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Physics and Current Help (page 2)

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By McGraw-Hill Professional
Updated on Sep 5, 2011

Electromotive Force

Current can flow only if it gets a “push.” This push can be provided by a buildup of electrostatic 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 (emf) exists. This effect, also known as voltage or electrical potential , is measured in volts (symbolized V).

Ordinary household electricity has an effective voltage of between 110 and 130 V; usually it is about 117 V. A car battery has an emf of 12 V (6 V in some older systems). The static charge that you acquire when walking on a carpet with hard-soled shoes can be several thousand volts. Before a discharge of lightning, millions of volts exist.

An emf of 1 V, across a resistance of 1 Ω, will cause a current of 1 A 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 law of electricity will be covered in detail a little later.

It is possible to have an emf without having current flow. This is the case just before a lightning bolt occurs and before you touch a metallic object after walking on the carpet. It is also true between the two prongs of a lamp plug when the lamp 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 can flow if there is a conductive path between the two points.

Even a 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), not many coulombs of charge normally can accumulate on an object the size of your body. Therefore, not many electrons flow through your finger, in relative terms, when you touch the metallic object. Thus you don’t get a severe shock.

Conversely, if there are plenty of coulombs available, a moderate voltage, such as 117 V (or even less), can result in a lethal flow of current. This is why it is dangerous to repair an electrical device with the power on. The utility power source can pump an unlimited number of coulombs of charge through your body if you are foolish enough to get caught in this kind of situation.

Practice problems of these concepts can be found at: Direct Current Practice Problem

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