Practice portal › Magnetism and Matter › Bar Magnet and Dipole in a Field
Asked in RS Academy GUJCET booklet · Field of a bar magnet and earth's field
Idea: a magnetic needle sets itself along whatever part of the field lies in its plane of rotation. For a needle turning in the horizontal plane that is the horizontal component B_H=B cos δ, where δ is the angle of dip.
At a geomagnetic pole the dip is 90°: the Earth's field there is vertical.
So B_H=B cos 90°=0, and there is no torque about the vertical axis of the pivot.
Every orientation is then an equilibrium orientation - neutral equilibrium - and the needle stays wherever it is set.
So at a geomagnetic pole such a needle can stay in any position.
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