We have discussed very briefly what electric current is and how it works. Now we will move to adapting the theory to more practical applications. We will discuss conductors, resistors and insulators, and how electrical properties are measured. We will limit the scope in this section to DC applications only. In dealing with electricity, there some terms that you must know and the relationship between them. If you understand these basics you will be able to analyze what is happening in a circuit when you are trying to troubleshoot it.
E = Electromotive force, also called Voltage, measured in Volts.
I = The flow rate of electricity, also called Current, measured in Amperes.
R = Resistance, the opposition to current flow, measured in Ohms.
P = Power, the quantity of electric energy, measured in Watts.
To illustrate the relationships, it is often easier to visualize if you compare it to water. For example, if you had a dam and at the bottom you drilled a hole through it and put a pipe in it. If the pipe was capped you would have pressure on the pipe relative to the height of the column of water above it. This pressure in water is measured in PSI (pounds per square inch). Because the pipe is capped and there is no flow this is "potential energy". This corresponds to electromotive force (EMF) or Voltage, with the symbol V.
Next, let's uncap the pipe. We now have water flowing through it. This flow is measured in Gallons Per Minute (GPM). This corresponds to current or the flow of electrons measured in Amperes, with the symbol I.
Now, let's look at the size of the pipe. The larger the pipe the less resistance to the flow, so the more Gallons Per Hour (GPH). The longer the pipe, the more friction on the walls and the less water flows through it. So, both the size and the length of the pipe affect the flow rate (GPH). This corresponds to Resistance, measured in Ohms. The symbol for Ohms is the capital Omega symbol ,W. The longer the wire or the smaller its size, the higher its resistance to the flow of current.
If we want greater a volume of water delivered in a given time, we can do two things. One, we could raise the dam so the water column would be higher and the pressure would increase. Increased pressure would push more water through the pipe. The second option would to use a larger diameter pipe at the original level and by reducing the resistance we could increase the flow, or GPH. Electricity works in the same way. To increase the electrical current (measured in amperes), we can increase the electromotive force (measured in volts) or decrease the resistance (measured in ohms).
Next we need to look at Power. If we hook the pipe to a hydraulic motor connected to a load, the amount of work that can be extracted from the motor (horsepower) is relative to the pressure PSI on the water and the rate of flow GPH. This corresponds to electric power, measured in Watts (W) or in the case of our homes, Kilowatt Hours, or KWH. Kilowatt hours equal Watts X Hours X 1000. Horsepower is often expressed in Kilowatts, particularly in Europe. One often sees engines rated as so many kilowatts [KW]. One horsepower is equal to 746 watts of electricity. A 3KW engine would be about the same as 4HP.
A corresponding example would be a tractor you need to start. You have a 12v battery and you use a small wire and hook it to the starter. Because the small wire has a lot of resistance, most of the energy will be lost in the wire and the engine won't crank because not enough energy is getting to the starter. The wire will possibly burn because most of the energy is lost in the wire. Now if we take a large cable and hook it up, it will have a very low resistance and most of the energy will get to the starter and crank the engine. This works just like the small pipe and the large pipe, and the same with the length of the pipe. See, it is not that mysterious! It is just that we can see the water and we can't see the building blocks of atoms moving, so it is easier to comprehend examples using water.
In the next section, we are going to explore some basic circuits and some laws that govern what happens.