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 1 when I give you a deeper look at alternating current.
Understanding Power
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.
Power is measured in units called watts (abbreviated W). The definition of one watt is simple: One watt is the amount of work done by a circuit in which one ampere of current is driven by one volt.
This relationship lends itself to a simple equation. I promised myself when I started this book that I would use as few equations as possible, but I knew I’d have to include at least some of the basic equations. Fortunately, this one is pretty simple:
In other words, power (P) equals voltage (E) times current (I).
Thus, the light bulb is doing 1 watt of work.
Often, you know the voltage and the wattage of the circuit and you want to use those values to determine the amount of current flowing through the circuit. You can do that by turning the equation around, like this:
For example, if you want to determine how much current flows through a lamp with a 100-watt light bulb when it’s plugged into a 117-volt electrical outlet, use the formula like this:
Thus, the current through the circuit is 0.855 amperes.
Here are some final thoughts concerning the concept of power:
The term dissipate is often used in association with power. As the energy carried by an electric current is converted into another form such as heat or light, the circuit is said to dissipate power.
Did you notice that current and voltage are represented by the letters I and E, not the letters C or V as you might expect, but power is represented by the letter P? Sometimes you wonder if the people who make the rules are just trying to confuse everyone.Maybe the following table will help you keep things sorted out:ConceptAbbreviationUnitCurrentIAmp (A)VoltageE or EMFVolt (V)PowerPWatt (W)
Earlier in this chapter, in the section “Understanding Voltage,” I say that I can’t define “one volt” until you know what power is. Now that you know, you can see that the definition of a volt is simple: One volt is the amount of electromotive force (EMF) necessary to do one watt of work at one ampere of current.
Calculating the power dissipated by a circuit is often a very important part of circuit design. That’s because electrical components such as resistors, transistors, capacitors, and integrated circuits all have maximum power ratings. For example, the most common type of resistor can dissipate at most ¼ watt. If you use a ¼-watt resistor in a circuit that dissipates more than ¼ watt of power, you run the risk of burning up the resistor.
Chapter 3
Creating Your Mad-Scientist Lab
IN THIS CHAPTER