- Any excess charge in the conductor must lie on its surface.Using Gauss’s law, draw a Gaussian surface
just below the surface of the conductor. Since the electric field inside the conductor is zero, the electric
flux through this surface is zero. Then by Gauss’s law, the charge inside the surface is zero. Therefore,
any excess charge must lie on the surface. (Another way to think of this is that since the charges repel,
they will want to get as far away from each other as possible, so they will end up on the surface.) - Electric field lines are perpendicular to the surface of the conductor.If the electric field lines intersected
the surface of the conductor at some angle, then there would be a tangential component of the electric
field present, which would cause the electrons to accelerate parallel to the surface. Therefore electric
field lines must meet the surface of the conductor at right angles.
16.7 Dielectric Breakdown
It is possible for materials that are normally insulators (dielectrics) to become electrically conducting, if they
are in the presence of a sufficiently large electric field. For example, air is normally in insulator, but the
presence of an electric field of at least 3 106 N/C creates channels of ionized gas through which electrons
can flow; the result is the familiarspark. This phenomenon is calleddielectric breakdown.
16.8 Lightning
Another example of dielectric breakdown islightning. During a thunderstorm, falling water drops and snow
pellets cause the clouds to acquire a negative charge, while the ground becomes positively charged; this
creates an electric field pointing upward. Electrons from the thundercloud carve a channel of ionized gas
that makes its way to the ground in a series of steps; this channel is called thestepped leader. At the same
time, a number of shorter ionized leader channels reach from the ground to a short distance upward. At some
point the downward-moving stepped leader connects with one of the upward leaders, and forms a complete
conducting path of ionized gas from the cloud to the ground. This causes a powerful ionizing wavefront,
called thereturn stroke, to move very quickly from the ground back up to the cloud, producing the flash we
see. The return stroke heats the surrounding air to a very high temperature, causing it to expand at supersonic
speed. This creates a shock wave that produces the sound we hear as thunder. Typically several such strokes
carry current between the ground and the earth during a single flash of lightning.