# Physics Class 12 NCERT Solutions: Chapter 3 Current Electricity Part 11 (For CBSE, ICSE, IAS, NET, NRA 2023)

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Q: 23. Figure shows a potentiometer circuit for comparison of two resistances. The balance point with a standard resistor is found to be , while that with the unknown resistance . Determine the value of X. What might you do if you failed to find a balance point with the given cell of emf?

Resistance of the standard resistor,

Balance point for this resistance,

Current in the potentiometer wire

Hence, potential drop across

Resistance of the unknown resistor

Balance point for this resistor,

Hence, potential drop across

The relation connecting emf and balance point is,

Therefore, the value of the unknown resistance, is .

If we fail to find a balance point with the given cell of emf, then the potential drop across Rand X must be reduced by putting a resistance in series with it. Only if the potential drop across R or X is smaller than the potential drop across the potentiometer wire AB, a balance point is obtained.

Q: 24. Figure shows a potentiometer used for the determination of internal resistance of a cell. The balance point of the cell in open circuit is . When a resistor of is used in the external circuit of the cell, the balance point shifts to length of the potentiometer wire. Determine the internal resistance of the cell.

Internal resistance of the cell

Balance point of the cell in open circuit,

An external resistance is connected to the circuit with

New balance point of the circuit,

Current flowing through the circuit

The relation connecting resistance and emf is,

Therefore, the internal resistance of the cell is .