Question

A battery of emf 21 V and internal resistance 3 Ω is connected to a resistor. If the current in the circuit is 3 A, find:

1. The resistance of the resistor.
2. The terminal voltage of the battery.

Answer 1

Given: E = 21 V

r = 3 Ω

I = 3 A

(i) Resistance of external resistor

`I = E/(R + r)`

`3 = 21/(R + 3)`

`3 = 21/(R + 3)`

R + 3 = `21/7`

R + 3 = 7

R = 7 − 4

= 3 Ω

(ii) Terminal voltage

V = E − Ir

= 21 − (3 × 3)

= 21 − 9

= 12 V