The aim of this instcutable is to give a crash course in high school physics, by explaining the basics of electronics hopefully it should make it alot easier for beginners to undertake instructables that they never thought they knew how to before.

as a quick start you will need to know :
voltage is measured in volts (V)
and current in amps or amperes (A)

now, Circuits.

Step 1: Current and Voltage Fundamentals

Voltage changes throughout a circuit and differs across components. voltage is measured by putting a voltmeter in parallel across a component as pictured below, this is because a voltmeter has infinite resistance and if put in series would break the circuit stopping the flow of electricity.

current (amps) is constant throughout a series circuit and splits in a parallel circuit evenly.
current is measures using an ammeter in series and the placement of the ammeter does not matter as the current is the same anywhere in a circuit. (pictured below)

Step 2: Series Circuits

OK, you should know that electricity needs a completed circuit to flow, or a loop if you will.
however it doesn't stop there, there are two types of circuit you will need to know and those are Series and Parallel.

A Series circuit (pictured below) operates like a race track, one continuous route for the flow of electricity to take.
the images below show a simple circuit where a battery is connected to two lamps, this is a series circuit as the two lamps are connected with inverse polarity. (positive to negative).
in series the voltage is split between components, for example say in the image below each lamp consumes 2.5v and the power source (battery) is 5v then the total voltage consumed in the lamps would be 5v which is equal to the source and both lamps would function correctly.
however say the power source is 6v then this will be split between the two lamps (assuming they are the same) equally meaning each one received 3v, this is too much for the lamps and will cause them to light brighter and greatly reduce their life span.

this is why we use resistors, to "absorb" some voltage (and current) to protect components.

now if the components are different then the voltage is distributed accordingly.

say i have a blue led and a white led, these are connected to a 5v source in series
the blue led runs on 3v and the white on 2v.
this scenario would be fine as both the components voltage requirements are the same as the source and there would be 3v across the blue led and 2v across the white led.

the last thing you need to know is that when in series, if the circuit is broken, all components will not receive power.
so our series circuit is a race track and some rude person just dumped a huge barrier right in the middle of it. this mean no more cars can drive all the way around the race track.
similarly if say a bulb blows (pictured below) no more electricity can flow through the circuit and the path for the electricity is broken resulting in both lamps failing to light.

i will go into further detail on resistors later, but next up is a parallel circuit.

Step 3: Parallel Circuits

in a parallel circuit components are connected with common polarities (positive to positive and negative to negative)
you can see this in the image below.
unlike a series circuit however in parallel the voltage is not shared through the components, in fact each component receives the same voltage as the source.
However in a parallel circuit

considering the image below each lamp runs on 1.5v and you want to run 2 off of a AA battery (1.5v) but in series each would consume only 0.75v and neither would glow very bright if at all.
so you use a parallel circuit, Both lamps now receive 1.5v and glow at their desire brightness.

Now if say each lamp needed 0.6 amps to light but the battery was only rated for 1.5v at 1amp then each lamp would only be receiving 0.5 amps as the current is split equally between the circuits.

so remember:
in series voltage is distributed through out the components and the voltage across everything in the circuit always adds up to the supply voltage.
in parallel voltage is the same throughout the parralel components but current is split.

One thing to remember is that you can consider one bridge of a parallel circuit a mini series circuit.
meaning that if say for instance the power source was 6v and i have 4 lamps that all ran at 3v i could connect them in parallel but in pairs, this results in each bridge of the circuit receiving the full 6v which is then distributed throughout the lamps as 1.5v each.

lastly, in a parallel circuit is a component in one bridge breaks and stops the flow of electricity throughout that bridge the other bridge will still operate, this is illustrated by the image below.

ok so now we know the basics of circuits lets move onto resistors.

Step 4: Resistors

resistance is measure in Ohms ( Ω )
Resistors, as the name suggest resist the flow of electricity.
example:
i have an led that runs on 3v at 20 milli amps (0.02A)
but my battery outputs 5v at 2 amps, if i connected the led straight to the battery it would definitely be VERY bright but get hot incredibly fast and burn out . A resistor can be used to limit the voltage and current going through the led and protect it ultimately extending its life span.

to work out the resistor i need i will use this formulae

V=IR (Voltage = Current x Resistance)
but i dont know resistance so i rearange this equasion to get
R = V / I

important! V is the voltage that needs to be dissipated across the resistor. we figure this out by taking the voltage that the led runs on away from the source voltage

so using the example above that's

5 - 3 = 2 V

Now we have voltage we need current, this is simple. its the current the led runs on. as per the example above thats 20MA

so the final equasion is

R = 2 / 0.02
R = 100 Ω

Note: most leds come with their current and voltage ratings on the packet.

in series the resistance of all the resistors add up. so say i have a lamp in a circuit that is currently running off of to high a voltage, so i need to add say... 130 ohms of resistance to the negative side of the circuit to protect the lamp. but i only have a collection of 50Ω and 30Ω resistors.
well i can put all of these in series and make the values add up to 130Ω
so i use 2 x 50Ω and 1 x 30Ω
50 +50 +30 = 130Ω

however if you put resistors in parallel the value of resistors decrease the more you add.
If the value of the resistors you are putting in parallel are all the same then the equation is
Value of one resistor / Number of resistors
so 2 50Ω resistors in parallel would be
50 / 2 = 25Ω
But if the values of the resistors are the same then this equation must be used

1/RT = 1/R1 + 1/R2 + 1/R3 ect.
Rt being the total resistance and r1,2,3 ect being resistors

ok so say i have a 10ohm, 100ohm and 30ohm resistors in parallel. (in series these would give a total resistance of 140ohms).
1/10 + 1/100 + 1/30 =1/rt
0.1433= 1/rt so 1/0.1433 = rt
rt = 7ohms (rounded)

Step 5: Lastly

I hope this instructable made sense, if not please do leave a comment stating what bit confused you and why and i will change it accordingly.

If this all did make sense please have a read of my breadboard instructable

Thanks alot guys, all comment and ratings appreciated, constructive or not.