Current Electricity 4 Question 26
26. A microammeter has a resistance of $100 \Omega$ and full scale range of $50 \mu A$. It can be used as a voltmeter or as a higher range ammeter provided a resistance is added to it. Pick the correct range and resistance combination (s)
(1991, 2M)
(a) $50 V$ range with $10 k \Omega$ resistance in series
(b) $10 V$ range with $200 k \Omega$ resistance in series
(c) $5 mA$ range with $1 \Omega$ resistance in parallel
(d) $10 mA$ range with $1 \Omega$ resistance in parallel
Integer Answer Type Question
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Solution:
- To increase the range of ammeter a parallel resistance (called shunt) is required which is given by
$$ S=\frac{i _g}{i-i _g} G $$
For option (c)
$$ S=\frac{50 \times 10^{-6}}{5 \times 10^{-3}-50 \times 10^{-6}}(100) \approx 1 \Omega $$
To change it in voltmeter, a high resistance $R$ is put in series, where, $R$ is given by $R=\frac{V}{i _g}-G$
For option (b), $R=\frac{10}{50 \times 10^{-6}}-100$
$$ \approx 200 k \Omega $$
Therefore, options (b) and (c) are correct.
