If the electric field inside a capacitor exceeds about 3 × 106 V/m, the few free electrons in the air are accelerated enough to
trigger an avalanche and make a spark. In the spark shown in Figure 24.117, electrons are accelerated upward and positive
ions are accelerated downward.
(a) Qualitatively, explain the directions and relative magnitudes of the radiative electric fields at location A, to the left
of the capacitor, due to the indicated motions of the electrons and of the ions (actually, in a spark there are also
accelerations in the opposite direction when the electrons and ions collide with air molecules).
(b) Repeat the analysis for locations B (to the right) and C (to the right and down a bit).
(c) If you are at location A, 3 meters to the left of the capacitor, how long after the initiation of the spark could you
first detect a magnetic field? What is the direction of the radiative magnetic field? (Before the spark occurs there is
no magnetic field anywhere in this region.)
(d) In addition to producing electromagnetic radiation, the moving charges in the spark produce electric fields
according to Coulomb's law and magnetic fields according to the Biot–Savart law. Explain why these fields are
much smaller than the radiative fields, if you are far away from the capacitor.