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Physics · 2025 · 5 marks
CBSE 2025 · Region 7 · Set 1 · Q32
(a) (i) With the help of a labelled diagram, explain the principle of working of a moving coil galvanometer. Write the purpose of using (i) radial…
(i)
With the help of a labelled diagram, explain the principle of working of a moving coil galvanometer. Write the purpose of using (i) radial magnetic field, and (ii) soft iron core, in it.
(ii)
Define current sensitivity of a galvanometer. "Increasing the current sensitivity may not necessarily increase the voltage sensitivity." Give reason.
(i)
(I)
Write Ampere's circuital law in mathematical form and explain the terms used.
(II)
As the current carrying solenoid is made longer, the magnetic field produced outside it approaches zero. Why ?
(III)
A flexible loop of irregular shape carrying current when located in an external magnetic field, changes to a circular shape. Give reason.
(ii)
A galvanometer of resistance G is converted into a voltmeter to measure up to V volts, by connecting a resistance $\displaystyle \mathrm{R}_{1}$ in series with the coil. If $\displaystyle \mathrm{R}_{1}$ is replaced by $\displaystyle \mathrm{R}_{2}$, then it can only measure up to $\displaystyle \frac{\mathrm{V}}{2}$ volt. Find the value of the resistance $\displaystyle \mathrm{R}_{3}$ (in terms of $\displaystyle \mathrm{R}_{1}$ and $\displaystyle \mathrm{R}_{2}$ ) needed to convert it into a voltmeter that can read up to $\displaystyle 2$ V .
Marking-scheme solution
(a) 
Principle:A current carrying coil placed in uniform magnetic field, experiences a torque.
(i)
Radial magnetic field makes the scale linear Alternatively: Radial magnetic field provides maximum Torque.
(ii)
Use of soft iron core is to increase the strength of magnetic field/ increase sensitivity of the galvanometer.
(ii)
Current sensitivity is defined as deflection per unit current. Alternatively:
\[\mathrm{I}_{\mathrm{s}}=\frac{\Phi}{\mathrm{I}}=\frac{N A B}{k}
\]
Voltage sensitivity \(\displaystyle \mathrm{V}_{\mathrm{s}}=\frac{\Phi}{\mathrm{V}}=\left(\frac{N A B}{k}\right) \frac{\mathrm{I}}{\mathrm{V}}=\left(\frac{N A B}{k}\right) \frac{1}{\mathrm{R}}\) Increase in number of turns, increases the current sensitivity and resistance of the galvanometer in the same proportion of current sensitivity therefore Voltage sensitivity remains unchanged.
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