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Electric current is what happens when the random exchange of electrons that occurs constantly in a conductor becomes organized and begins to move in the same direction.

When current flows through a conductor such as a copper wire, all those electrons that were previously moving about randomly get together and start moving in the same direction. A very interesting effect then happens: The electrons transfer their electromagnetic force through the wire almost instantaneously. The electrons themselves all move relatively slowly — on the order of a few millimeters a second. But as each electron leaves an atom and joins another atom, that second atom immediately loses an electron to a third atom, which immediately loses an electron to the fourth atom, and so on trillions upon trillions of times.

The result is that even though the individual electrons move slowly, the current itself moves at nearly the speed of light. Thus, when you flip a light switch, the light turns on almost immediately, no matter how much distance separates the light switch from the light.

Here are a few additional points that may help you understand the nature of current:

 One way to illustrate this principle is to line up 15 balls on a pool table in a perfectly straight line, as shown in Figure 2-2. If you hit the cue ball on one end of the line, the ball on the opposite end of the line will almost immediately move. The other balls will move a little, but not much (assuming you line them up straight and strike the cue ball straight).FIGURE 2-2: Electrons transfer current through a wire much like a row of pool balls transfers motion.This is similar to what happens with electric current. Although each electron moves slowly, the ripple effect as each atom loses and gains an electron is lightning fast (literally!).

 It’s no coincidence that moving water is also called current. Many of the early scientists who explored the nature of electricity believed that electricity was a type of fluid, and that it flowed in wires in much the same way that water flows in a river.

  The strength of an electric current is measured with a unit called the ampere, sometimes used in the short form amp or abbreviated A. The ampere is nothing more than a measurement of how many charge carriers (in most cases, electrons) flow past a certain point in one second. One ampere is equal to 6,240,000,000,000,000,000 electrons per second. That’s 6,240 quadrillion electrons per second. (That’s a huge number, but remember that electrons are incredibly small. To give it some perspective, though, imagine that each electron weighed the same as an average grain of sand. If that were true, one amp of current would be equivalent to the movement of nearly 350 tons of sand per second.)

 Most electric incandescent light bulbs have about one amp of current flowing through them when they are turned on. A hair dryer uses about 12 amps.

 Current in electronic circuits is usually much smaller than current in electrical devices like light bulbs and hair dryers. The current in an electronic circuit is often measured in thousandths of amps, or milliamps (abbreviated mA).

 Current is often represented by the letter I in electrical equations. The I stands for intensity.