Question

A car has a fresh storage battery of e.m.f $12\,V$ and internal resistance $2 \times 10^{-2} \Omega$. If the starter motor draws a current of $80\,A$. Then the terminal voltage when the starter is on is

• A. 12V
• B. 8.4V
• C. 10.4V
• D. 9.3V

Answer 1

The Correct Option is C

The resistance \(R\) of a conductor is given by the formula: \[ R = \rho \cdot \frac{L}{A} \] Where:
\(\rho\) is the resistivity of the material (a constant),
\(L\) is the length of the conductor,
\(A\) is the cross-sectional area through which current flows. For a rectangular cross-section, the area \(A\) is the product of the two dimensions of the cross-section.
1. When the battery is connected across the 1 cm \(\times\) \( \frac{1}{2} \) cm faces, the cross-sectional area \(A = 1 \times \frac{1}{2} = \frac{1}{2} \, \text{cm}^2\).
2. When the battery is connected across the 10 cm \(\times\) \( \frac{1}{2} \) cm faces, the cross-sectional area \(A = 10 \times \frac{1}{2} = 5 \, \text{cm}^2\).
3. When the battery is connected across the 10 cm \(\times\) 1 cm faces, the cross-sectional area \(A = 10 \times 1 = 10 \, \text{cm}^2\).
Since the resistance is inversely proportional to the cross-sectional area, the resistance will be maximum when the cross-sectional area is minimum.

Therefore, the resistance is maximum when the battery is connected across the 1 cm \(\times\) \( \frac{1}{2} \) cm faces.