Potential energy is a nice way to understand the reason that
lightning happens. During a thunder-storm, the bottoms of clouds tend
to become negatively charged, which causes the ground (where we live)
to become positively charged. In the language or electricity we say
that there is a potential difference between the clouds and the ground,
which just means that the positive charges on the ground (and the
negative charges on the cloud bottoms) have lots of potential energy.
Usually when things have lots of potential energy they will start
moving if you let them. In the case of the charges in the cloud, they
will try to move to the ground. Normally, this is difficult since air
is a very good electrical insulator, but if the difference in potential
between the clouds and the ground gets big enough (often many millions
of volts) the charges can jump, resulting in lightning.
As the first electrons jump, they will leave a small trail of
ionized gas in the air. This trail is a pretty good conductor of
electricity, so the rest of the charges in the cloud will quickly
follow down this path, causing the lightning bolts that we often see.
The potential energy that was present to begin with will be reduced
by this motion of charges, and instead there will be lots of kinetic
energy (heat) produced as the lightning ionized the air and slams into
(republished on 08/02/06)