Q. Two identical cells connected in series send 1.0 A current through a 5$\Omega$ resistor. When they are connected in parallel, they send 0.8 A current through the same resistor. What is the internal resistance of the cell?
A) $0.5\ \Omega$
B) $1.0\ \Omega$
C) $1.5\ \Omega$
D) $2.5\ \Omega$
Explanation
(d) Case (i) $E + E = (r + r + 5) \times 1.0$
or $2E = 2r + 5$ …(i)
Case (ii) $E = \left( \frac{r \times r}{r + r} + 5 \right) \times 0.8$
or $E = \left( \frac{r}{2} + 5 \right)0.8\ or\ E = 0.4r + 4.0$ …(ii)
Multiplying Eq.(ii) by 2 and equating with Eq.(i), we get
$2r + 5 = 00.8r + 8$
or $1.2 = 3\ or\ r = \frac{3}{1.2} = 2.5\Omega$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.