Doug Lowe - Electronics All-in-One For Dummies

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 8of 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 .

An electric current that flows continuously in a single direction is called a direct current, or DC. The electrons in a wire carrying direct current move slowly, but eventually they travel from one end of the wire to the other because they keep plodding along in the same direction.

The voltage in a direct-current circuit must be constant, or at least relatively constant, to keep the current flowing in a single direction. Thus, the voltage provided by a flashlight battery remains steady at about 1.5 V. The positive end of the battery is always positive relative to the negative end, and the negative end of the battery is always negative relative to the positive end. This constancy is what pushes the electrons in a single direction.

Another common type of current is called alternating current, abbreviated AC. In an alternating-current circuit, voltage periodically reverses itself. When the voltage reverses, so does the direction of the current flow. In the most common form of alternating current, used in most power distribution systems throughout the world, the voltage reverses itself either 50 or 60 times per second, depending on the country. In the United States, the voltage is reversed 60 times per second.

Alternating current is used in nearly all the world’s power distribution systems for the simple reason that AC current is much more efficient when it’s transmitted through wires over long distances. All electric currents lose power when they flow for long distances, but AC circuits lose much less power than DC circuits.

The electrons in an AC circuit don’t really move along with the current flow. Instead, they sort of sit and wiggle back and forth. They move one direction for 1/60th of a second and then turn around and go the other direction for 1/60th of a second. The net effect is that they don’t really go anywhere.

For your further enlightenment, here are some additional interesting and useful facts concerning alternating current:

A popular toy called Newton’s Cradle might help you understand how alternating current works. The toy consists of a series of metal balls hung by string from a frame, such that the balls are just touching each other in a straight line, as shown in Figure 2-4. If you pull the ball on one end of the line away from the other balls and then release it, that ball swings back to the line of balls, hits the one on the end, and instantly propels the ball on the other end of the line away from the group. This ball swings up for a bit, and then it turns around and swings back down to strike the group from the other end, which then pushes the first ball away from the group. This alternating motion, back and forth, continues for an amazingly long time if the toy is carefully constructed.Alternating current works in much the same way. The electrons initially move in one direction but then reverse themselves and move in the other direction. The back and forth movement of the electrons in the circuit continues as long as the voltage continues to reverse itself. FIGURE 2-4:Newton’s Cradle works much like alternating current.

If you want to see a Newton’s Cradle in action, go to YouTube and search for Newton’s Cradle .

The reversal of voltage in a typical alternating current circuit isn’t instantaneous. Instead, the voltage swings smoothly from one polarity to the other. Thus, the voltage in an AC circuit is constantly changing. It starts out at zero, then increases in the positive direction for a bit until it reaches its maximum positive voltage, and then it decreases until it gets back to zero. At that point, it increases in the negative direction until it reaches its maximum negative voltage, at which time it decreases again until it gets back to zero. Then the whole cycle repeats itself.

The fact that the amount of voltage in an AC circuit is always changing turns out to be incredibly useful. You learn why in Book 4, Chapter 1when I give you a deeper look at alternating current.

At the start of this chapter, I mention the three key concepts you need to know about electricity before you can start to work with your own circuits. The first two — current and voltage — are described earlier in this chapter. To recap, current is the organized flow of electric charges through a conductor, and voltage is the driving force that pushes electric charges to create current.

The third piece of the puzzle is called power (abbreviated P in equations) . Simply put, power is the work done by an electric circuit. Electric current, in and of itself, isn’t all that useful. It becomes useful only when the energy carried by an electric current is converted into some other form of energy, such as heat, light, sound, or radio waves. For example, in an incandescent light bulb, voltage pushes current through a filament, which converts the energy carried by the current into heat and light.