Question

A microammeter has a resistance of $100\Omega$ and a 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 combinations.

• A. $50V$ range with $10$ $k\Omega$ resistance in series
• B. $10V$ range with $200$ $k\Omega$ resistance in series
• C. $5mA$ range with $1\Omega$ resistance in parallel
• D. $10mA$ range with $1\Omega$ resistance in parallel

Answer 1

Answer: (B), (C)

The correct options are
B $10V$ range with $200$ $k\Omega$ resistance in series
C $5mA$ range with $1\Omega$ resistance in parallel

Given the Microammeter has a resistance of $100ohms$ and a full range of $50\mu amps$

Ammeter can function as a voltmeter when it is connected in series with a large resistance $R$. The value of $R$ is determined by the maximum voltage that will be measured.

Checking for option A $50V$ to produce a full-scale deflection of a voltmeter containing a $100-ammeter$ with a $50-A$ sensitivity. Then $50V$ applied to the meter must produce a current of $50A$.

$R_{total}=R+r=\frac{V}{i}=\frac{50}{50\times {10}^{-6}}\simeq 1000k\Omega$

Checking for option A $10V$ to produce a full-scale deflection of a voltmeter containing a $100-ammeter$ with a $50-A$ sensitivity. Then $10V$ applied to the meter must produce a current of $50A$.

$R_{total}=R+r=\frac{V}{i}=\frac{10}{50\times {10}^{-6}}\simeq 200k\Omega$

Microammeter has a resistance of $100ohms$ and a full range of $50\mu amps$

Substituting them in the formula for shunt of Ammeter , we have

$S=\frac{G}{\frac{i}{i_g}-1}=\frac{100}{\frac{i}{50\times {10}^{-6}}-1}$

Option B and C has values of $i$ and $S$ that satisfy this equation.

Option B and C are correct