Resistance of a Piece of Wire

Planning

In this piece of coursework, I am going to investigate what affects the resistance of a piece of wire. Wire is made up of atoms.

Electric current is a flow of electrons, and it is these electrons that collide with the nucleus of the atoms. Every time this happens, it causes resistance. The build up of friction is what produces the heat.

There are many variables that could affect the resistance of the piece of wire:

• Length

• Potential difference

• Tension

• Temperature

• Diameter

• Shape

• Material of the wire

I have chosen to do length because I think this will give me the biggest range of results. My prediction is-

The longer the wire, the higher the resistance

I think my prediction will be correct because the longer the wire gets, the more atoms there are for the electrons to pass. This means they have more chance of colliding, which will make it harder for the current to flow.

I also think that the length of the wire and the resistance may be proportional and that if the length of the wire doubles, the resistance will also double. This is because if the wire was twice as long it would have twice as many atoms for the electrons to collide with making it twice as hard for the current to flow.

To see if my prediction is correct, I am going to do an experiment.

(diagram)

• The ammeter will be connected in series- current flowing through the wire

• The voltmeter will be connected in parallel- voltage (potential difference) across the wire

• The crocodile clips make sure that only the certain amount of wire needed e.g. 10cm, is connected in the circuit.

I have already planned which measurements I am going to take for the length of the wire. I will start at 10cm, and then keep increasing the length by 10cm until I get to 90cm. This will give me 9 results which should give me a big enough range of results.

I will first set up my experiment (as shown in the diagram above). I will measure the piece of wire and then attach the crocodile clips to the amount of wire needed, changing this length each time. I will switch on the power, and then measure the current and the potential difference. I can use these measurements to calculate the resistance by using the formula:

After I have all the measurements from 10cm up to 90cm, I will repeat the whole experiment again. This will make my results more reliable as I will have 2 sets. If after I have repeated the experiment, all the results are similar I do not have to repeat it a 3rd time because my results will be reliable enough. However, if they are different I will have to repeat the experiment for a 3rd time. If I only have results that are different for 1 length, I will only repeat that 1 length for a 3rd time instead of the whole experiment.

Fair testing

The only thing that I am going to change in this experiment is the length of the wire. I will keep everything else the same.

• I am going to keep the wire made out of the same material. This is because different materials have different resistances and so would affect my results.

The wire that I am going to use is constantan wire (copper/nickel alloy).

• I am going to keep the diameter of the wire the same. This is because different diameters let electrons flow through differently.

• I am going to keep the temperature of the wire the same. If the wire got hotter, the particles would have more energy and so vibrate more. This would increase the resistance because there is more chance of the electrons colliding with the particles.

To keep the temperature of the wire the same I will leave the wire to cool after each one by turning off the power pack.

Preliminary experiment

Before I am going to do the experiment, I am first going to do a preliminary experiment. I will connect a piece of wire into a simple circuit.