Before the switch is turned on
- The battery does not supply current to the light bulb
- Voltmeter reading = E.m.f. of battery
After the switch is turned on
- The battery supplies current which flows around the circuit
- Voltmeter measures the potential difference across the terminals of the battery
- The voltmeter reading drops due to internal resistance of the battery
EMF = V + Ir
EMF = I (R + r)
where EMF = electromotive force [V]
I = current flowing through the circuit [A]
R = total resistance of the circuit [Ω]
r = internal resistance of the batteries [Ω]
V = potential difference of the circuit [V]
Worked example:
What is the internal resistance of a battery if its emf is 12 V and the voltage drop across its terminals is 10 V when a current of 4 A flows in the circuit when it is connected across a load?
Solution:
E = V + Ir
12 = 10 + 4(r)
r =0.5Ω
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