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Ohm's Law for Beginners (or The Amp-Hour Fallacy) 2012-03-14

Just about once a day, somebody writes in with, “My boom box has a dead battery that says 12 volts, 1.8 amp-hours. The new battery says 12 volts, 2.4 amp-hours. Will the extra amp hours burn up my radio?”

No, gentle reader, amp-hours are a measure of capacity. Your new battery has an extra quart of electricity, not a high pressure nozzle.

It's like standing next to a swimming pool with a glass of water. Water represents current and you represent the radio. You (the radio) can drink a glass of water (an amp of current) from either container, and no matter which one you choose to drink from, you are not going to drink the entire swimming pool and the swimming pool is not going to try to force you to drink it. You (the radio) only take as much as you need.

The first thing you need to know about electricity is, “Don't touch it with your fingers, it bites.”
The second thing you need to know is Ohm's Law.

Ohm's Law is an equation.

The simplest equation in the world has 2 symbols and an equal sign.


The next level is 3 symbols and an equal sign.

Distance = (miles per hour) x hours
inches = feet x 12

That's as bad as Ohm's Law gets. It is entirely within your grasp.

The “E” means “electromotive force” or volts.
“I” means current, as measured in amps.
“R” means resistance...the resistance to the flow of electricity, and those resistance things are called ohms.

E=IR says, voltage equals current times resistance.
Sounds stupid at first.
How can you multiply current times resistance? What are amp-ohms? I've never heard of that!

Trust me. It works. What Ohm's Law really says is, “current equals voltage divided by resistance”.
12 volts divided by 12 ohms is one amp.
A 12 volt battery will push one amp through a 12 ohm radio, no matter if it's a 12 volt gel-cell or a 12 volt fork lift battery. That's because the radio has resistance.

In fact, any circuit that is working properly has resistance. Only the broken ones don't have resistance, and that's why the fuse blows.

Getting a bit closer now...

E=IR is an equation, a statement of equality. It can be written as E/I =R or E/R=I

Ohms are just ohms. However many ohms you have, the voltage will be divided by that number to find out how many amps will flow.

Volts are always volts as compared to some other place. Volts are a unit of pressure, as in, how hard will the battery push on the ohms to get current through. Through to where? To the other end of the battery.

Amps aren't just lumps, like a pound of clay or a box of cookies. You can't have a box with an amp in it. An amp is a flow, defined as current per second. Like, water might flow in gallons per minute, but when you say, “amps” you are already saying, “per second”. When an amp flows for an hour, that's an amp-hour, like the label on the battery. All the amp-hours don't come out of the battery at the same time. If they did, the battery would be empty in a millionth of a second. That's not very useful, is it?

Electrons are real. They have weight and take up space, but they're really tiny! Electrons are so small that they actually sneak between the atoms in a copper wire. Electrons are laying around, all over the place, by the millions. Whenever two photons combine, they make an electron. You know that light is abundant on this planet. When you think about that, you must realize that electrons are abundant. This whole planet is fairly slopping over with electrons. The job called, “electronics” is about how to get electrons to do stuff...useful stuff.

How to get electrons to do useful work:

The first thing to do is to get some electrons in a pile. (Maybe that's why the first batteries were called, “piles”?) Anyway, it is the difference in concentration of electrons that makes voltage. You see, electrons don't like each other. They “repel” each other. When you get a bunch of electrons in a pile, they want to leave. If you give electrons the opportunity, they will all scatter about until they are uniformly distributed. Any place that has more electrons than the nearest other place will find the electrons repelling each other and trying to become the same concentration of electrons per area, in all the areas that they can get to. The desire to leave is called, “voltage”. That's where Ohm's Law becomes useful. When you let the electrons leave through a resistance, it takes time and they do work. If you make it complicated enough, it can be really interesting. For now, it's going to be a bit boring, but you have to start somewhere.

There are several ways to get a pile of electrons. Just walking across a carpet can scrape off enough electrons to make a spark. I don't know whether you get excess electrons all over yourself, or you scrape electrons off yourself, but it doesn't matter. It's the difference in concentration that makes voltage, and voltage makes sparks. The problem with this approach is that these electrons will try to escape to just about anywhere, and that isn't useful, unless maybe zapping your little brother is what you call useful. One of the reasons the electrons pile up is that air has a lot of resistance (ohms). Electrons don't flow through air like water does. They flow through air, and that's called lightening or static electricity, but it's not very useful.

Batteries cause electrons to pile up at the negative connector, and they want to leave, and, at the same time, the chemicals in the battery reduced the concentration of electrons on the positive terminal, and... that's where the electrons want to the place with a less concentrated number of electrons. With a battery, you have two defined points that you can connect together through a device that has resistance and get a dependable flow to do something interesting or useful.

When you get a difference in the concentration of electrons, you have voltage. You can measure the voltage with a volt meter. You can connect a resistance between the two points and current will flow in terms of electrons per second, or amps. The voltage divided by the resistance will give you a number that we use as a measure of electron flow and we call that amps.

This is what Ohm's Law is about. Voltage divided by resistance equals amps. Amps times resistance will tell you the voltage that is driving the electrons per second through the resistance. If you know the volts by reading the label on the battery, and you can measure the amps with a meter, you can calculate the ohms that must be resisting the flow of electrons.

There are a lot of new words in this writing. You have to use the words until you begin to understand them. It takes time for your brain to make sense of this. You are allowed to read this as many times as you want to. We all go back and read stuff we have read before. Electronics is just too complicated to hold it all in your head, but Ohm's Law is small enough that you can learn it and keep it in your head. You will use Ohm's Law a dozen times a day when you are studying electronics, building a circuit, or fixing the brake lights on your pick-up truck. It's that important. You really must learn this in order to make any progress because there is no sub-section of electronics that does not use Ohm's Law.

Disclaimer: It will only be a matter of weeks until you can spot where I used generalizations and left out exceptions. It is not my purpose to write the definitive, all inclusive, perfect description of electricity. I am only trying to get new people to become functional, and this is only the first step of a very long journey.

Most people think I'm weird. I expect a lot of people will think my take on electrons is weird, but it's functional. As far as I know, all these things I said here are true...a bit colorful, but true. If something is seriously wrong with my take on electrons, I expect the moderators of this forum to delete this.

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like your style - thanks for taking the time to write this up

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