Electromotive force

Current can only flow if it gets a “push.” This might be caused by a buildup of static electric charges, as in the case of a lightning stroke. When the charge builds up, with positive polarity (shortage of electrons) in one place and negative polarity (excess of electrons) 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 systems). 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 Law. If the EMF is doubled, the current is doubled. If the resistance is doubled, the current is cut in half. This important law of electrical circuit behavior is covered in detail a little later in this book.
It is possible to have an EMF without having any current. This is the case just before a lightning bolt occurs, and before you touch that radiator after walking on the carpet. It is also true between the two wires of an electric lamp when the switch is turned off. It is true of a dry cell when there is nothing connected to it. There is no current, but a current is possible given a conductive path between the two points. Voltage, or EMF, is sometimes called potential or potential difference for this reason.
Even a very large EMF might not drive much current through a conductor or resistance. A good example is your body after walking around on the carpet. Although the voltage seems deadly in terms of numbers (thousands), there are not that many coulombs of charge that can accumulate on an object the size of your body. Therefore in relative terms, not that many electrons flow through your finger when you touch a radiator so you don’t get a severe shock.
Conversely, if there are plenty of coulombs available, a small voltage, such as 117 volts (or even less), can result in a lethal flow of current. This is why it is so dangerous to repair an electrical device with the power on. The power plant will pump an unlimited number of coulombs of charge through your body if you are foolish enough to get caught in that kind of situation.