Question

A moving-coil instrument gives full-scale deflection for 1 mA and has a resistance of 5 ohms. If a resistance of 0.5 ohms is connected in parallel to the instrument, what is the maximum value of current it can measure?

• A. 5 mA
• B. 10 mA
• C. 50 mA
• D. 100 mA

Hint:

When resistance is reduced by adding a parallel resistor, the maximum current that can be measured increases according to Ohm’s law.

Answer 1

The Correct Option is B

The total resistance of the parallel combination is: \[ R_{\text{total}} = \frac{5 \, \Omega \times 0.5 \, \Omega}{5 \, \Omega + 0.5 \, \Omega} = \frac{2.5}{5.5} = 0.4545 \, \Omega \] Now, the maximum current is calculated using Ohm's law: \[ I_{\text{max}} = \frac{V}{R_{\text{total}}} = \frac{1 \, \text{V}}{0.4545 \, \Omega} \approx 10 \, \text{mA} \] Thus, the maximum current is 10 mA. Thus, the correct answer is 10 mA.