Question

Figure shows an arrangement to measure the emf $ϵ$ and internal resistance $r$ of a battery. The voltmeter has a very high resistance and the ammeter also has some resistance. The voltmeter reads $1.52V$ when the switch $S$ is open. When the switch is closed the voltmeter reading drops to $1.45V$ and the ammeter reads $1.0A$. Find the emf and the internal resistance of the battery.

Answer 1

a) When switch is open, no current passes through the ammeter. In the upper part of the circuit the Voltmenter has $\infty$ resistance. Thus current it is $0$.

$\therefore$ Voltmeter read the emf. (There is not Pot. Drop across the resistor).

b) When switch is closed current passes through the circuit and if its value of $i$.

The voltmeter reads

$\in -ir=1.45$

$\Rightarrow 1.52-ir=1.45$

$\Rightarrow ir=1.07$

$\Rightarrow 1r=0.07\Rightarrow r=0.07\Omega$