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