THE ESSENCE OF OHMS LAW

by Jud Williams, Performance Power Technologies

A lot of folks do not have a feeling of what the difference is between voltage and current or, for that matter, resistance. And then there is the term "power". Let's look at these terms and work toward a sense of feeling about them.

Voltage is the hardest of the terms to understand because we have developed misconceptions due to hearing the term misused so often. Voltage is merely stored energy and really does not do anything useful. About all we can do with voltage is measure it. The thing that does all the work is current.

Current is well named because it describes what it really does. Look at the current in a river. It is basically the same concept. As current flows, it is capable of doing work like floating some object down the river or turning a water wheel.

Now, voltage does not do that. It just sits there as a potential. For instance, the water held back by a dam is just like the voltage in a battery. The voltage in the battery is not doing anything until something is hooked to the terminals which will allow current to flow. Then it is the current that does the work.

That's simple, isn't it?

As for power, it relates to how much work the current is doing. Take a light bulb, for instance. They are always rated in Wattage and we know that they get very hot. That heat is caused by the current flowing through the filament. The power is then related to the amount of heat that is generated. The hotter an object gets is generally related to the amount of power consumed.

So now we can presume that if a device were to have current passing through it, it would become warm to some degree, if not hot. If we were to increase the amount of current through a device, it would become hotter. How would we cause the current to increase through the device in question? Well, let's say that we are using one battery to begin with and then add a second battery which would double the voltage. This additional battery causes an increase of current through the device we are working with. Let's see how this happens.

And now, we will get a bit technical. This will explain Ohms law and clear up some mysteries surrounding it. Take a resistor of some value such as 10 Ohms. Note that we capitalize the word Ohm. That's because it is a person's name. But to go on with the story, let's apply 12 volts across the resistor. Knowing these two values we can now calculate the amount of current that would be flowing through the resistor. Ohms law states that current (Amperes) is equal to the voltage, divided by the value of the resistor. Divide 12 Volts by 10 Ohms and we get 1.2 Amperes. Remember, we suggested that if we were to increase the voltage by adding another battery, additional current would flow. So let's add another 12 Volts and see what happens. 24 Volts divided by 10 ohms gives us 2.4 Amperes of current, just twice as much as before. Do you think the resistor will get warmer as a result of this?

Remember that the heat is a result of the power consumed by the resistor. Ohms law for power is current multiplied by Voltage. P equals I x E or "pie" (just an easy way to remember the formula). If the current with 12 Volts (one battery) is

1.2 Amperes, we would calculate the power to be 14.4 Watts (Watt is another persons name just as Volta and Ampere are). Now let's again increase the voltage by adding a second battery which we have already discovered increases the current flow. 24 Volts times 2.4 Amperes gives us 57.6 Watts. That is four times the wattage increase for just twice the voltage increase. Now you know why some devices get so hot so quickly.END

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