω
