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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)            ...