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)

3.9V

Answer 2

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
$\, \, \, \, \, R' = r + R$
$\, \, \, \, \, \, R' = 0.1 + 3.9$
$\Rightarrow \, \, \, \, R' = 4.0 ?$
Hence, current in the circuit is $\, \, \, \, \, \, I = \frac{E}{R'}$
$\Rightarrow \, \, \, \, I = \frac{4}{4} = 1 A$
Now, form E (i)
$V = 4 - 1 \times 0.1$
$V = 4 - 0.1$
$V = 3.9 volt$