# A compass needle

(a) Number of turns in the coil (n) = 30

Radius of coil (r) = 12cm = 0.12m

Current in the coil (I) = 0.35A

Angle of dip (δ) = 45o

Magnetic fields due to current carrying coils (B) =

where, μ0 = Permeability of free space = 4π × 10-7TmA1

B =

= 5.49 × 10-5 T

Horizontal component of the earth’s magnetic field = B sinδ

= 5.49 × 10-5 T × sin45o

= 5.49 × 10-5 T × 0.707

= 3.88 × 10-5 T

(b) When the current in the coil is reversed, and the coil is rotated about its vertical axis by an angle of 90° in the anticlockwise sense looking from above, then the needle will reverse its original direction (i.e. the needle will point from East to West).

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