3.11 A storage battery of emf 8.0 V and internal resistance 0.5 Ω is being charged by a 120 V dc supply using a series resistor of 15.5 Ω. What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?
Given,
Emf of battery, E = 8 V
Internal resistance of battery, r = 0.5 Ω
Supply Voltage, V = 120 V
The resistance of the resistor, R = 15.5 Ω
Let V' be the effective voltage in the circuit.
Now, V' = V - E
V' = 120 - 8 = 112 V
Now, Current flowing in the circuit is:
I = V' / (R + r)
Now, Using Ohm ’s Law:
Voltage across resistor R is v = IR
v = 7 x 15.5 = 108.5 V
Now, Voltage supplied, V = Terminal Voltage of battery + v
Terminal Voltage of battery = 120 -108.5 = 11.5 V
The purpose of having a series resistor is to limit the current drawn from the supply.