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MIT Physics 8.02 - Electricity & Magnetism

SECTION : Magnetostatics       

SUBJECT: Magnet Suspended Between Two Coils  

This animation shows the magnetic field of a permanent magnet suspended by a spring in the TeachSpinTM apparatus (see TeachSpin visualization), plus the magnetic field due to current in the two coils (here we see a "cutaway" cross-section of the apparatus). The magnet is fixed so that its north pole points upward, and the current in the two coils is sinusoidal and in phase. When the effective dipole moment of the top coil points upwards, the dipole moment of the bottom coil points upwards as well. Thus, the magnet the magnet is attracted to both coils, and as a result feels no net force (although it does feel a torque, not shown here since the direction of the magnet is fixed to point upwards). When the dipole moments are reversed during the second half of the cycle, the magnet is repelled by both coils, again resulting in no net force.

This process can also be described in terms of tension along, and pressure perpendicular to, the fieldlines of the resulting field. When the dipole moment of the coils is aligned with that of the magnet, there is a tension along the fieldlines as they are "pulled" from both sides. Conversely, when their moments are anti-aligned, there is a pressure perpendicular to the fieldlines as they are "squeezed" from both sides.




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