CBSE 2025 · Region 5 · Set 1 · Q29 · 4 marks

A galvanometer is an instrument used to show the direction and strength of the current passing through it. In a galvanometer, a coil placed in a magnetic field experiences a torque and hence gets deflected when a current passes through it. The name is derived from the surname of Italian scientist L. Galvani, who in $\displaystyle 1791$ discovered that electric current makes a dead frog's leg jerk. A spring attached with the coil provides a counter torque. In equilibrium, the deflecting torque is balanced by the restoring torque of the spring and we have: $\displaystyle \mathrm{NBAI}=\mathrm{k} \phi$ where N is the total number of turns in the coil A is the area of cross-section of each turn B is the radial magnetic field k is the torsional constant of the spring $\displaystyle \phi$ is the angular deflection of the coil As the current ( $\displaystyle \mathrm{I}_{\mathrm{g}}$ ) which produces full scale deflection in the galvanometer is very small, the galvanometer cannot as such be used to measure current in electric circuits. A small resistance, called shunt, of a suitable value is connected with the galvanometer to convert it into an ammeter of desired range. By using a higher resistance, a galvanometer can also be converted into a voltmeter.

(i)

The value of the current sensitivity of a galvanometer is given by :

(A)

$\displaystyle \frac{\mathrm{k}}{\text { NBA }}$

(B)

$\displaystyle \frac{\text { NBA }}{\mathrm{k}}$

(C)

$\displaystyle \frac{\mathrm{kBA}}{\mathrm{N}}$

(D)

$\displaystyle \frac{\mathrm{kNB}}{\mathrm{A}}$

(ii)

A galvanometer of resistance $\displaystyle 6 \Omega$ shows full scale deflection for a current of $\displaystyle 0.2$ A . The value of shunt to be used with this galvanometer to convert it into an ammeter of range ( $\displaystyle 0-5 \mathrm{~A}$ ) is :

(A)

$\displaystyle 0 \cdot 25 \Omega$

(B)

$\displaystyle 0 \cdot 30 \Omega$

(C)

$\displaystyle 0.50 \Omega$

(D)

$\displaystyle 6 \cdot 0 \Omega$

(iii)

The value of resistance of the ammeter in case (ii) will be :

(A)

$\displaystyle 0 \cdot 20 \Omega$

(B)

$\displaystyle 0 \cdot 24 \Omega$

(C)

$\displaystyle 6 \cdot 0 \Omega$

(D)

$\displaystyle 6 \cdot 25 \Omega$

(iv)

(a)OR(b)

A galvanometer is converted into a voltmeter of range ( $\displaystyle 0-\mathrm{V}$ ) by connecting with it, a resistance $\displaystyle \mathrm{R}_{1}$. If $\displaystyle \mathrm{R}_{1}$ is replaced by $\displaystyle \mathrm{R}_{2}$, the range becomes $\displaystyle (0-2 \mathrm{~V})$. The resistance of the galvanometer is :

(A)

$\displaystyle \left(\mathrm{R}_{2}-2 \mathrm{R}_{1}\right)$

(B)

$\displaystyle \left(\mathrm{R}_{2}-\mathrm{R}_{1}\right)$

(C)

$\displaystyle \left(\mathrm{R}_{1}+\mathrm{R}_{2}\right)$

(D)

$\displaystyle \left(\mathrm{R}_{1}-2 \mathrm{R}_{2}\right)$

CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.