Ohm's Law for Noobies...or The Amp Hour Fallacy

Discussion in 'General Electronics Chat' started by #12, May 6, 2012.

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  1. #12

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    Nov 30, 2010
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    Just about once a week, somebody posts a question like, “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 electricity 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. Any properly working electrical device only takes what it needs. If that answers your question, good-bye. If you want to know why, keep reading.

    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.

    1=1
    2=2
    number of jelly beans = 5

    The next level is 3 symbols and an equal sign.

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

    That's as bad as it gets.

    “V” means 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.

    V=IR says, voltage equals current times resistance.
    Sounds stupid at first.
    How can you multiply current times resistance? There are no such things as amp-ohms!

    Trust me. It works. Ohm's Law really says, “current equals voltage divided by resistance”. I=V/R
    12 volts divided by 12 ohms is one amp. (V/R = I)

    A 12 volt battery will push one amp through a 12 ohm radio, no matter if it's eight “D” cells connected in series or a 12 volt fork lift battery. That's because the radio has resistance.

    In fact, any circuit that is working properly has resistance.

    Getting a bit closer now...

    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. That's why all volt meters have two wires. 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” (literally, amp-seconds per second). When an amp flows for an hour, that's an amp-hour. You CAN have a box with amp-hours in it. It's usually called a 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? If your battery has 2.4 amp-hours in it, and you are using half an amp, the useful charge in the battery will last 4.8 hours.


    How to get electrons to do useful work:

    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. They are laying around, all over the place, by the millions. They are in the air, the water, and all throughout the solid mass of the Earth, in numbers too large to easily count. We have plenty of them and we know where to find them. The job called, “electronics” is about how to use electrons to do stuff...useful stuff.

    The first thing to do is to get some electrons in a pile. Maybe that's why the first batteries were called, “piles”? (joke) 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, and the desire to leave is called, “voltage”. 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. 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 scrape electrons off the carpet, 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 reduce the concentration of electrons on the positive terminal, and that's where the electrons want to go...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.

    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.
     
    Last edited by a moderator: May 11, 2012
  2. #12

    Thread Starter Expert

    Nov 30, 2010
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    You can't see electricity, you can't smell it, you can't hear it, and you better not touch it (if there are more than a few volts). All the ways we use our human senses to discover what electricity is doing are really about detecting the effects of electricity. Transformers hum, light bulbs light up, electric stoves smell. These are all clues about what the electricity is doing, but the right way is to use a meter. You can buy a Digital Multimeter for less than $10. I recommend that. You're going to blow up or melt down a couple of them before you get good at this. Even if you don't intend to get good at working with electricity, without a meter you have almost nothing to work with.

    Ohm's Law is about quantities of resistance, current, and voltage. You can use a meter to measure those quantities, then you can use the math (Ohm's Law) to verify that you measured correctly or expose the fact that something is wrong. One of the first rules of troubleshooting is that electricity will either be where it shouldn't be or not be where it should be. How are you going to know if you can't measure it?


    Nonlinear (or LEDs and diodes)

    Ohm's Law works very well with what I call, “basic light bulbs and batteries”. Voltage, resistance, and current, all obeying the law in an orderly manner. The next level includes components with polarity, directional flow. The simplest part in this series is a diode. It's like a flapper valve. Electricity can only flow properly in one direction through a diode. A rectifier diode is built to allow (up to) some maximum amount of current in the forward direction and refuse current in the backwards direction. In the forward direction, a small bit of voltage is required to push the flapper open. Once you get that much voltage applied, the flapper opens like an avalanche. For a common silicon rectifier, seven tenths of a volt will get a hundredth of an amp to flow, eight tenths of a volt will push ten times as much current through it, and nine tenths of a volt will push a hundred times as much current through it. This is important because:

    The most interesting diode is the light emitting diode or, LED.

    Electronic devices don't use light bulbs very much now because LEDs are way more efficient (lumens per watt) and durable. Drop an LED on the floor and it just bounces, no harm, no foul. The important part of “Light Emitting Diode” is, “diode”. LEDs aren't resistors. They don't work like Ohm's Law. An LED that is barely on at three volts will be completely smoked if you apply four volts to it. It has this “breakover” characteristic like a rectifier diode, because it IS a diode. Once you get to the breakover voltage it starts letting current through in an almost unlimited manner. That is why you must use some way to limit the current through an LED. The easiest way is to use a resistor. Say you have a 9 volt battery and an LED that needs about 3 volts to start up. You use Ohm's Law to calculate a resistance that will keep the current under the usual .02 amp limit. Calculating from the fact that you have 6 leftover volts that the LED doesn't need, the math goes like this:

    R = V/I
    R = 6 volts/.02 amps
    R = 300 ohms.

    Any resistor more than 300 ohms will protect that LED from smoking.

    Another way is to use a constant current circuit, but that's getting a bit complicated for beginners. The way it works is that it allows .02 amps to flow and the LED just uses up the amount of voltage it needs. That's the opposite of a radio where you give it some voltage and it only takes the amount of current it needs. The cool part is that an LED gives you some result besides numbers on a meter. They actually DO something. Bill Marsden has posted a lot of LED circuits on this site: LEDs, 555s, Flashers, and Light Chasers. It is also a more comprehensive tutorial on LEDs.

    It won't be long until you have blinking LEDs all over your work bench and you can tell whether the battery in your car is dead or the alternator isn't charging the battery.

    By #12

    _________________________________________________

    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. Besides, it's the theory of electricity. Even if I tried to make it perfect, it wouldn't be. This is nothing more than a practical method of starting to learn how to manipulate electricity.

    If you wish to discuss this please make a new thread on the General Electronics Chat forum, or go to the master document in the "Feedback and Suggestions" forum to suggest changes.
     
    Last edited by a moderator: May 7, 2012
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