# NCERT Physics Class 12 Exemplar Ch 5 Magnetism and Matter Part 7

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Q.23 Assume the dipole model for earth’s magnetic field which is given by vertical component of magnetic field Horizontal component of magnetic field

– lattitude as measured from magnetic equator. Find loci of points for which (i) is minimum; (ii) dip angle is zero; and (iii) dip angle is .

Solution:

(I)

, minimum at

is minimum at magnetic equator.

(ii)

At dip angle vanishes. Magnetic equator is again the locus.

(iii) Dip angle is when

is the locus.

Q.24 Consider the plane formed by the dipole axis and the axis of earth. Let be point on the magnetic equator and in . Let be the point of intersection of the geographical and magnetic equators. Obtain the declination and dip angles at and .

Solution:

Refer to the adjacent Fig.

1. is in (needle will point both north) Declination

is also on magnetic equator.

2. is on magnetic equator.

But declination

Q.25 There are two current carrying planar coils made each from identical wires of length . is circular (radius ) and is square (side ). They are so constructed that they have same frequency of oscillation when they are placed in the same uniform and carry the same current. Find a in terms of .

Solution:

(Moment of inertia about an axis through the diameter)