NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 18
Total: 25
A long straight horizontal cable carries a current of 2.5 A in the direction 10° south of west to 10° north of east. The magnetic meridian of the place happens to be 10° west of the geographic meridian. The earth’s magnetic field at the location is 0.33 G, and the angle of dip is zero. Locate the line of neutral points (ignore the thickness of the cable). (At neutral points, magnetic field due to a current-carrying cable is equal and opposite to the horizontal component of earth’s magnetic field.)
Solution:  

The neutral point can be achieved at a location above cable, where magnetic field of cable is balanced by earth’s magnetic field BH.
B =
µ0
4π
2I
r
= BH
107 ×
2×2.5
r
= 0.33 × 104
or r =
5×103m
0.33
= 15.15 × 103 = 15.15 mm
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