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In its natural state, the electrons in a conductor such as copper freely move from atom to atom but in a completely random way. To get them to move together in one direction, all you have to do is give them a push. The technical term for this push is electromotive force, abbreviated EMF, or sometimes simply E. But you know it more commonly as voltage.

A voltage is nothing more than a difference in charge between two places. For example, suppose you have a small clump of metal whose atoms have an abundance of negatively charged atoms and another clump of metal whose atoms have an abundance of positively charged atoms. In other words, the first clump has too many electrons and the second clump has too few. A voltage exists between those two clumps. If you connect those two clumps with a conductor such as a copper wire, you create what is called a circuit through which electric current will flow.

This current continues to flow until all the extra negative charges on the negative side of the circuit have moved to the positive side. When that has happened, both sides of the circuit become electrically neutral and the current stops flowing.

Here are some additional points to ponder concerning voltage:

 Whenever there’s a difference in charge between two locations, there’s a possibility that a current will flow between the two locations if those locations are connected by a conductor. Because of this possibility, the term potential is often used to describe voltage. Without voltage, there can be no current. Thus, voltage creates the potential for a current to flow.

 If current can be compared to the flow of water through a hose, voltage can be compared to water pressure at the faucet. It’s water pressure that causes the water to flow in the hose.

 Voltage is measured using a unit called, naturally, the volt, usually abbreviated V. The voltage that’s available in a standard electrical outlet in the United States is about 117 V. The voltage available in a flashlight battery is about 1.5 V. A car battery provides about 12 V.

 You can find out how much voltage exists between two points by using a device known as a voltmeter, which has two wire test leads that you can touch to different points in a circuit to measure the voltage between those points. Figure 2-3 shows a typical voltmeter. (Actually, the meter shown in the figure is technically called a multimeter because it can measure things other than voltage. For more information about using a voltmeter, see Chapter 8 of this minibook.)FIGURE 2-3: You can use a multimeter like this to measure voltage.

 Voltages can be considered positive or negative but only when compared with some reference point. For example, in a flashlight battery, the voltage at the positive terminal is +1.5 V relative to the negative terminal. The voltage at the negative terminal is –1.5 V relative to the positive terminal.ARE YOU POSITIVE ABOUT THAT?For the first 150 years or so of serious research into the nature of electricity, scientists had electric current backward: They thought that electric current was the flow of positive charges and that electric current flowed from the positive side of a circuit to the negative side.It wasn’t until around 1900 that scientists began to unravel the structure of atoms. They soon figured out that electrons have a negative charge, and current is actually the flow of these negatively charged electrons. In other words, they discovered that current flows in the opposite direction from what they had long thought.Old ideas die hard, and to this day most people think of electric current as flowing from positive to negative. This concept of current flow is sometimes called conventional current. Modern electronic circuits are almost always described in terms of conventional current, so the assumption is that current flows from positive to negative, even though the reality is that the electrons in the circuit are actually flowing in the opposite direction.

  I’d like to tell you the exact definition of a volt, but I can’t — at least not yet. The definition of a volt won’t make any sense until you learn about the concept of power, which is described later in this chapter in the section titled “Understanding Power.”

  Although current stops flowing when the two sides of the circuit have been neutralized, the electrons in the circuit don’t stop moving. Instead, they simply revert to their natural random movement. Electrons are always moving in a conductor. When they get a push from a voltage, they move in the same direction. When there’s no voltage to push them along, they move about randomly.

 In electrical equations, voltage is usually represented by the letter E, which stands for electromotive force.