Question

Two identical cells each of emf $1.5\, V$ are connected in parallel across a parallel combination of two resistors each of resistance $20\, \Omega$. A voltmeter connected in the circuit measures $1.2 \, V$. The internal resistance of each cell is

• A. $2.5 \, \Omega$
• B. $4 \, \Omega$
• C. $5 \, \Omega$
• D. $10\, \Omega$

Answer 1

Answer: (C)

$V = E - ir / 2$
$1.2=1.5- i \left(\frac{ r }{2}\right)$
$i \frac{ r }{2}=0.3$
$i =\frac{1.5}{10+\frac{ r }{2}} $
$\Rightarrow 10 i +\frac{ ir }{2}=1.5$
$10 i =1.5-0.3$
$i =0.12 A$
$\Rightarrow r =\frac{0.6}{0.12}=5 \,\Omega$