What is the efficiency of a cell with internal resistance of \(3 \Omega\), when it supplies current to a \(7 \Omega\) resistor ____________
A. \(70 \%\)
B. \(30 \%\)
C. \(20 \%\)
D. \(10 \%\)
Correct Answer: A
Explanation
efficiency of a cell \(=\frac{\text { energy output }}{\text { energy input }} \times 100 \%\)
\(=\frac{\text { P.D. }}{\text { E.m.f. }}=\frac{V}{B}=\frac{I R}{I(R+r)}=\frac{R}{R+r}\)
efficiency \(=\frac{7}{3+7} \times 100 \%\)
\(=\frac{7}{10} \times 100 \%=70 \%\)
