The battery of a car has an internal resistance of 0.10 Ω and an electromotive force of 12 V.

Question 39

The battery of a car has an internal resistance of 0.10 Ω and an electromotive force of 12 V. When the battery is connected to the starter motor, the potential difference across the battery terminals is 7.0 V.

What is the current supplied to the starter motor?

A 50 A                         B 70 A                         C 120 A                       D 190 A

Reference: Past Exam Paper – March 2016 Paper 12 Q37

Solution:

Answer: A.

Internal resistance r = 0.10 Ω

e.m.f. of battery = 12 V

When connected to the motor, the p.d. across the battery terminals is 7.0 V.

Terminal p.d. = 7.0 V

e.m.f of battery = terminal p.d. + lost volts

12 = 7 + Ir       where I is the flowing in the circuit

12 = 7 + (I×0.10)

0.1 I = 12 – 7

0.1 I = 5

Current I = 5 / 0.1 = 50 A