Motorhome and Caravan News Australia

Presented here, in a very easy to understand format, is basic electrical theory. This is the stuff that I learned in my first year as an apprentice electrician (and that was just after they invented digital watches). This basic knowledge is an absolutely essential grounding if you are to have any hope of installing, repairing or even understanding your own motorhome or caravan electrical system.

Water and Electricity

Electricity flowing down a length of wire acts almost exactly like water flowing down a garden hose. It is really helpful to explain new terms by relating them to concepts you already understand (like water pressure for example) – for this reason we will use the similarities between the way water flows and electricity flows to explain some of the theory.

Voltage – what is voltage?

“Voltage is pressure”

Electricity all revolves around some very tiny things called electrons. When we move lots of additional electrons into a piece of metal, these electrons get pushed together. Electrons all have the same negative charge, so they tend to repel each other. This is very much like a lot of tiny magnets with all the South poles facing each other.

The more electrons we cram together the more they want to repel each other and the greater the pressure they force upon each other. This pressure is measured in units called volts.

So what about the water analogy? Volts is like water under pressure. More volts, more pressure! Imagine you have this huge 100,000 litre tank of water on top of a hill, the higher you fill it, the more the tank weighs, and the more pressure is available at the bottom of the tank. The weight of the water makes the water want to get out of the tank. More water and weight gives the water more pressure.

With water, we measure pressure in terms of its weight in kilopascals or Kpa . With electricity we measure the pressure in terms of Volts.

So the terms Kpa and Volts are units of measurement for water and electricity respectively.

So the answer to the question “what is a volt?” can simply be answered like this: Volts in a battery is like water pressure in a tank. The more pressure (volts) you have, the easier you will be able to get it to move through even the smallest hose (wire).

So Volts is pressure – that’s easy to understand, so then, what is an Amp?

Amps of current

If a volt is the pressure of electricity, then the ampere (amp for short) is the flow of electricity. That should be really easy to remember.  Electricity flows through a wire in much the same way that current flows down a river. But for the sake of this discussion we are going to stick with talking about how water flows down a hose. It is easier and more meaningful to equate a garden hose to a piece of wire.

Let’s get back to our tank and let’s place a valve at the bottom of the tank so that we can let water out and empty it all the way if we want to. Connected to this valve will be a hose. The hose will simply dump the water into the ground.

The shut off valve on the tank acts like an electrical switch. It either stops the water current from flowing or it allows it to pass. An electrical switch does the same thing to the flow of electrons. When the switch is on, we have current flowing when we shut it off, we have no current.

That was too simple, wasn‘t it?

Ok, so when are you going to get to what an amp is? Well, here it is….

The rate of water that flows down a hose is similar to the rate of electrons that flow down a wire. We might measure the flow in a garden hose in terms of litres per hour or millilitres per minute. However we measure current in terms of electrons per second.

Actually one amp is technically defined as 62,420,000,000 electrons going past a point of wire per second (more or less (aren’t you glad we have meters that save us from having to count electrons by hand?)).

More amps mean more electrons per second. Fewer amps mean less flow.

So how do you get more amps you ask? Well I am glad you asked that…

Well, there are two ways. First, if you have more volts (remember volts = pressure) you will provide more motivation for the little electrons to zip down the wire in a hurry. This is just like putting more water into our holding tank, giving us more weigh, and therefore giving us more pressure. The flow of water or the amps of current will be greater with more water pressure or voltage.

The second way to get more current with water, without increasing the pressure, is simply to get a bigger hose. This is one reason why bigger wires can carry more current. Put a fire hose and a bigger valve on the water tank and more litres per minute will flow.

Now here is an interesting concept… Put a tap on that water tank and now you can reduce how many litres per hour will flow even though you may still have a big fire hose. This is why the size of wire you have can limit the maximum current you can deliver.

Yes, you can use a pair of automotive jumper cables to power a cell phone but people may think that is quite a waste of wire.

Jump starting a car requires many – many amps of current so that is why the wires are big for that application.

The bigger the wires are, the greater the ability they will have to carry a high flow of current.

And, just like forcing water down a long skinny hose, lots of Kpa (pressure) at the tank side can turn into very little Kpa dribbling out the end.

A thin wire can restrict flow and reduce the apparent pressure. That is why in high current applications we want as big a wire as practical.

So getting back to the tap, we now have this way of reducing the flow of current by simply turning the tap. As we all know, a tap merely makes the opening for the water current smaller and smaller offering more resistance until it is fully shut off. Turning a tap completely off is the ultimate resistance.

So what is the analogy to this tap resistance in terms of electricity? Well, this one is easy, the scientific electronics term is called resistance, yes resistance!

Resistance to electricity flowing is measured in a unit called Ohm. An ohm is a term that you have to understand when you start talking about amps. Why? Because Ohms and amps are inversely related!

So what on earth does that mean?

Simply that when you have more Ohms (resistance) you have less Amps (current flow).  This is like when you turn down the flow of water with the tap! Also, when you have less Ohms (resistance) you get more amps. This is like opening up the tap and/or using a big fire hose! (This whole discussion, of course, assumes that the volts (pressure) remain constant.)

OK so we now know that Ohms and Amps affect each other in opposite directions. On the other hand if you leave Ohms the same and you change the volts (pressure) up and down, you will find that more volts (pressure) means more amps (current flow) and less volts (pressure) means less amps (current flow).

Now if for some reason the discussion above was confusing to you in any way, it is simply because we are using new words to describe concepts that you no doubt already understand (pressure, flow etc). Take the time to re-read the paragraph above until you get it. It’s not really hard to understand, plus you will have mastered the fundamentals of electricity when it becomes clear to you.

Quick Review:

Volt = unit of measurement of electrical pressure.  (Volts is like Kpa of water pressure)

Amp = unit of measurement of the rate of electrical current flow in a conductor. (Amps is like l/hr of water flow).

Ohm = unit of measurement of a conductors ability to resist current flow. (Ohms is like resistance to the flow of water – or how much the tap is turned).

A tiny bit of math

• Volts (V) are equal to the number of Amps multiplied by the number of Ohms
• Amps (I)  are equal to the number of Volts divided by the number of Ohms
• Ohms (R) are equal to the number of Volts divided by the number of Amps

The letters in brackets are the letters used in formulas to represent that item – eg I (as in India) is the letter that represents amps of current. So we could express the same three formulas like this

• V = I x R
• I = V / R
• R = V / I

The above formulas collectively are called “Ohms Law” and they describe the basics of all electricity and electronics.

This is the end of part one. Don’t tell anyone, but you are half way to understanding the basics of electrics. Really it is just new words for stuff you already understood.

In the next section we will look at what is a watt – all solar panels are rated in watts … so how does a watt fit into all this? We will also look at batteries – series or parallel?

Writing stuff like this stakes time – don’t get me wrong, I enjoy writing it.  But I really enjoy getting feedback too – if you like this article (or even if you don’t) I’d like to hear your opinion. Why not leave a comment in the “Leave a Reply” box below.

Part two of this tutorial can be found here.

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This entry was posted on Thursday, July 1st, 2010 at 3:24 pm and is filed under Caravan, Electrics, Motorhome, Power, Solar. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.