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Voltage – By definition, voltage is a measure of electro-motive pressure. But what does that mean to us? I’ve often used plumbing references to illustrate the flow of electricity in a circuit as it seems to be easier to visually grasp the concepts involved. Describing electrical voltage would be like the water pressure in a plumbing pipe. It is the force needed to push the water through our pipe, just as the voltage is the pressure that pushes electrons through a wire. Voltage is the measurement of potential difference between any two points and is measured in units called Volts. As an example, typical flashlight batteries are rated at 1.5 volt, however, if you stack 2 batteries end to end the voltage is now 3 volts from end to end, but still only 1.5 volt from either end to the middle. Also, voltage does not indicate the quantity of electricity, only the potential difference of electro-motive force available. It only describes the electrical pressure needed to create electron flow, but it does not mean there is electricity flowing. To describe the quantity of electricity flow we have another term.
Current – Current is a measure of the total electron flow over a certain amount of time. Current is measured in units of Amperes or Amps. If we continue with the plumbing analogy, the current would be the amount of water flowing through the pipe or the number of gallons per minute for example. Almost intuitively, we know that if there was no water pressure in our pipe, there would be no water flowing through it. We need pressure (Volts) to push the water (Amps) through the pipe. In electrical circuits that is voltage! So, we need a voltage difference to create the force to get things moving and therefore creating the current flow of electrons through a circuit.
Conductors and Insulators -Now we need something for the electrons to flow through, the pipes in our plumbing analogy. As we have already discussed, atoms have electrons orbiting around their nucleus. Different materials have different numbers of electrons and behave differently when asked to either give or take any electrons away. Materials such as copper, gold, and silver can very easily share electrons back and forth and therefore make very good electron carriers. These are called Conductors. Meanwhile, materials such as rubber or plastic do not like to share their electrons and are therefore very good at blocking any electron flow. These are called Insulators. So, in electricity, our “pipes” or conductors have usually insulated wires. These wires are usually made up of a core of a good conductor, like copper, and an outer layer of insulation, like plastic, that keeps the electrons from spilling out onto another conductor. Air can also be an insulator. Most High Voltage power transmission line are uninsulated metal cables, but they are suspended far above the ground and kept far apart from each other so the air between them act as the insulator.
One thing to note is that no conductor is a perfect conductor. Though usually very small, there is always some resistance to current flow. In any wire carrying a current, there will always be some internal friction. This friction is what can make a wire overheat or melt if we try to force too much current through a wire that is too small to carry the current. This is why, for example, a high-current device like a starter has a much larger conductor or wire connected to it than you would use for a low current device like a tail lamp. The “pipes” need to be larger to accommodate the larger flow. Just keep in mind that in any operating circuit there are small voltage and current losses from the circuit conductors themselves.
Source – These electrons must come from somewhere, like a battery or a generator. We call this a Source. A Source can be any device providing voltage and current. On the vehicles we work on our source is usually a battery. Very simply put, a battery is a device driven by an internal chemical reaction that produces an abundance of extra electrons on one of the battery’s terminals and a shortage of electrons on the other battery terminal. This creates a charge difference potential (volts) to cause electricity (electron flow) but they will only move if we allow the flow of electrons from the abundant side to the shortage side. This is what we need a conductor for.
Now, we could connect a conductor directly from a battery’s positive terminal to the battery’s negative terminal. That would actually be a complete circuit. However, since the conductor is typically very low resistance, connecting the two directors would cause an unrestricted flow of electrons which not only could be very dangerous but would not accomplish anything except, maybe creating a fiery spectacle as anyone who has ever accidentally dropped a tool across both battery terminals knows. (Don’t ask me how I know.)
Normally we want the flow of electrons to do some work for us before we allow them to return back to the battery. A headlamp, a fan motor, a fuel pump, etc. or any other device we need to operate could be placed in between our two battery posts. These are the components we are trying to operate. Usually, they convert electrical energy to some different form of energy; like heat, light, or mechanical force. These are called Loads and are the whole purpose of the electrical circuits. We want the electrons to do work for us.
Let’s review, so far in our circuit, we have a Source of EMF Voltage and Current (the battery), a Conductor to carry the electrons from the source and back again, and we have a Load doing work for us. Every load device places some sort of restriction on the electron flow through the circuit. This restriction of electron flow we call Resistance.
Resistance – Resistance is any opposition or restriction of current flow. If a circuit had no resistance electrons would flow unrestricted until the conductor overheated and failed. Also, the component or Load ideally is the only resistance in the circuit and therefore the sole factor in determining circuit amperage. The Resistance of the component being operated will be what determines how much current (Amps) the circuit will draw. You cannot force more Amps into a circuit and a component will not draw more current than its Resistance allows. (Unless you applied more pressure/voltage.)
Alright, we have covered quite a bit of electrical theory and the electrical terms commonly used. Here is a quick review:
Electricity – the flow of electrons.
Electrons – atomic particles with a negative charge.
Voltage – Electro-Motive Force (EMF) the moving force or push.
Current – electron flow over a period of time, the quantity of electricity.
Resistance – a restriction or opposition to electron flow.
Conductor – material that can easily carry electron flow.
Insulator – material that prevents electron flow.
Source – what provides the voltage/current. (Battery, Generator, Solar Cell, etc.)
Load – the circuit device doing work. (Lamp, Motor, Solenoid, etc.)
That’s probably enough for this week’s lesson. Thanks for reading and be sure to check back, as our next lesson will cover a couple more electrical terms and then we’ll start looking at circuits and circuit problems.