What is Electromotive force

Chapter Electromotive force

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • electricity. It’s called “static” because it doesn’t go anywhere. You don’t feel this until youtouch 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 isthe 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, becauseevery 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 sixinches. Then it would more than startle you; you could get hurt. This doesn’t happenwith 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.16 Basic physical concepts1-7Simple diagram of a Van de Graaff generator for creatinglarge static charges.In the extreme, lightning occurs between clouds, and between clouds and groundin the earth’s atmosphere. This spark is just a greatly magnified version of the littlespark you get after shuffling around on a carpet. Until the spark occurs, there is a staticcharge in the clouds, between different clouds or parts of a cloud, and the ground. InFig. 1-8, cloud-to-cloud (A) and cloud-to-ground (B) static buildups are shown. In thecase at B, the positive charge in the earth follows along beneath the thunderstorm cloudlike 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 hundredsof thousands, of amperes. But it takes place only for a fraction of a second. Still, manycoulombs of charge are displaced in a single bolt of lightning.Electromotive forceCurrent 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-