Page 36 - Teach Yourself Electricity and Electronics

16 Basic physical concepts

electricity. It’s called “static” because it doesn’t go anywhere. You don’t feel this until you
touch some metallic object that is connected to earth ground or to some large fixture;
but then there is a discharge, accompanied by a spark that might well startle you. It is
the current, during this discharge, that causes the sensation that might make you jump.
If you were to become much more charged, your hair would stand on end, because
every hair would repel every other. Like charges are caused either by an excess or a de-
ficiency of electrons; they repel. The spark might jump an inch, two inches, or even six
inches. Then it would more than startle you; you could get hurt. This doesn’t happen
with ordinary carpet and shoes, fortunately. But a device called a Van de Graaff gen-
erator, found in some high school physics labs, can cause a spark this large (Fig. 1-7).
You have to be careful when using this device for physics experiments.

1-7 Simple diagram of a Van de Graaff generator for creating
large static charges.
In the extreme, lightning occurs between clouds, and between clouds and ground
in the earth’s atmosphere. This spark is just a greatly magnified version of the little
spark you get after shuffling around on a carpet. Until the spark occurs, there is a static
charge in the clouds, between different clouds or parts of a cloud, and the ground. In
Fig. 1-8, cloud-to-cloud (A) and cloud-to-ground (B) static buildups are shown. In the
case at B, the positive charge in the earth follows along beneath the thunderstorm cloud
like a shadow as the storm is blown along by the prevailing winds.
The current in a lightning stroke is usually several tens of thousands, or hundreds
of thousands, of amperes. But it takes place only for a fraction of a second. Still, many
coulombs of charge are displaced in a single bolt of lightning.

Electromotive force

Current can only flow if it gets a “push.” This might be caused by a buildup of static elec-
tric charges, as in the case of a lightning stroke. When the charge builds up, with posi-