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Yet to be completed I would like beginners to comment on the level.
INTRODUCTION
This guide is not intended to make you into electricians. It is about being able to understand what electricians tell you, to enable you to read a circuit diagram and to get you to the stage where you can do simple calculations. It has been aimed at those with very little knowledge of science or electricity and where possible has avoided stuff you don't need to know (yet). If you already know about electricity you will recognise that to make the explanations easier I have lied a little here and there. Although I have tried to keep it simple you will be challenged to think about it, no spoon feeding here
SAFETY
Electricity can be dangerous there is no point at the moment telling an absolute beginner the whys and hows. So for the moment don't touch anything!
There is a separate safety section at the end where you will be more equipped to understand it.
DIAGRAMS and SYMBOLS
I intend to make use of diagrams, don't worry quite yet about what the following items are or do (or indeed if the symbol I use conforms to any particular standard). If I need any more I'll explain them later.
A SIMPLE CIRCUIT
Here is the simplest circuit I could think of. Its the circuit of a torch, take a few moments to identify the components from the symbols diagram:
Not too surprisingly when the switch is closed the lamp lights. So what is happening? Well a flow of electrical current (Amps), shown by the red arrows, is going from the battery though the wires, switch and lamp and back to the battery. It is this current that heats up the lamp and makes the light. Note that the diagram doesn't alter if, for example, one of the wires is actually the casing of the torch or is a metal strip rather than a wire.
First important point: the same amount of current that leaves the battery goes back into it. If you could measure the current anywhere in this circuit it would be the same, it is not used up.
Hang on batteries go flat I can hear you say. To that I reply yes you are correct the chemical processes inside the battery get used up and eventually there will be no current. However in the circuit all the current is the same. Current (which is referred to in formula as 'I' - yes I know!) is measured in Amps (A)
This is an example of a series circuit, as all the current goes through each component one after another.
Second important point: Why does the current circulate? Simple answer it is pushed by the voltage difference between the two terminals of the battery. Voltage (V) is measured in Volts (V)
Third important point: Why doesn't all the current come out in one go? Answer the circuit has resistance to current flow. Some of that resistance is in the wires some in the switch but most is in the lamp. Resistance (R) is measured in Ohms. (Ω)
YOUR FIRST SUMS
Current, Voltage and Resistance are interrelated by a simple formula. Voltage divided by resistance equals current.
So in the torch example if it was a 3V battery and the lamp was 30 Ohms then the current would be 'about' a tenth of an amp. Why 'about'? Well remember I told you there is also resistance in the wires and switch but we will for the purposes of calculation ignore those from here on in.
It is such a basic building block of electrical knowledge that it is at the heart of most electrical theory and calculations.
A SLIGHTLY MORE COMPLICATED CIRCUIT
This is still a series circuit ie all the current passes through all the components but I have done away with the really useful components ie the switch and lamp and have substituted two resistors. Now resistors can be just thought of as things that restrict the flow of current. This circuit doesn't do anything except flatten your battery but I'm using it to illustrate the principle and cement Ohms law in place. You will find out that this circuit illustrates a very useful concept in fault finding so read on.
Although we won't cover how to use a meter in this guide there is one point further to cover about voltages. Although you might refer to a car battery as a 12V battery what you are really saying is that there is 12 volts (nominally) between the positive and negative plates ie if you say that the positive terminal is 12V then it follows that the negative terminal is zero volts. Its a sort of convention that you refer all the voltages in a dc circuit to the negative terminal of the battery unless otherwise stated. This will become clearer in this example
So if we say that point A is 12V then point B is 0V and my question will be what is the voltage at point C?
You can't do any sums unless you know some more facts, so lets say the current is the same 0.1 A that we had in the earlier example. Use of Ohms law (this time voltage divided by current will give you resistance) gives an answer of 120 Ohms but that isn't what we were looking for and shows the futility of just blindly putting numbers into equations 120 Ohms is the resistance of the complete circuit but tells us nothing yet about the voltage at point C.
Lets say though we know that the resistance of the bottom resistor is 100 Ohms. Now we have a value of resistance and a value of the current (remember the current is the same everywhere in a series circuit) and applying Ohms law (this time Current times Resistance) will give an answer of 10V.
This is so fundamental to the understanding of electricity that we need to pause to make sure we all agree on the values calculated and learn that in a series circuit the current is the same but the voltages around the circuit differ. Ok now if its only 10V at point C what has happened to the missing 2V? Well it has been dropped by the other resistor. If you do the sum 2V divided by 0.1A you get 20 Ohms. This shows another characteristic of a series circuit: resistances just add up. ie 20Ohms and 100 Ohms equals the 120 Ohms that we calculated earlier.
SUMMARY SO FAR
In a series circuit the current is the same throughout but the voltages vary.
Ohms law enables us to do simple calculations about what is happening
In a series circuit the total resistance is the sum of all the individual resistances.
to be concluded
Last edited by sallytrafic on Mon Mar 01, 2010 8:51 am; edited 5 times in total ______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
Just a friendly comment, trying to view the reading of this from a novice's point of view.
Your first use of the word 'current' could be interpreted by the novice as 'flow' of electricity, without distinguishing between volts and amps.
Later it is necessary to use 'current' to describe the amperage.
Maybe it would be helpful to differentiate the two uses of the word 'current' earlier in the tutorial.
I take it you have decided to reject the popular analogies with Hydrology (flow, volume, pipe sizes, restrictions (resistors) etc.). That sort of explanation does require an assumption of the basics of Physics, which you are trying to avoid.
On a VERY positive note - I was struggling with land law until I found a book with DIAGRAMS about the relationship between Freeholder - Leasee _Coveantor etc.
I've amended it taking into account the comments so far. This is only the beginning so I need to get it right (level and approach) before doing the rest.
PS I haven't used the water analogy because at some point the analogy breaks down or becomes less useful (normally for me when considering pressure through a narrow opening which can be counter intuitive)
______________________________________________________________ Regards Frank - - please follow me on twitter @FrankieBryant
Denn wir haben nichts in die Welt gebracht; darum offenbar ist, wir werden auch nichts hinausbringen.
RIP Peter and Fiona Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray
Plusnet, Safari 5, G5 PowerPC iMac running OSX 10.5.8, Salisbury UK : Get behind early - it gives you more time to catch up.
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France
Our thoughts are with you Chris and Graham. Fighting with Mavis and Ray 
