The aim of this experiment is to find how resistance changes in a circuit containing bulbs in series or parallel and how they are related.

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Physics Coursework                                                                        James Taylor 11MA

An Investigation of Resistors

In Series and Parallel


The aim of this experiment is to find how resistance changes in a circuit containing bulbs in series or parallel and how they are related.

For this experiment I will need knowledge of electricity and physics. A famous physicist Ohm made an important discovery. Known as Ohms law, from his discovery he stated that: ‘the current flowing through a conductor is directly proportional to the potential difference across its ends provided that the temperature is constant’. In equation form this is: R = V/I. From his discovery we are know able to work out the resistance of a circuit from knowing the voltage across the circuit and the current flowing (amps). After his discovery the units with which we measure resistance were named after him – Ohms, the symbol for which is the Greek letter omega (Ω).


In this experiment to investigate the resistance of light bulbs in series and parallel there are a number of variables, which I could change or keep the same. The dependent variable, which I will change is the voltage across the circuit, I am able to change this using a variable resistor.   

I this experiment to find out the resistance of light bulbs in a circuit there are a few things, which I have to consider to do it safely.  As my experiment involves the use of electricity I have to be careful, as it can be deadly when misused. I must take care when connecting wires, and make sure that the mains are off when I am connecting them. Also as I will be using light bulbs, which can get very hot when left on for long periods of time so I must be careful not to burn myself.


The equipment that we will use for this experiment is:

  • 1 Power Pack
  • 3 Filament Lamps (Bulbs)
  • 1 Rheostat
  • 1 Ammeter
  • 1 Voltmeter
  • A collection of wires

Diagram – Series

Diagram – Parallel


I predict that as the current is increased the voltage across the bulbs will increase. I am expecting that with the circuit with the bulbs in series, as I increase the current, then the overall resistance will increase. This is because resistors in series add together, because the current must go through one resistor, then the other, losing more power each time. Therefore I predict that as the number of resistors doubles, the total resistance will double. When resistors are in parallel the current in the main circuit divides, part passing through each resistor, I predict that as I double the amount of resistors, then the total resistance will halve, because the current has to split between the two resistors, therefore losing less power.

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For resistors in series, the total resistance, RT, is given by:

RT = R1 + R2

This can be proved by the fact that when resistors are joined in series the same current I flows through them all. The potential differences across individual resistors are:

V1 = IR1        V2 = IR2        V3 = IR3

The total drop in potential V = IR, were R is the total equivalent resistance.

Since V = V1 + V2  + V3

         IR = IR1 +  IR2 + IR3


RT = R1 + R2

For resistors in parallel, the total resistance is given by:


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