Ncert Exercises & Solutions

3.12 In a potentiometer arrangement, a cell of emf 1.25 V gives a balance point at 35.0 cm length of the wire. If the cell is replaced by another cell and the balance point shifts to 63.0 cm, what is the emf of the second cell?

Emf of the storage battery, E = 8.0 V

Internal resistance of the battery, r = 0.5 $Ω$

DC supply voltage, V = 120 V

Resistance of the resistor, R = 15.5 $Ω$

Effective volate in the circuit = V¹

R is connected to the storage battery is series. Hence, it can be written as V¹ = V - E

V¹ = 120 - 8 = 112 V

Current flowing in the circuit = I, which is given by the relation,

$l = \frac{V^{1}}{R + r}$
$= \frac{112}{15.5 + 5} = \frac{112}{16} = 7 A$

Voltage across resistor R given by the product IR = 7 $\times$ 15.5 = 108.5 V

DC supply voltage = Terminal voltage of battery + Voltage drop across R

Terminal voltage of battery = 120 - 108.5 = 11.5 V

A resistor in a charging circuit limits the current drawn from the external source. The current will be extremely high in its absence. This is very dangerous.

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