Alternating Current - as in the electrical outlets in our homes, the flow (current) of electricity is always changing direction (60 times every second in the U.S., 50 times every second in other countries), through the loads such as lamps, water heaters, televisions, and all the things we plug into the wall outlets. These items also have resistance, but because of the alternating current, we have to change our mathematics starting with R or RESISTANCE being replaced by a thing called IMPEDANCE or the letter Z. Now, if we think about water flowing in the river again, but this time with the flow (current) changing direction 50 or 60 times in every second, we can imagine some very big differences in the DC and AC theories. The AC theory is mostly used in journeymanship levels through upper engineering people.
For practical purposes, an electrical current always meets some opposition when flowing through a circuit, even if that circuit is merely a straight copper wire. The opposition to alternating current is called impedance because it impedes, or hinders, the flow of electrons. Impedance is represented by the letter Z in equations, and it's measured in ohms. Impedance is closely related to resistance, but the two are not identical. Impedance is how much a device resists the flow of an AC signal, such as audio. Impedance is similar to resistance which is how much a device resists the flow of a DC signal.
Once you know the resistance and reactance of a circuit, the impedance is actually the overall opposition to current presented by the circuit. The impedance of a circuit is also expressed in ohms; Unfortunately, you cannot simply add the resistance and the reactance to get the impedance.
The total impedance of a circuit is the square root of the sum of the squares of the resistance and reactance. That is, impedance (symbolized by the letter Z) is the square root of (resistance squared plus reactance squared).
(keep scrolling down...) You May Be An Engineer IF.....
YOU MAY BE AN ENGINEER... If you are always asking your friends from marketing to hold two leads to a giant capacitor,
If you find your head nodding up and down every time you read Dilbert,
If your pocket is full of too many mechanical pencils, If when your 3-year old asks "Why is the sky blue?" you start explaining it to them,
If you can explain which direction the water spins as you flush the toilet and why,
If you go to the air show, and you start calculating how fast the sky divers are falling, you may be an engineer; if you start telling all the people around you, you definitely are.
If you need a spreadsheet to figure out who owes what for lunch,
If you plan your family vacation on a Gantt chart,
If you pre-plan your route on a map of the exhibits through the annual computer show at Moscone Center, If you read PC World and Popular Mechanics while on vacation,
If you refer to your spouse as "\woman at home.wife,"
If your favorite TV show is "Mr. Wizard" instead of "Baywatch,"
If when your family is expecting, you are more interested in the ultra-sound equipment than the test results,
If when someone asks "What's new?" you answer "C over lambda,"
If you know Bill Gates' e-mail address, and don't remember your own,
If you are willing to debate for two hours the possible results of an experiment that takes five minutes to run,
If you know the altitudes at which you must turn off electronic devices on an airplane, and why,
If on a camping trip, your spouse starts complaining about bug "bites" and you respond that "Yes, we do need more memory in our computer,"
If Dilbert is your hero
If you stare at an orange juice container because it says CONCENTRATE
If you want an 8X CDROM for Christmas
If you can name 6 Star Trek episodes
If the only jokes you receive are through e-mail
If your wrist watch has more computing power than a 486DX2-50
If your idea of good interpersonal communication means getting the decimal point in the right place
If you look forward to Christmas only to put together the kids' toys
If your spouse sends you an e-mail instead of calling you to dinner
If you can quote scenes from any Monty Python movie
If you use a CAD package to design your son's Pine Wood Derby car
If you have used coat hangers and duct tape for something other than hanging coats and taping ducts
If, at Christmas, it goes without saying that you will be the one to find the burnt-out bulb in the string
If you window shop at Radio Shack
If your ideal evening consists of fast-forwarding through the latest Sci-Fi movie looking for technical inaccuracies
If you have "Dilbert" comics displayed anywhere in your work area
If you carry on a one-hour debate over the expected results of a test that actually takes five minutes to run
If you are convinced you can build a phazer out of your garage door opener and your camera's flash attachment
If you don't even know where the cover to your personal computer is
If you have modified your can-opener to be microprocessor driven
If you know the direction the water swirls when you flush
If you own "Official Star Trek" anything
If you have ever taken the back off your TV just to see what's inside
If a team of you and your co-workers have set out to modify the antenna on the radio in your work area for better reception
If you ever burned down the gymnasium with your Science Fair project
If you are currently gathering the components to build your own nuclear reactor
If you own one or more white short-sleeve dress shirts If you have never backed-up your hard drive
If you are aware that computers are actually only good for playing games, but are afraid to say it out loud
If you truly believe aliens are living among us
If you have ever saved the power cord from a broken appliance
If you have ever purchased an electronic appliance "as-is"
If you see a good design and still have to change it If the salespeople at Circuit City can't answer any of your questions
If you still own a slide rule and you know how to work it
If the thought that a CD could refer to finance or music never enters your mind
If you own a set of itty-bitty screw drivers, but you don't remember where they are
If you rotate your screen savers more frequently than your automobile tires
If you have a functioning home copier machine, but every toaster you own turns bread into charcoal
If you have more toys than your kids
If you need a checklist to turn on the TV
If you have introduced your kids by the wrong name
If you have a habit of destroying things in order to see how they work
If your I.Q. number is bigger than your weight
If the microphone or visual aids at a meeting don't work and you rush up to the front to fix it
If you can remember 7 computer passwords but not your anniversary
If you have memorized the program schedule for the Discovery Channel and have seen most of the shows already
If you have ever owned a calculator with no equal key and know what RPN stands for
If your father sat 2 inches in front of your family's first color TV with a magnifying lens to see how they made the colors, and you grew up thinking that was normal
If you know how to take the cover off of your computer, and what size screw driver to use
If you can type 70 words a minute but can't read your own handwriting
If people groan at the party when you pick out the music=7F
If you can't remember where you parked your car for the 3rd time this week
If you did the sound system for your senior prom
If your checkbook always balances
If your wristwatch has more buttons than a telephone
If you have more friends on the Internet than in real life
If you thought the real heroes of "Apollo 13" were the Mission Controllers
If you think that when people around you yawn, it's because they didn't get enough sleep
If you spend more on your home computer than your car
If you know what http:/ stands for
If you've ever tried to repair a $5.00 radio
If you have a neatly sorted collection of old bolts and nuts in your garage
If your three year old son asks why the sky is blue and you try to explain atmospheric absorption theory
If your 4 basic food groups are: 1. Caffeine 2. Fat 3. Sugar 4. Chocolate
A very cool electrical engineer equation cartoon that was commissioned and copyrighted by TechNote Time:
