126 Week 3: Potential Energy and Potential
−q strong (point−charge−like) field!glow (corona)weak field at sidesE+QgroundFigure 38: External charge +Qinduces a charge -q on the sharp tip of a nearby conductor. Electric
fields lines leave the tip at right angles, producing a field that looks like that of a very largepoint
chargewhich is extremely strong very close to the tip. This in turn ionizes nearby air molecules,
creating thecorona(and spraying/repelling negatively charged ions out into the air where they are
attracted to +Qand eventually neutralize it).
Those molecules quickly pick up charge from the tip and are thenrepelledby it. They literally
spray away from it, carrying charge and momentum and flowing towards the inducing charge. This
is a process calledcorona dischargeand is how lightning works. A lightning rod does notattract
lightning (youneverwant to attract lightning) itneutralizes itby allowing charge tograduallybe
pulled up from the ground and sprayed onto an approaching strongly charged cloud andslowly
neutralize it.