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A galvanometer of resistance 20\(\Omega\) is to be provided with a shunt that \(\frac{1}{10}\) of...

Physics
JAMB 1987

A galvanometer of resistance 20\(\Omega\) is to be provided with a shunt that \(\frac{1}{10}\) of the whole current in a circuit passes through the galvanometer. The resistance of the shunt is

  • A. 2.00\(\Omega\)
  • B. 2.22\(\Omega\)
  • C. 18.00\(\Omega\)
  • D. 18.22\(\Omega\)
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Correct Answer: Option B
Explanation

 Let the total current be \( I \). The galvanometer current is \( I_g = \frac{1}{10}I \), and the shunt current is \( I_s = I - I_g = \frac{9}{10}I \).
  Since the galvanometer resistance \( R_g = 20 \Omega\) and shunt (resistance \( R_s \)) are in parallel, the voltage across them is equal:
    \( I_g R_g = I_s R_s\)
    \(\left( \frac{1}{10}I \right) \cdot 20 = \left( \frac{9}{10}I \right) \cdot R_s\)
    Divide through by \( I \) (since \( I \neq 0 \)):
    \(2 = \frac{9}{10} R_s\)
    Solve for \( R_s \):
    \(R_s = 2 \cdot \frac{10}{9} = \frac{20}{9}\) \(\approx\) 2.222\(\Omega\)


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