The factor I have chosen to investigate
I have decided to investigate how resistance is affected depending on the length of wire.
Control of variables
In order to keep this a fair test I will have to keep the variables constant throughout the experiment. This means that the wire will have to remain the same temperature, have the same cross section and be made of the same material each time. I will use the same equipment throughout the experiment as some of the wires will have different resistance. The power level will remain unaltered otherwise the readings would be useless.
Prediction
The longer the wire, the higher the resistance. This is because the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions (as heat).
Preliminary experiments
Before the real experiment I carried out some preliminary tests to check a few things. I checked how much power would be needed to keep the ammeter below 2 amps. I then did some tests involving the wire I would be testing. I found that if I use thick wire with a high voltage, the wire gets very hot and this would affect the resistance. So I then decided to use a thinner wire with a low voltage. The current was so low I decided to use a mille ammeter as it is more accurate. I had to make sure the current didn’t go above 200 mA at all lengths. If it did it would mean the current is too high.
I fastened the wire to the meter ruler at three different points. I noticed that when I attached the crocodile clips the wire began to coil up. This would mean that the length I would be testing is inaccurate.
Proposed method
(Diagram on the page after this)
To take my first reading of 100cm I will fasten the two ends of the circuit, which I have attached to crocodile clips to, to each end of the wire. I will then read off the readings from the volt meter and mille ammeter. I will go down in 10cm intervals and repeat this three times for each length. After this I will test for ohms using and ohmmeter only twice.
I will try to make sure all the readings are as accurate and precise as possible. I will do this by doing a number of things through out. The first little detail I will look for is that the wire is not coiling up and I have the exact length. The next is for when I am reading off the results, I will wait until the numbers have set and are not swapping between digits. These are the main things I will especially be checking for but there are lots of really small things to do to make the readings as accurate as possible.
Controlling the variables
To control the different variables I will have to do different things. To control the starting temperature I brought in a small thermometer that I can wrap around the wire. For the length I will just make sure that the wire doesn’t coil up and that I put on the clips carefully. The cross section of the wire will not be changed as I am using the same wire throughout this is also the same for the material.
I will only repeat a set of results if the all three times I test each length the readings are completely different.
Safety precautions
- I will make sure that the voltage doesn’t go to high as it my blow the fuse in the multimetre as I found out.
- I will make sure I do not leave the circuit on unnecessarily as it will waste power.
- I will make sure the wire does not get to hot as it could burn someone or set something on fire.
Analysis
From my graph I can see that the length of a wire does affect the resistance. I can tell this because when I doubled the length the resistance more than doubled. We say that the resistance is directly proportional to the length. This means that when we double the length the resistance also doubles. This can be seen by the straight line on the graph.
The results I obtained agree with the prediction I made, in the way that as the length of wire increases so will the resistance.
The line of best fit clearly shows that the results followed the expected pattern very well. The points are very close if not touching the line. This shows how the results were directly proportional through out, as the gradient remained the same.
Evaluation
My experiment may have been slightly inaccurate, as there are a few minor details that I did not check. These include that the wire may not have been the exact length as it is very hard to get it exactly the right length just by looking at it. I could have used a more accurate measure and got it exact every time. Also the metre stick was old and may not have been accurate. Over night 2 of my wires had been taken so I had to replace them and start over. If I hadn’t the wires could have different resistances. All these potential problems could have very slightly affected the results but as it was so insignificant it didn’t matter too much. The resistance of the crocodile clips and wires linking everything together would affect the results as their resistance would have added to the resistance of the nichrome wire. think my experiment went very well as I did not have to repeat any lengths. At first I didn’t know how to set up the circuit but after I figured it out it was fairly easy.