Question

A galvanometer with its coil resistance $25\Omega$ requires a current of $1mA$ for its full deflection. In order to construct an ammeter to read up to a current of $2A$, the approximate value of the shunt resistance should be

• A. $2.5\times {10}^{-2}\Omega$
• B. $1.25\times {10}^{-3}\Omega$
• C. $2.5\times {10}^{-3}\Omega$
• D. $1.25\times {10}^{-2}\Omega$

Answer 1

The correct option is D $1.25\times {10}^{-2}\Omega$

formula : S= G*I/ (I-Ig)

where S= shunt resistance , (I-Ig)= current through the shunt resistance , I=Current in the circuit.

Here, galvanometer resistance ,G= 25 ohm ; Ig= 0.001 A , I= 2A

S= (25* 0.001 )/( 2- 0.001)

= $1.25\times {10}^{-3}\Omega$

A shunt resistance of $1.25\times {10}^{-2}\Omega$ must be connected to make the galvanometer in the range of 0 to 2A