Before I start my experiments I will do a test run to try out my experiment, this will help me to find any problems and improved on my experiment. When doing this I found that for both experiments the voltage has to be kept low, at 2V dc from the power pack, because when the voltage is to high the wire melts. I will use copper wire in both experiments to ensure a fair test. Both experiments will be done twice and then I will work out an average, so I will have more accurate results.
Then using my results I can work out the resistance (R = V/I).
Also I have to take safety precautions:
- Before I make any changes to the wiring in my circuits I will switch of the power pack.
Experiment 1
Metals conduct electricity because they have a free flow of electrons that have a negative charge. Resistance is caused when these electrons flowing towards the positive terminal have to flow against the atoms of the wire. So if we double the length of a wire, the current (flow of electrons) has to be pushed between twice as may atoms in the wire, so doubling the length you will also double the resistance. This is ohms law, that the voltage is proportional to the current in the resistance of a wire (if temperature is kept constant). I predict that the longer the wire the more resistance it will have.
Experiment 2
The thinner the wire is there is less space for the electrons in the current to flow, so the resistance will be higher. So if the area of the wire doubles so does the space for the current to flow, therefore resistance will half. So I predict the thicker the wire the more resistance it will have.
You can measure the resistance of a wire by using the formula:
Resistance = Voltage/Current.
Experiment 1
Experiment 2
Experiment One
The points on my graph of experiment one are a little scattered, but the line of best fit is a straight line which does go through the origin, which means the resistance is directly proportional to the length. So if the length is 50cm, and resistance is 1ohms, when the length doubles to 100cm, the resistance also roughly doubles to 2.01ohms. There was also a few anomalous points, 0.1ohms and 1.65ohms. I probably did not read the results accurate. My graph proves ohms law, that the voltage is proportional to the current (if temperature is kept constant). This agrees with my original hypothesis that doubling the length of wire ought to make it twice as difficult to push electrons through.
Experiment Two
The graph of experiment two is an inversely proportional curve. This is because the resistance is directly proportional, this means when the thickness of the wire doubles, the resistance will halve. For example when the diameter of the wire is 0.36mm the resistance is 0.13ohms. When the diameter roughly doubles to 0.73mm, the resistance roughly halves to 0.07ohms. This is because when the area of the wire doubles, so does the number of possible routes for the current to flow down. This also agrees with my original hypothesis that the thinner the wire is there is less space for the electrons in the current to flow, so the resistance will be higher.
Experiment One
My results are not completely accurate for many reasons:
- The temperature of the wire will rise which will affect the resistance. The temperature will increase because when there is a current flowing through a resistance, this will cause the atoms in the wire to vibrate, stopping the flow of electrons therefore the resistance will increase. I could of over came this by putting the wire in a beaker of cold water.
- When measuring the lengths of the wire the length might not be accurate as the wire might not be completely straight.
- There could have been a different thickness in the length of wire.
Even though of all these problems my results are still accurate enough to support my prediction. If I did this experiment again, I would use newer, with a more sensitive range ammeters and voltmeters, a more accurate way of measuring, and I would take a wider range of readings, and more readings so I could get a more accurate average. I would also investigate other factors such as, temperature, voltage and type of wire, to see how these affect the resistance.
Experiment Two
My experiment is not completely accurate for many reasons:
- When measuring the lengths of the wire, it is difficult to get an accurate reading of length because the ruler may not be accurate.
- The wire might not be completely straight.
- It may also be of different thickness throughout the length.
Because of these problems, and that I did not have enough results so my experiment was not accurate enough to completely support my prediction. If I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings, and more readings so that a more accurate average can be taken. I would also investigate other factors, such as temperature, voltage and current, and see how these affect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance. But my graph is unclear and hard to see if it is straight line or curve. It would be better if I had more readings, and more points on the graph so I can see a pattern to support or disapprove my prediction. My results do not clearly support my prediction but it does show that the thicker the wire the less resistance there is.
To help me with this investigation I used the following:
- Complete Physics by Stephen Dople
- Letts GCSE Physics
- CGP Physics by Richard Parsons
- Encyclopaedia – Britannica 99 CD-rom
