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 180
degrees out of phase. When the effective dipole moment
of the top coil points upwards, the dipole moment of
the bottom coil points downwards. Thus, the magnet is
attracted to the upper coil and repelled by the lower
coil, causing it to move upwards. When the conditions
are reversed during the second half of the cycle, the
magnet moves downwards.
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 one
of the coils is aligned with that of the magnet, there
is a tension along the fieldlines as they attempt to
"connect" the coil and magnet. Conversely,
when their moments are anti-aligned, there is a pressure
perpendicular to the fieldlines as they try to keep
the coil and magnet apart.