Static Electricity

Charge carriers, particularly electrons, can build up, or become deficient, on things without flowing anywhere. You’ve experienced this when walking on a carpeted floor during the winter, or in a place where the humidity was low. An excess or shortage of electrons is created on and in your body. You acquire a charge of static 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.

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 deficiency of electrons; they repel. The spark might jump an inch, 2 inches, or even 6 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 generator, found in physics labs, can cause a spark this large (following Figure). Be careful when using this device for physics experiments!

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Simplified illustration of a Van de Graaff generator. This machine can create a charge buildup large enough to produce a spark several centimeters long.

In the extreme, lightning occurs between clouds, and between clouds and ground in the earth’s atmosphere. This spark, called a stroke, is a magnified version of the spark you get after shuffling around on a carpet. Until the stroke occurs, there is a static charge in the clouds, between different clouds or parts of a cloud, and the ground. In Following Fig. cloud-to-cloud (A) and cloud-to-ground (B) static buildups are shown.

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In the case at B, the positive charge in the earth follows along beneath the storm cloud. 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.