A positive point charge moves out of the page rigin
in a time-changing external field. This external field
is uniform in space but varies in time with a period
T as indicated by the blue vector in the animation.We
assume that the variation of the external field is so
rapid that the charge moves only a negligible distance
in one period of this field.
When the external magnetic field is zero, we see only
the magnetic field of the moving charge. Since the charge
is moving out of the page, its field circulates counter-clockwise.
When the vertically downward external magnetic field
is at its maximum, we see to the left of the charge
an enhancement of the the magnetic field. This enhancement
arises because the field of the charge is in the same
direction as the external magnetic field (downward)
on that side. To the right of the charge, where the
field of the charge is opposite that of the external
magnetic field, we see a reduction in the total magnetic
This overall field configuration indicates a net force
to the right on the moving charge. This occurs because
the pressure of the magnetic field is much higher on
the left as compared to the right. Note that if the
charge had been moving into the page instead of out
of the page, the force would have been to the left,
because the magnetic pressure would have been higher
on the right.
The animation shows dramatically the inflow of energy
into the neighborhood of the charge as the external
magnetic field grows, with a resulting build-up of stress
that transmits a sideways force to the moving positive
charge. The white vector indicates direction and magnitude
of this force.