Apparatus
100cm of constantan wire
Ammeter
Voltmeter
Battery
Crocodile clips
Diagram
Method
1. The apparatus was set up as in the diagram above and the voltage across the wire was set to be 3Volts.
2. The initial length of the Constantan wire was set to be 1metre. The current was measured. The distance between the crocodile clips was adjusted until it was 90 cm as measured with the metre rule. The voltage adjusted and the current recorded.
The wire was laid out in a straight line on the desk so that any heat generated would be lost to the surroundings.
3. The value of the current through the wire was measured using the ammeter and recorded.
This is repeated with 10cm intervals (each length 3 times.)
4. The length of wire between the crocodile clips was finally reduced to 30 cm, the voltage adjusted and the current recorded.
Safety
I will make my experiment safe by maintaining minimal direct contact with the wire, as it will heat up due to the current passing through it.
I shall make sure all surfaces are free from water. This is because water is a conductor of electricity and contact with electricity may cause electric shack to present parties.
I shall wear goggles and conduct the experiment standing up, making sure my chair is under the table out of the way, I am wearing no loose clothing and my hair is tied back.
I shall make sure all equipment is placed so that it is near no edges of the table.
Fair Test
I shall make it a fair test by conducting the experiment on the same day, in the same room, with the same wire material (constantan), and by repeating each measurement three times.
It is hard to maintain temperature that is why constantan is used, it is not greatly affected by temperature and therefore fluctuations in temperature will not affect the results of my experiment.
I will repeat the experiment three times in order to make sure my results are accurate. I shall also make it a fair test by measuring the distance with the same ruler each time and placing the crocodile clips in place as accurately as possible.
Prediction
I believe that as the length of wire increases the resistance will increase and that that doubling the length of wire will equal to double the resistance. Doubling the length of a prism such as a wire doubles its volume, therefore doubling the number of atoms it contains, doubling its resistance. An increase in resistance should therefore be due to an increase in the length of wire. The longer wire has a greater number of free electrons that will collide with other free electrons, metal atoms and any metal impurities. Each electron passing through the wire must lose a certain amount of energy every time an atom obstructs it. Therefore if it hits twice as many atoms it looses twice as much energy.
Results
Graph
Analysis
The results show that the longer the wire, the more the resistance. The graph shows that as the length of the wire is increased, the resistance increases steadily. The graph has a positive gradient, which indicates that the length of the wire is increasing in direct proportion to the resistance. I believe this occurred because, the longer the wire, the greater the resistance. This is because a long wire has a greater number of free electrons that will collide with other free electrons, metal atoms and any metal impurities.
The results support my prediction, because I stated that the longer the wire, the greater the resistance. As shown in the results, it is possible to see that as the length of the wire increased from 30cm to 40cm, the resistance also increased from 1.32Ω to 1.76Ω. I also discovered in my research that this increase (according to Ohm’s Law) would be proportional. The resistance depends on the number of collisions there are between the electrons and the electrons, atoms and impurities in the metal wire per second. The number of collisions per second doubles, when the length of the wire does, then there is heat energy lost from the collisions, which means double the resistance. The results show this to be true.
I predicted that if the length of wire doubled, the resistance would also double. This occurred as shown in my results. As I doubled the length of the wire from 40cm to 80cm, the resistance also approximately doubled, from 1.76Ω to 3.52 Ω. Therefore, my results match my prediction precisely. This was because as the length of wire was doubled, the number of metal atoms, which could collide, was doubled. Also the distance that the free electrons and metal atoms could collide through was increased. Therefore, there would be more collisions between the metal atoms and the free electrons, and therefore the resistance also doubled.
Conclusion
My investigation was a fair test. I drew a line of best fit using the 7 different lengths of wire resistance results, which helped me to prove my theory. The temperature was kept constant by performing the experiments in the same room on the same day.
The length of the wire was measured using a metre rule. The smallest scale division on the rule was 1mm. This gives an uncertainty of + or - 1mm. There is an uncertainty of + or - 0.5mm at each end of the length of wire. It was difficult to ensure that no kinks occurred in the wire. Kinks in the wire would have meant that the wire was actually longer than the measured value. Faulty connection may have lead to inconclusive data.
The contacts between the crocodile clips and the wire may have introduced extra resistance into the circuit. The amount of extra resistance cannot be estimated and will have changed during the course of the investigation..
Constantan wire was selected for the investigation because its resistance does not change very much as the temperature changes. The wire was laid out on the desk so that any heating effect would be minimised. The heat generated would have been lost to the surroundings.
The resistance of a metal wire increases as the temperature goes up. It should be possible to develop a technique for connecting the wires into the circuit, which would eliminate any uncertainty due to the contacts. This might involve soldering connections to the wire under test. This would involve quite a lot of extra work, which would not be justified by the increase in accuracy obtained.
My prediction was proven almost accurate, the length of a wire and resistance are directly proportional. Thus Ohm’s law has been proven right.
To extend my investigation I could explore a different variable like temperature or material. Different metals have different values of resistance. I could experiment to test the resistance of different metals. A series of experiments could be carried out to measure the change in resistance of a fixed length of Constantan wire as the temperature of the wire is changed. Placing the wire under test in a water bath and changing the temperature of the water bath by heating it with a Bunsen burner could do this.
Bibliography
World Book Encyclopaedia
Letts GCSE Physics
CGP Physics
WWW Search engines
Encyclopaedia Britannica
GCSE Bitesize Revision
Ama Oliver Resistance Coursework of
