Electromotive Force:

Current can flow only if it acquires a "push." This push can be given by a buildup of electrostatic charges, as in the situation of a lightning stroke. Whenever the charge builds up, with positive polarity (i.e., shortage of electrons) in one place and negative polarity (surplus of electrons) in the other place, a powerful electromotive force (emf) exists. This result, also termed as voltage or electrical potential, and is measured in volts (represented by V).

Common household electricity has an effective voltage of between 110 and 130 V; generally it is around 117 V. A car battery has an emf of 12 V. The static charge which you get whenever walking on a carpet with hard-soled shoes can be numerous thousand volts. Before discharge of lightning, millions of volts exist.

An emf of 1 V, across a resistance of 1 ?, will cause a current of 1 A to flow. It is a classic association in electricity and is stated commonly as Ohm's law. When the emf is doubled, the current is doubled. When the resistance is doubled, then current is cut in half.

It is possible to have an emf without containing current flow. This is the situation just before a lightning bolt takes place and before you touch a metallic object after walking on the carpet. It is also true among the two prongs of a lamp plug whenever the lamp switch is turned off. It is true for a dry cell when there is nothing connected to it. There is no current, though a current can flow if there is a conductive path among the two points.

Even a large emf may not drive much current via a conductor or resistance. A good illustration is your body after walking around on the carpet. Though the voltage seems deadly in terms of numbers, not many coulombs of charge generally can accumulate on an object the size of your body. Thus, not many electrons flow through your finger, in relative terms, whenever you touch the metallic object. Therefore you do not get a severe shock.

On contrary, if there are plenty of coulombs available, a moderate voltage, like 117 V (or even less), can outcome in a lethal flow of current. This is why it is hazardous to repair an electrical device with the power on. The utility power source can pump an infinite number of coulombs of charge via your body when you are foolish adequate to get caught in this type of situation.