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.

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