question_answer

A cell supplies a current of 0.9 A through a 20. resistor and a current of 0.3 A through a 70. resistor. The internal resistance of the cell is:

A)  1.2\[\Omega \]

B)  2.0\[\Omega \]

C)  0.5\[\Omega \]

D)  1.0\[\Omega \]

Correct Answer: C

Solution :

 \[{{i}_{1}}=0.9amp.\] \[{{R}_{1}}=2\Omega \]   \[E={{i}_{1}}{{R}_{1}}+{{i}_{1}}r\] ?(i) \[{{R}_{2}}=7\Omega {{i}_{2}}=0.3amp\] \[E={{i}_{2}}{{R}_{2}}+r{{i}_{2}}\] \[E=0.3\times 7+0.3r\] ?(ii) eq.(i) is equal to eq. (ii) \[0.9\times 2+0.9r=0.3\times 7+9.3r\] \[0.6r=2.1-1.8\] \[r=\frac{0.3}{0.6}\] \[=0.5\Omega \]