Electromotive force

Current can flow if it gets a "push." This can be created by a buildup of static electric charges, similar to the case of a lightning stroke. When the charge builds up, with

Figure:  Cloud-to-cloud (A) and cloud-to-ground (B) charge buildup can both occur in a single thunderstorm.

Positive polarity in one place and negative polarity in another place, a powerful  electromotive force exists. It is abbreviated as EMF. This force can be measured in units known as volts.

Ordinary electricity which is used in household has an effective voltage of between 110 and 130; usually it is about 117. A battery car has an EMF of 12 volts. The static charge that you acquire when walking on a carpet with hard-soled shoes is several thousand volts. Before the discharge of lightning, several millions of volts exist.

An EMF of 1 volt, across a resistance of 1 ohm, will cause a current of 1 ampere to flow. This is a classic relationship in electricity, and can be stated as Ohm's Law. If EMF is doubled, the current is doubled. This significant law of electrical circuit behavior is covered in detail later.

It is in fact possible to have an EMF without having any current. This is the case before a lightning bolt occurs, and before you touch that radiator after walking on the carpet.  It is true between the two wires of the electric lamp when switch is turned off. It is true of a dry cell when there is nonentity connected to it. There is no current, but the current is possible given the conductive path between the 2 points. Voltage, or EMF, is sometimes called as potential or potential difference for this very reason.

Even a very large EMF may not drive much current through a conductor or resistance. A good example of it is your body after walking around on the carpet. Although the voltage seems deadly in terms of numbers, there are not that many coulombs of charge which 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 do not get a severe shock.

Conversely, if there are plenty of coulombs available, a small voltage, such as 117 volts, can result in a lethal flow of current. This is why it is so unsafe to repair an electrical device with the power on. The power plant will pump an infinite number of coulombs of charge through your body if you are foolish enough to get caught in that kind of situation.