Solution:

Ans. (i) Importance and production of radial magnetic field : In a radial magnetic field, magnetic torque remains maximum everywhere. $1/2+1/2$ It is produced by cylindrical pole pieces and soft iron core.

(ii) Voltmeter : A high resistance in series is required when moving coil galvanometer is used as voltmeter to make sure that a low current should travel through voltmeter without changing the potential difference then needs to be measured. $1/2$ Ammeter : A shunt is used when a moving coil galvanometer is used as ammeter as to make sure about not much change in total resistance of the circuit with actual current value of flowing current. $1/2$

[AI Q. 2. Explain giving reasons, the basic difference in converting a galvanometer into (i) a voltmeter and (ii) an ammeter.

U [O.D. I, II, III 2012]

Ans. (i) Try yourself, Similar to Q. 1, (ii) Short Answer Type Questions-I (AI Q. 3. A square loop of side $20\mathrm{cm}$ carrying current of $1\mathrm{A}$ is kept near an infinite long straight wire carrying a current of $2\mathrm{A}$ in the same plane as shown in the figure.

Calculate the magnitude and direction of the net force exerted on the loop due to the current carrying conductor.

A [O.D. I, II, III 2015] OR

A square shaped plane coil of area $100{\mathrm{cm}}^2$ of 200 turns carries a steady current of $5\mathrm{A}$. It is placed in a uniform magnetic field of $0.2\mathrm{T}$ acting perpendicular to the plane of the coil. Calculate the torque on the coil when its plane makes an angle of ${60}^{\circ }$ with the direction of the field. In which orientation will the coil be in stable equilibrium ?

We have

$$F=\frac{{\mu }_0{I}_1{I}_2}{2\pi d}l$$

$\therefore$ Net force on sides $ab$ and $cd$

$=\frac{{\mu }_{0}2\times 1}{2\pi }\times 20\times {10}^{-2}[\frac{1}{10\times {10}^{-2}}-\frac{1}{30\times {10}^{-2}}]\mathrm{N}$

$=4\times {10}^{-7}\times 20\left[\frac{20}{10\times 30}\right]\mathrm{N}$

$=\frac{16}{3}\times {10}^{-7}\mathrm{N}$

$=5.33\times {10}^{-7}\mathrm{N}$

This net force is directed towards the infinitely long straight wire.

Net force on sides $bc$ and $da=$ zero.

$\therefore$ Net force on the loop $=5.33\times {10}^{-7}\mathrm{N}$

The force is directed towards the infinitely long straight wire.

$$\begin{array}{rlrlrlrl}& \text{ OR }& \text{ Torque }=\overrightarrow{{\mu }_m}\times \overrightarrow{B}& \left|\overrightarrow{{\mu }_m}\right|=nI\times A=200\times 5\times 100\times {10}^{-4}\text{ A.m }{}^2& =10{\mathrm{A}}^2& \text{ Angle between }\overrightarrow{{\mu }_m}\text{ and }\overrightarrow{B}={90}^{\circ }-{60}^{\circ }={30}^{\circ }& \mid \text{ Torque }\mid =10\times 0.2\times \sin {30}^{\circ }& =1\mathrm{N}.\mathrm{m}\end{array}$$