Question

A voltmeter is connected in parallel with a variable resistance are which is in series and an ammeter and call. For one value of R, metres read 0.3 A and 0.9 volt. For another value of R the readings are 0.25 ampere and 1.0 volt. What is the internal resistance of the cell ?

• A. $0.5\Omega$
• B. $2\Omega$
• C. $1.2\Omega$
• D. $1\Omega$

Answer 1

The correct option is B $2\Omega$

Let E be the emf of the cell emf r be its internal resistance.

In the circuit the voltage equation can be written as,

$E-I_r-I_R=0$, where $I=$ammeter reading

but $I_R=v=$voltmeter reading

$\Rightarrow E-I_r-V=0$

$\Rightarrow E=I_r+V$

From case i,

$E=0.3r+0.9$

From case ii,

$E=0.25r+1$

Equating both the equation,

$0.3r+0.9=0.25r+1$

$\Rightarrow 0.05r=0.1$

$\Rightarrow r=\frac{0.1}{0.05}=2$ohms

Hence the internal resistance of the cell is $2$ ohms.