Question

Explain the determination of the internal resistance of a cell using voltmeter.

Answer 1

1. The emf of the cell ξ is found by connecting a high resistance voltmeter across it. Without connecting the external resistance R.
2. Since the voltmeter draws very little current for deflection, the circuit is called an open circuit.
3. Voltmeter reading gives the emf of the cell.
4. Resistance ‘R is included in the circuit and current I is established in the circuit.
5. The potential drop across the resistor.
   R is V = IR …………(1)
6.  Due to internal resistance (r) the voltmeter reads a value V, less than the emf of cell (E).
7.  V = ξ – Ir
8. Ir = ξ – V …………(2)
9. Dividing (2) by (1)
   (a)
   
   (b)
   
10. Internal resistance of the cell
    `"Ir"/"IR" = (ξ - "V")/"V"`
    r = `((ξ - "V")/"V")`R
11. Since ξ, V and R are known, internal resistance r can be calculated.