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Electromotive force 17
1-8 Cloud-to-cloud (A) and cloud-to-ground (B) charge buildup can both occur in a single
thunderstorm.
tive polarity (shortage of electrons) in one place and negative polarity (excess of elec-
trons) in another place, a powerful electromotive force exists. It is often abbreviated
EMF. This force is measured in units called volts.
Ordinary household electricity has an effective voltage of between 110 and 130;
usually it is about 117. A car battery has an EMF of 12 volts (six volts in some older sys-
tems). The static charge that you acquire when walking on a carpet with hard-soled
shoes is often several thousand volts. Before a discharge of lightning, many millions of
volts exist.
An EMF of one volt, across a resistance of one ohm, will cause a current of one ampere
to flow. This is a classic relationship in electricity, and is stated generally as Ohm’s
