Current Electricity Ques 89
- 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
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Answer:
Correct Answer: 89.(b,c)
Solution:
Formula:
- 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.
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