Question

The internal resistance of a cell of emf 4 V is $0.1\Omega$ It is connected to a resistance of $3.9\Omega$. The voltage across the cell will be

• A. 3.9V
• B. 2V
• C. 0.1V
• D. 3.8V

Answer 1

Answer: (A)

emf of a cell (E)=4 V internal resistance of a cell $(r)=0.1\Omega$ external resistance $(R)=3.9\Omega$
The potential drop across the cell $V=E-1.r......(i)$
Now, the total resistance of the circuit
$R^{\prime }=r+R$
$R^{\prime }=0.1+3.9$
$\Rightarrow R^{\prime }=4.0\Omega$
Hence, current in the circuit is $I=\frac{E}{R^{\prime }}$
$\Rightarrow I=\frac{4}{4}=1A$
Now, form Eq. (i)
$V=4-1\times 0.1$
$V=4-0.1$
$V=3.9volt$