Question 114: Current Electricity

Question

A moving coil galvanometer of resistance $100 \Omega$ shows a full scale deflection for a current of 1 mA . The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA , is $\_\_\_\_$ $\Omega$

Accepted Answer

To convert a galvanometer into an ammeter of a higher range, a small shunt resistance $(S)$ must be connected in parallel with the galvanometer.
Galvanometer resistance $=\mathrm{G}=100 \Omega$
Full-scale deflection current $=\mathrm{I}_{\mathrm{g}}=1 \mathrm{~mA}=1 	imes 10^{-3} \mathrm{~A}$
The required ammeter range $=\mathrm{I}=5 \mathrm{~mA}=5 	imes 10^{-3} \mathrm{~A}$
In a parallel circuit, the potential difference across the galvanometer is equal to the potential difference across the shunt:
$$ V_g=V_s $$
$$\Rightarrow $$ $\mathrm{I}_{\mathrm{g}} 	imes \mathrm{G}=\left(\mathrm{I}-\mathrm{I}_{\mathrm{g}}ight) 	imes \mathrm{S}$
$\Rightarrow \mathrm{S}=\frac{\mathrm{I}_{\mathrm{g}} 	imes \mathrm{G}}{\mathrm{I}-\mathrm{I}_{\mathrm{g}}}$
$$ \Rightarrow S=\frac{1 	imes(100)}{5-1}=25 \Omega $$
To measure a current of 5 mA with a $100 \Omega$ galvanometer that deflects at 1 mA , a shunt resistance of $25 \Omega$ is required. Hence, the correct option is (A).

Answer

25

Answer

2.5

Answer

0.5

Answer

10

Current Electricity

A moving coil galvanometer of resistance 100 \Omega shows a full scale deflection for a current of 1 mA . The value of resistance required to convert this galvanometer into an ammeter, showing full scale deflection for a current of 5 mA , is \_\_\_\_ \Omega

Choose the correct answer: