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NCERT Class XII Chapter
Magnetism and Matter
Questions With Solutions

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Question : 19
Total: 25
A telephone cable at a place has four long straight horizontal wires carrying a current of 1.0 A in the same direction east to west. The earth’s magnetic field at the place is 0.39 G, and the angle of dip is 35°. The magnetic declination is nearly zero. What are the resultant magnetic fields at point 4.0 cm below the cable?
Solution:  

Let us first decide the directions which can best represent the situation.
Here, BH = B cos δ = 0.39 × cos 35° G
BH = 0.32 G
and BV = B sin δ = 0.39 × sin 35° G
BV = 0.22 G
Telephone cable carry a total current of 4.0 A in direction east to west. We want resultant magnetic field 4.0 cm below.

Bwire = √(BH−Bwire)2+BV2
Bnet = √(0.12)2+(0.22)2 = √0.0144+0.0484 = 0.25 G
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