the same
The factor I’m going to change is the length. I’m going to keep the cross sectional area and material the same and the temperature as constant as I can.
Fair test
I am going to use identical Nicrome wire throughout the investigation and use the same ammeter and voltmeter. I will also turn the power supply off for the same length of time during each trial.
Prediction
The variable I’m going to change is the length. I predict that when I increase the length the resistance will also increase in proportion to the length.
Resistance occurs when the electrons travelling along the wire collide with atoms in the wire. These collisions slow down the flow of electrons causing resistance.
I believe that the resistance will increase as I increase the length because the longer the wire, the more atoms there will be for the electrons to collide with.
I think they will be proportional because a wire twice the length of another wire would have double the amount of atoms causing resistance.
Trial experiments
I did a preliminary test to find out which lengths of wire would be best to use in this experiment. First of all I tried 10cm. This wasn’t a good length to use because it heated up very quickly and even when the power pack had been switched off for a couple of minutes it was still rather hot. I decided it would take to long to cool down and the high temperature could affect the results by increasing the resistance.
I then tried 20cm which worked well and didn’t get so hot. This will be my starting length.
I then tried 120cm as I thought it would be good to test 10 lengths. A problem with this was it was harder to measure because we were using a meter ruler so I decided that my finishing length would be 100cm.
Plan
Equipment:
- Meter ruler
- Crocodile clips
- Connecting wires
- Ammeter
- Voltmeter
- Power pack
- Nicrome wire
I’m going to set up the circuit below
- I’m going to set the power supply to 6 Volts.
- I’m going to start off with a 20cm wire. I will make sure this wire is pulled taught to get rid of the little kinks and loops.
- I will connect this length of wire to the circuit using crocodile clips as close to 0cm and 20cm as I can.
- I will then read the ammeter and voltmeter readings and record them in a table. I will use a digital ammeter and voltmeter because they won’t be affected if knocked.
- I will then turn off the power pack for 5 minutes.
- I will repeat the last three steps with lengths of wire increasing 10cm in size each time up to 100cm.
I will repeat the experiment three times at each length and take the average. I am doing this so any anomalous results I might have will not show up on my graph.
I have decided on the lengths 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm. This is because they give a good range and are measured easily with a ruler.
Safety
My starting wire is 20cm long because anything smaller than that will heat up to quickly and anyone touching it will be in danger of being burnt. I will also switch off the power pack supply for 5 minutes after I have recorded my results each time because if it’s left on the wire will heat up and I could burn myself.
It also affects my results if it gets to hot because the atoms vibrate faster than they would at room temperature which also increases the resistance.
My formula for working out the resistance is:
Results
Above: A table with three voltage readings and three current readings for each length of wire.
Below: A table with the average voltage and average current readings with the calculated resistance.
Resistance calculated
with the formula:
Analysis of my evidence
From my graph I can clearly see that the resistance of the wire is directly proportional to the length of the wire. I know this because the line of best fit is a straight line which passes through co-ordinates (0,0). This shows that as the length of the wire is increased the resistance in the wire also increases.
The graph shows a very strong positive correlation.
I predicted that as the length is increased the resistance will also increase. My graph shows my prediction is correct as my line of best fit is straight, passing through (0,0) proving that the wire length is proportional to the resistance in the wire.
The reason for this is briefly explained in my prediction;
In a wire there are free moving electrons and fixed atoms. Electricity is the movement of these electrons through the wire. Resistance is created when the electrons collide with atoms which do not release more electrons so slows down the flow of the current.
This means for these materials the more atoms, a larger number of collisions which will increase the resistance in the wire. If a length of wire contains a certain amount of electrons, then when the length is increased the number of atoms will also increase. This is shown in my diagrams below.
(Current ‘energy’ absorbed by more atoms)
Evaluation
The resistance for 20cm is the only result that isn’t really on the line of best fit. I think this could be because my first reading for this result has quite a big difference from the other two tries compared with all my other results.
I think the method I chose for this investigation was a good one as most of my results are very close together showing consistency and all are grouped very closely around the line of best fit.
Although I quickly switched off the power pack after recording my results to prevent the wire from heating up and affecting my results if I did the experiment again I would leave it for 10 minutes and not 5 because then there would be no chance of the temperature affecting my results as I probably got some slight errors.
I could also do more than three tries and get a more accurate average.
I would do this experiment at 2 Volts and 4Volts as well to make sue my results were the same for every voltage.
I could also try different lengths up to 200cm to see the length of the wire continues to be directly proportional.
