Q70 / 90 · JEE Main 2024 (09 Apr Shift 1) · Physics

A galvanmeter has a coil of resistance $200 \Omega$ with a full scale deflection at $20 \mu \mathrm{A}$. The value of resistance to be added to use it as an ammeter of range $(0-20) \mathrm{mA}$ is ;

  • A
    $0.40 \Omega$
  • Correct answer
    B
    $0.20 \Omega$
  • C
    $0.50 \Omega$
  • D
    $0.10 \Omega$

Solution

$\begin{aligned} & \mathrm{G}=200 \Omega \\ & \mathrm{i}_{\mathrm{g}}=20 \mu \mathrm{A} \\ & \mathrm{i}=\mathrm{i}_{\mathrm{g}}\left(\frac{\mathrm{G}}{\mathrm{S}}+1\right) \\ & \Rightarrow 20 \times 10^{-3}=20 \times 10^{-6}\left(\frac{200}{\mathrm{~S}}+1\right) \\ & \Rightarrow \frac{200}{\mathrm{~S}}=999 \\ & \Rightarrow \mathrm{S} \approx 0.2 \Omega\end{aligned}$

Hence, the correct answer is option: 'B'.

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