From the ‘physics for you’ textbook, page 254, I found out the following statements:
‘As the length increases, the resistance increases, as cross-sectional area increases, the resistance decreases, as temperature increases, the resistance of wire increases’.
I also believe that the material will alter the resistance but by how much is unpredictable.
Plan
- Make sure the equipment works.
- Set up equipment as show in circuit diagrams - Make sure ammeter is connected in series and voltmeter is connected in parallel.
- Turn on power source
- Connect voltmeter, one end at 0cm and the other at 100cm
- Take reading off of the ammeter
- Take reading off of the voltmeter
- Record results in the table
- Work out the average potential difference by adding the results from the all the runs and dividing by how many runs there were.
- Work out resistance of each length using the formula R=V/I - Make sure you use the average potential difference when working out the resistance
Hypothesis
After researching on the Internet, books and CD ROMs I have found that the factors that affect the resistance of resistance wire are Temperature, cross sectional area, length of wire and material.
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Temperature: If the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance. I am not going to investigate this factor because it would be hard to control accurately, so the results wouldn’t be fair.
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Material: The type of material will affect the amount of free electrons that are able to flow through the wire. The number of electrons depends on the amount of electrons in the outer energy shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase but we cannot predict how much it will increase or decease. I have chosen not to investigate this factor because we have only two alloys to work with.
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Length of wire: If the length of the wire is increased then the resistance will also increase as the electrons will have a longer distance to travel and so more collisions will occur. Due to this the length increase should be proportional to the resistance increase. This will be the factor I will investigate because it is easy to control and the results will be fair and accurate.
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Cross sectional area: If the wire cross sectional area is increased the resistance will decrease. This is because of the increase in the space for the electrons to travel through. Due to this increased space between the atoms there should be less collisions. I am not investigating this factor because I only have to different thickness to experiment with.
The length of the wire increases so will the resistance. So if I double the length I believe the resistance will double this is because if the length is doubled there will be twice as many delocalised ‘mobile’ electrons for the current to pass through. Therefore the current will have to go through more wire and this will create more particle collisions, which means slowing down the current. I also believe this is true the other way round, so if the length is halved the will be a decrease in resistance.
This can be explained by using the formula:
R=V/I
Resistance (in Ω) = Potential difference (in V)
Current (in A)
Because of the information above the graph I predict is:
Resistance
Length
The length is directly proportional to the resistance
Preliminary work
Equipment
- Power pack
- Ammeter
- Voltmeter
- Ruler
- Sticky tape
- Eureka wire Ǿ 0.37
- 6 wires
Using the set up on the pervious page. I will switch the power pack on. Then I will touch the end of wire B along the eureka wire and take measurements at intervals of ten centimetres. I will read both meters and record the readings on a table. To make it a fair test I will be careful to keep the current, thickness and temperature all constant.
The temperature rose once the current was passed through it, which will cause the atoms in the wire to vibrate, and so obstruct the flow of electrons, so the resistance will increase creating an error. In my final experiment to make it fair I have to keep the temperature constant, I will accomplish this by adding a variable resistor to the keep current the same. Therefore if the current is the same the wire will not heat up.
Final Experiment
I will perform this experiment the same as I did in my preliminary experiment except I have added a variable resistor to keep the current the same and the temperature constant.
I will do this by putting the variable resistor on full resistance then decreasing it until the ammeter reads 1 amp. This is the size of the current I intend to maintain. Then I will take the reading. I will do this before every interval/reading.
The results I got are:
Conclusion
As you can see from my results table and graph my hypothesis was correct. The line on my graph for final results is almost totally loyal to my prediction length is directly proportional to the resistance. If my predicted graph (orange line) is compared with the final results graph it can be seen that they are very similar. This shows that there is a definite trend between length and resistance. Resistance is directly proportional to the Length. This means that if the Length is 50cm, and the resistance is 1.00 Ohms, then if the length doubles to 100cm then the resistance also doubles to 2.00 ohms.
Evaluation
Overall I think the experiment was a success the results were very accurate, proof of this is the results were nearly the same after all three runs and the graph was identical to my prediction. There were also no anomalous points. The biggest difference was a tenth of a volt. The length of the resistance wire could have been measured more accurate, the wire was extremely hard to keep straight. It could have been measure more accurately by using a ruler with more subdivisions and I could of spent more time straightening the wire. The resistance could have also been affect by the croc clips, this is because they were dirty and this might increase resistance. If I were to do the experiment again I would 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 have also found out there is a much easier and reliable way to perform the experiment (shown below)
In this experiment the resistance wire is used and set up in the same way in which a variable resistor is used. This would take a lot less time and the results would be a lot more accurate because nothing has to be adjusted you could just place the wire on wherever you need a reading.
I would also take readings at 5cm intervals instead of 10cm. This would obviously be more accurate and reliable, but it would also tell if the rule resistance is proportional to length if true at 5cm intervals. Ultimately I would use a lot of different types of resistance wire to see if the rule were true for all materials.