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MIT Physics 8.02 - Electricity & Magnetism

SECTION :    Magnetostatics       

SUBJECT: The Dip Needle  

A magnetic dipole in a "dip needle" oscillating in the magnetic field of the earth, at a latitude approximately the same as that of Boston. The magnetic field of the earth is predominantly downward and northward at these Northern latitudes. To explain what is going on in this visualization, suppose that the magnetic dipole vector is initially along the direction of the earth's field and rotating clockwise. As the dipole rotates, the magnetic field lines are compressed and stretched. The tensions and pressures associated with this field line stretching and compression results in an electromagnetic torque on the dipole that slows its clockwise rotation.

Eventually the dipole comes to rest. But the counterclockwise torque still exists, and the dipole then starts to rotate counterclockwise, passing back through being parallel to the earth's field again (where the torque goes to zero), and overshooting. As the dipole continues to rotate counterclockwise, the magnetic field lines are now compressed and stretched in the opposite sense. The electromagnetic torque has reversed sign, now slowing the dipole in its counterclockwise rotation. Eventually the dipole will come to rest, start rotating clockwise once more, and pass back through being parallel to the field, as in the beginning. If there is no damping in the system, this motion continues indefinitely.

Faraday understood the oscillations of a compass needle in exactly the way we describe here. In his words, "…To understand this point, we have to consider that a [compass] needle vibrates by gathering upon itself, because of it magnetic condition and polarity, a certain amount of the lines of force, which would otherwise traverse the space about it…"



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