Example of Magnetism | Learn Physics

Example of Magnetism

Example of Magnetism: a circular coil of radius 0.157 m has 50 turns. It is placed such that its axis is in magnetic meridian. A dip needle is supported at the centre of the coil with its axis of rotation horizontal and in the plane of the coil. The angle of dip is 30˚, when a current flows through the coil. The angle of dip becomes 60˚ on reversing the current. Find the current in the coil assuming that magnetic field due to the coil is smaller than the horizontal component of earth’s field. Take H = 3 × 10^-5 T.

 Solution:

If H is horizontal component and V is vertical component at the place, then true value of dip(∂) at the place is given by

Tan ∂ = V/H

If B magnetic field intensity at the centre of coil due to current I in circular coil and B is along H, then

Tan 30˚ = v/(H+B)

On reversing the direction of current, the direction of B is reversed.

Tan 60˚ = V/(H-B)                                 (B < H)

Dividing, we get tan 60˚/tan 30˚=(H+B)/(H-B)=√3/(1/√3)=3

B = H/2

B=(3 × 10^-5)/2=1.5× 10^-5 T

For a circular coil,  B = µ₀ni/2r

1.5× 10^-5 = ( 4π × 10^-7) × 50 ×i/(2 × 0.157)

i=0.75 A.

Ans : 0.75 A