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| This article is going to require that you do some math. It's not involved math, just simple division and multiplication. That's all that's needed to understand Ohm's Law. That, and the knowledge of the different forms of measurement for voltage, current and resistance. If you don't understand this, or you get greatly confused between volts and amps you must read the preceding section before you can have a prayer of understanding anything about Ohm's Law. |
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| Now, I'm going to ask you a question. If you have a DC circuit made up (or a single resistor) that has a total resistance of 10 ohms across a certain two points, and now you place a voltage of 10 volts across those two points, how much current will flow. The answer is 1 ampere. Let me restate this again. (Notice, also, that I'm using numbers instead of spelling out the numbers at this point on purpose-- it's easier to learn this way): |
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| 10 volts applied across 10 ohms equals 1 ampere of current. That's not really that tough, is it? It's child's play to see that in this example (and believe it or not, it's not just an overly simplified situation) what we did here. We divided 10 by 10 and got an answer of 1. We represent this mathematically as 10/10=1. Read this example as follows: "Ten over ten equals one." I got into the habit of reading a fraction such as 3/4 as "three over four" when I was ten years old and the habit hasn't failed me since. Sure, 3/4 is "three quarters" too, but at this juncture it might be best to order our minds to automatically think in a flash that we're working with a division process here. |
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| Will it take a genius to figure out how much current flow will result in a DC circuit when we apply 20 volts between two points that we just measured (with our VOM, of course) at 10 ohms of resistance? In this case, I'm sure you see, 20/10=2. 2 amperes of current flows in a DC circuit made up of 20 volts applied across a resistance of 10 ohms. As you can see, the math part isn't the hard part to introducing your mind to these new ideas, it's getting the mental idea of these quantities of energy working along with each other and getting an appreciation for how it all comes out in the wash that's the real tough part. |
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| Now, another one. Your car is sitting in your driveway. It's been shut off for a few days and your car battery has dropped down to 12 volts exactly just because it likes you and wants to give you a nice even number like 12 to work with so you can get Ohm's Law straight in your mind. You take out your VOM and sure enough, you measure exactly 12 volts across the car battery. Now the car, obliging you again, causes no voltage drop to occur between its battery posts that you just measured at 12 volts and the lighter socket inside the passenger compartment. Your good fortune has caused you to have exactly 12 volts of voltage to apply to whatever you want for your experiments! |
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| You decide to take a bulb that you measured at exactly one ohm (How nice and even things are coming out!) and light it with the 12 volts from your car battery. Now you know, that since 12/1 (twelve over one) equals 12, you should have 12 amperes of current flowing, right? (I hope your VOM measures that much current-- it could a 20 ampere limit is not unrealistic, however, it probably tops out at around 2 amperes.) You set your VOM to measure current (now the VOM is being used as an ammeter) place it in series with the bulb-- it lights-- and you measure 0.3 amperes of current! That's 300 milliamps. Has Ohm's Law failed? |
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| Why did you measure only 300 milliamps (0.300 amperes) of current on the lit bulb when it had 1 ohm of resistance across it? To have such a lower current flow that bulb would have to have 40 ohms of resistance across it! Did you measure something wrong? No. The answer is something that, without experience, is quite perplexing, but to any trained electronics technician is also predictable. The answer is that the filament of the bulb measures a much lower resistance when it is cold with no voltage applied than when it is lit with the 12 volts applied. Varying resistance! That's what a filament in a bulb does. How do I know? I've done this PLENTY of times. This is only one example of things that become second-nature to you when you play with electronics. |
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| How then, you ask, can you possibly know what the lighted resistance is of the bulb's filament when you have to have the 12 volts applied? Ohm's Law to the rescue once again. |
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| The question here is: "How much resistance makes a current flow of 0.3 amperes with 12 volts applied? The answer here is 12/0.3 equals 40. 12 volts applied across 40 ohms equals 0.3 (300 milliamperes) of current. A current flow of 300 milliamperes results from connecting a 12 volts source across a 40 ohm resistance. All this analysis, the new knowledge that we just gained about a bulb measuring at a much lower resistance when cold, and we just lit a bulb inside our car! That's what the study of electronics is all about. No more guessing and assuming! |
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| It gets worse: In the real world, you're going to start out with a certain level of inaccuracy in your VOM, so if you measured exactly 12 volts at the car battery it is sure to be off by SOME amount. Let's use the 12 volts as being exact. The wires from the battery, through the connectors, up to the lighter socket will have their own resistance and the result will be a lower voltage to light the bulb with. The ammeter used to measure the current flow in the circuit has it's own amount of resistance too, and the sad fact is that every time you measure current flow with a meter the meter itself upsets the circuit and causes the current flow to be a little less than it would be if the meter were not being used in the circuit to try to measure what's going on. So you can bet, that if you measured 300 milliamperes of current with the meter in-circuit the amount of current flow would be higher when you took the meter out of the circuit. The more money you spend on the meter the less the meter will upset the circuit when it is inserted in the circuit for measurement. A more technical way of saying this is that the more expensive the meter, the lower its internal resistance will be and the more accurate its current readings will be because the meter itself will upset the circuit less when it's inserted into the circuit to measure the amount of current flowing. |
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| Take the above paragraph to heart, that is, if you got through it completely. Let it change your mind and your life. When you study electronics, and Ohm's Law is, perhaps, the most important step that you can take in that direction, you study theory and numbers and only if you keep in mind what the real world is doing to the ideal situation you had planned will you get an accurate idea of what is going on in fact. You still have to know what the idea, the theory is, though. |
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| Take a breather now before you go on to the next section where you will have Ohm's Law solidified in your mind along with the knowledge that, in fact, this law is a true predictor of electrical activity in a circuit as long as you don't muddy things up in your mind. |
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