1. Resistance Physics – 12.1.5 Series and parallel resistor circuits
Understand how the resistance of components add up in a circuit
Summate resistor networks by applying the formulae in various situations

2. GCSE Background From GCSE physics we already know that; V = IR
So simply in this circuit the resistance of the bulb is 12. This is a measure of how much the bulb resists the flow of electrons.
We should also have some idea of these graphs which show a “characteristic” behaviour of different conductors when we graph at V & I

3. Circuit Rules
For resistance calculations at AS you can use the premise of the following rules;

4. Series Resistance

5. Parallel

6. Generalised Formulae
So all this leads us to a generalised formulae for any number of resistance.
In Series resistance is simple but for Parallel resistances we must use the reciprocal.
NB: when multiplying out the second formula make sure you treat all terms equally!

7. Test it out….
If all the bulbs are of equal resistance answer the following;
Find all the meter readings
What is the total resistance of the bottom branch
If I swapped all three bulbs for one bulb what resistance should it be so that the current flow in the main branch is still 3A (i.e. combine all resistances using formulae)