Solution:

Ans. Try yourself as Q. 1 SAQ II

[AI Q. 6. Define the following using suitable diagrams : (i) magnetic declination and (ii) angle of dip. In what direction will a compass needle point when kept at the (i) poles and (ii) equator?

R [Delhi I, II, III 2015]

Ans. (i) Magnetic declination : Angle between the magnetic axis and geographical axis.

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Alternatively : Angle between magnetic meridian and geographical meridian.

(ii) Angle of dip : It is the angle which the magnetic needle makes with the horizontal in the magnetic meridian.

Alternatively,

The angle which the total magnetic field of the earth makes with the surface of the earth.

The direction of compass needle is vertical to the earth’s surface at poles and is parallel to the earth’s surface at equator.

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Long Answer Type Question

[AI Q. 1. (a) A long solenoid with air core has $n$ turns per unit length and carries a current $I$. Using Ampere’s circuital law, derive an expression for the magnetic field $B$ at an interior point on its axis. Write an expression for magnetic intensity $H$ in the interior of the solenoid. (b) A (small) bar of material, having magnetic susceptibility $\chi$, is now put along the axis and near the centre, of the solenoid which is carrying a d.c. through its coils. After some time, the bar is taken out and suspended freely with an unspun thread. Will the bar orient itself in magnetic meridian if (i) $\chi<0$ (ii) $\chi>1000$ ? Justify your answer in each case.

A [CBSE SQP 2018-19]

Sol. (a) Derivation $\quad H=n I \quad 2^{\frac{1}{2}}$

The direction of $H$ is along the axis of the solenoid, directed along the direction of advance of a righthanded screw rotated along the direction of flow of current

(b) (i) Not necessarily.

Reason : material is diamagnetic. After removal of magnetising field, no magnetization will remain in the material and hence earth’s magnetic field will not affect it.

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Reason : The material is ferromagnetic. It will remain magnetised even after removal from the solenoid and hence align with magnetic meridian.

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