The opposition offered by the electrolyte of the cell to the flow of electric current through it is called the internal resistance of the cell.
The electro motive force is the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell.
The electric current in an external circuit flows from the positive terminal to the negative terminal of the cell, through different circuit elements. In order to maintain continuity, the current has to flow through the electrolyte of the cell, from its negative terminal to positive terminal. During this process of flow of current inside the cell, a resistance is offered to current flow by the electrolyte of the cell. This is termed as the internal resistance of the cell.
A freshly prepared cell has low internal resistance and this increases with ageing.
Determination of internal resistance of a cell using voltmeter : The circuit connections are made as shown in Fig. With key K open, the emf of cell E is found by connecting a high resistance voltmeter across it. Since the high resistance voltmeter draws only a very feeble current for deflection, the circuit may be considered as an open circuit. Hence the voltmeter reading gives the emf of the cell. A small value of resistance R is included in the external circuit and key K is closed. The potential difference across R is equal to the potential difference across cell (V).
The potential drop across R, V = IR ... (1)
Due to internal resistance r of the cell, the voltmeter reads a value V, less than the emf of cell.
Then V = E – Ir or Ir = E−V ... (2)
Dividing equation (2) by equation (1)
Since E, V and R are known, the internal resistance r of the cell can be determined.
Factors affecting internal resistance of a cell :
(1) Larger the separation between the electrodes of the cell, more the length of the electrolyte through which current has to flow and consequently a higher value of internal resistance.
(2) Greater the conductivity of the electrolyte, lesser is the internal resistance of the cell. i.e., Internal resistance depends on the nature of the electrolyte.
(3) The internal resistance of an electrolyte is inversely proportional to common area of the electrodes dipping in the electrolyte.
(4) Internal resistance of a cell depends on the nature of the electrolytes.
Comparison of emf and potential difference :
1. The difference of potentials between the two terminals of a cell in an open circuit is called the electromotive force (emf) of a cell. The difference in potentials between any two points in a closed circuit is called potential difference.
2. The emf is independent of external resistance of the circuit, whereas potential difference is proportional to the resistance between any two points.
