Answer :

Given:

Resistance of the wire, r=15 Ω

Length of potentiometer wire, l = 1 m

EMF of cell 1, E_{1} = 2 V

EMF of cell 2, E_{2} = 75 mV

Balancing point, L=30 cm=0.3 m

Calculation EMF per unit length is given by,

Current is given by,

Potential is given by,

EMF per unit length of the wire is given by,

(ii) The reasons for selecting potentiometer over voltmeter for comparison are as follows:

• The potentiometer is more sensitive in comparison with the voltmeter.

• The voltmeter measures the terminal voltage whereas the potentiometer measures the actual emf.

• The potentiometer is used to measures the internal resistance of the cell but the voltmeter cannot measure the internal resistance.

(iii)

**OR**

(i) The graph showing variation of voltage vs the current drawn from the cell is given by.

The potential of the cell is given by,

Where,

E=EMF of the cell

I = Current

r = internal resistance

(ii) When two cells are connected in parallel combination, Applying Kirchhoff’s law

I =I_{1} +I_{2}

The equivalent emf of two cells connected in parallel is given by,

The equivalent internal resistance,

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