|
|
|
 |
|
|
|
|
| Since Ohm's Law deals with voltage, current and resistance, why don't we make things a bit tougher by introducing three new symbols to represent these quantities? Let's use E for voltage, I for current and R for resistance. Why? Tradition, mainly. These terms of E, I and R being used to represent voltage, current and resistance go back to the beginning of the serious study of electronics. It's really not too tough to remember E can stand for "Electromotive force" which, as was mentioned when we were talking about voltage is another term that can be used. R, of course, is the easy one-- you shouldn't have to remember that R stands for resistance. I stands for current. Hmmm... maybe you just have to remember that one. |
|
| So now, with our "new" terms of E, I and R being used, let's state all forms of Ohm's Law! |
| E=I/R |
| I=ExR |
| R=E/I |
| That's all folks. There you have it. All of Ohm's Law in three easy formulas that will make the great predictor perform. |
|
| In the previous article, you'll recall that we needed to figure out what the resistance of a light bulb filament was when it was lit. We did this by measuring the applied voltage to the bulb and the resulting current. So, we found R by using the formula R=E/I. We took 12 volts, place it over (divided it by) 0.3 amperes and figured out our resistance like this: 12/0.3=40. If you know two quantities in a circuit, you can figure out the other quantity EXACTLY. After a while of working with circuits at a certain applied voltage, such as a car mechanic working with a 12 to 14 volt source all the time, you'll intuitively get a feel for how much circuit action is going on where. |
|
| There's another subject that we should touch upon here before I leave you to go on your own to read about basic electricity and Ohm's Law from a much better source than I. That is the subject of working equations for power. |
|
| Think of power as the measure of the amount of real work being done in a circuit. Power is just an overall idea of how much energy is being dissipated in a circuit, regardless of the exact voltage and current of that circuit. It is a great equalizer. The idea of Power is to give you an idea of exactly how much work is going on in a circuit, whether that circuit uses, let's say 12 volts as it's source or 120 volts. Here's a good example: |
|
| You buy a transistorized audio amplifier for your car stereo that puts out 100 watts per speaker (a 200 watter!), or so the manufacturer claims. You turn on the stereo in your house, which has an audio amplifier made with tubes, and it, too puts out 100 watts per speaker. Which one has more power output? |
|
| This question is like the old saw of asking what's heavier- a ton of feathers or a ton of lead! They both are a ton. Both of these amplifiers are 100 watt per channel amplifiers. Yet, the amp in the car uses 12 volts DC and the one in the house has roughly ten times that voltage, 120 volts AC available to it as a voltage source. I say roughly here because there is, as you might suspect, some differences in accounting for so many volts DC when stacked against so many volts AC that must be taken into account because of the effects of AC. |
|
| By use of the idea of a power measurement, you immediately know that you can expect about the same level of sound pressure to come out of both of these amplifiers, all other things being equal, such as the response of the speakers, the acoustics of the speaker enclosures, etc. How do we come to this great equalizer of looking at a circuit or device without worrying how much voltage we had to work with in the beginning? |
|
| Just as there are three equations for Ohm's Law, you have to remember three forms of power equations. The symbol for Power, by the way is pretty easy to remember, it's P! |
| P=ExI |
| P=E2/R |
| P=I2xR (The "2" here should be a small "2" and read as "squared".) |
| In these equations, P is power measured in watts, E is voltage measured in volts and I is current measured in amperes. Yeah, I snuck another form of measurement in on you-- watts. (Look up James Watt.) |
|
| So then, if you know two quantities in a circuit, you can figure the power that the circuit is dissipating. In our bulb example, carried over from the previous section, if I use the equation P=ExI to figure out the power being used by the light bulb, I'll take 12 volts times 0.3 amperes (12x0.3) makes 3.6 watts. So now, I can ask you a question, how would you expect the bulb you lit up inside the passenger compartment of your car, which we now know uses about three-and-one-half watts of power up might compare to the incandescent bulb inside your house that says it's a 60 watter? Divide 60/3.6 to tell you exactly how many of these bulbs you would have to plug into your car battery to equal the amount of energy that a 60 watt bulb sucks up in your home, without worrying about which bulbs use how much voltage or current. As a comparison, you know that the bulb in your home use roughly 16 times more energy than the bulb you were holding in your hand in your car. Now you know why the power company charges for how many watts (actually, kilowatts per hour) of energy they meter going into your humble abode. |
|
| A knowledge and a feel for voltage, current, resistance and the power used by an electrical circuit or device is paramount to coming out of the fog. The more you look at the electronic things around you and ask yourself, as well as those of us who have spent a good portion of their lives studying electronic circuit actions the right questions, your only problem is knowing whose answers you can trust. Do you know enough and have you had enough experience with electronic circuits to give yourself direction? What do you do if you have, let's say, a 50 watt device, made to run on 12 volts DC, and want to know if you can plug it into your car's lighter socket when you know that there's a 10 ampere fuse inline with it? Will it work? -George Beloin |
| |
|
|
|
