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

Electromotive force and internal resistance

Although electrons move quite freely in wires, they do not flow in one direction unless they are connected to a cell or some other source of energy. Consider an ordinary dry cell as a source of electrical energy. A chemical reaction occurs in the dry cell that results in a potential difference is produced between the positive and negative terminals. If a current path is provided then current can flow throughout the circuit. As the electrons move around the circuit, they transfer their energy to the bulb and the connecting wires.

When the electrons reach the cell again after flowing around the circuit, they would have lost almost all their initial potential energy. Energy is transferred to the electrons from the chemical reaction that takes place inside the cell. The electrons gain potential energy and the process is repeated.

The electromotive force (e.m.f) is defined as the work done by a source in driving one coulomb of charge around a complete circuit.

The unit of e.m.f is the volt, V which is equivalent to J C-1