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NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 13 of 25
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A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at 14 cm from the centre of the magnet. The earth’s magnetic field at the place is 0.36 G and the angle of dip is zero. What is the total magnetic field on the normal bisector of the magnet at the same distance as the null-point (i.e., 14 cm) from the centre of the magnet? (At null points, field due to a magnet is equal and opposite to the horizontal component of earth’s magnetic field.)
Solution:  
Angle of dip is zero at the place, so earth’s magnetic field is uniform with magnitude 0.36 G in the direction geographical south to geographical north.
At axial position for null point
BHB_H = BAB_A ⇒ 0.36 G = BAB_A
At equatorial position the field due to bar magnet is BA2\frac{B_A}{2}
BEB_E = BA2\frac{B_A}{2} = 0.18 G
Total magnetic field of earth and magnet adds at equatorial position
BTotalB_{Total} = BH+BEB_H+B_E = 0.36 G + 0.18 G = 0.54 G
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