Electromotive force and Internal Resistance

Electromotive force and Internal Resistance

1. A cell can be modeled as an e.m.f., E connected in series with an internal resistor, r.
2. When a high resistance voltmeter is connected across the terminals of the cell, the reading of the voltmeter gives the e.m.f., E of the cell.
3. If a resistor, R is then connected to the terminals of the cell, the voltmeter reading is the potential difference, V across the terminals of the cell.
4. The value of potential difference, V is less than e.m.f. of the cell. The difference between E and V is due to the potential difference needed to drive the current, I through the internal resistor, r of the cell.
Hence,

E-V = Ir ═> E = V + Ir

5. The internal resistance, r is given by:
r = E-V / I