I predict that at 0.2m the resistance will be higher than that of 0.1m, because of reasons previously stated. There will be twice the amount of wire particles (as 0.2 is twice 0.1), which should technically mean that there would be twice the amount of collisions. Although we cannot measure the amount of collisions, we can measure the resistance and I would expect this to be higher than that of 0.1m.
I predict that the resistance will increase steadily as the length becomes greater (i.e. 0.3m’s resistance will be more than in 0.2m, increasing in 10cm increments up to 1m, which I predict will have the highest resistance of all.
Plan
Apparatus/ diagram
J power supply
J wires x 6
J metre stick
J light bulb
J sellotape
J analogue voltmeter
J analogue ammeter
J crocodile clips x 2
J just over 1m thin nichrome wire
Method
This experiment requires safety considerations. The bulb may get hot, so do not handle it. The wire may also get hot so take care with that. Also, only use low voltages, because the wire will not be able to cope with very high ones. Do not use more than 4V for very long at a time.
Set up the circuit as shown on the above diagram, excluding the voltmeter. Place the crocodile clip closest to the bulb at 0.1m on the metre stick. Make sure it is clipped firmly to the wire (i.e. the teeth are touching it). Turn on the power supply and test to see if the bulb is working. If not check that all wires are connected correctly and that the bulb is not faulty. Also check that the ammeter is picking up some current. If not, again check to make sure the wires are properly connected and adjust accordingly. Turn off the power supply.
Now connect the voltmeter. Ensure that the wire is connected at 0.1m. Turn on the power supply to approximately 2V. Take readings from the voltmeter and ammeter. Turn the power supply off. Repeat this paragraph 3 times, using 2V, 3V and 4V.
Move the crocodile clip to 0.2m. Make sure the teeth of the clip are firmly on the wire. Turn on the power supply to 2V. Take readings from the voltmeter and ammeter. Turn off the power supply and repeat this paragraph 3 times, using 2V, 3V and 4V.
Next, repeat the previous paragraph, except replacing “0.2m” with “0.3m”, followed by “0.4m”, then “0.5m”, “0.6m”, “0.7m”, “0.9m”, and finally “1.0m”. Each time use 2V, 3V and 4V on the repeats.
It is important that you turn off the power supply in between readings because otherwise the wire will become too hot. Temperature would be another variable, and would make the test unfair. With heat a wire has more resistance, because the wire particles vibrate with the heat, making it more difficult for electrons to flow through the wire. There are more collisions, meaning less current for the same voltage and more resistance as a result.
We did some preliminary work to (a) find out how these factors affect resistance: temperature, cross-sectional area and resistivity and (b) to find out what material and width would be most suitable for this particular experiment. We needed to know what we would have to keep the same in order for our experiment to be a fair test; what our controlled variables would have to be.
The first experiment I did was to find out how temperature affects resistance. I did this by using a light bulb as a gauge for temperature. We translated bright as a high temperature and dim as a cold temperature. The light bulb provided the wire. We found that the current still increased with potential difference, but not proportionally. As the bulb got brighter (and therefore hotter) the current struggled to increase. This is because at a higher temperature, a wire is more resistant. So we then knew that we would have to keep the temperature constant throughout the experiment, to keep it a fair test.
Since we also knew that the cross-sectional area of the wire and the resistivity affected resistance in a wire, we decided to tackle both these factors in one experiment. We wanted to find out how those two factors affected resistance so we would know what to control in our coursework experiment. The other aim of the experiment was to find out which material and thickness of wire would be the most suitable for our experiment. We used nichrome wires and copper wires, with one thin, one medium and one thick in each material. We found that the nichrome wire was much more resistant than copper, and that the thinnest wires also had the most resistance. The reason that the copper wire had a very low resistance is due to a large amount of loose, or free electrons in the wire. These electrons are more easily able to carry electricity, therefore needing less push (provided by the voltage) to produce the same current as a different material. We also found out that a thicker wire has less resistance. This is because in a thin wire there is not very much space for the electrons to space out in. This leads to more collisions between wire particles and electrons, meaning a higher resistance.
So we then knew what our controlled variables would have to be: temperature, the thickness of the wire and the material of the wire. We also knew what the most suitable material and thickness were for the wire. We decided that it was most suitable to use a thin nichrome wire, as this had the highest resistance. Having a high resistance means needing a lot of push (provided by the potential difference) for the same amount of current as a different material (such as copper). This way more P.D would show up on the voltmeter, making our results more accurate. Because we will be using an analogue voltmeter, it is very difficult for them to pick up such low voltages, and the thin nichrome wire requires a higher voltage to produce the same current as copper, meaning more reliable readings.
We will use a range of 0.1m to 1.0m, with 0.1m increments. We will repeat each reading once with 2V, once with 3V and once with 4V. This is to make our readings more accurate. If we just used the same voltage for all 3 readings and then took an average all the readings would be the same, so we will vary it. We will repeat our results 3 times to ensure accuracy.
The independent variable is the resistance of the wire.
The dependant variable is the length of the wire.
The controlled variables are the potential difference, the power pack, the voltmeter, the ammeter, the piece of wire used, the temperature.
Results
Graph
Conclusion
I conclude that the longer the wire, the higher the resistance. This is because in a longer wire, there are more wire particles. Resistance is caused by electrons colliding with wire particles. Where there are more particles electrons are obviously more likely to have collisions, leading to a higher resistance
In a longer circuit, it is more of a struggle for electrons to get around the circuit without any collisions. There are lots more particles to avoid. Less electrons were able to get past at any one time in the wire, meaning that less current showed up on the ammeter. This means higher resistance.
The wire with the highest resistance was the longest one – 1m long. This had a mean average resistance of 13.27Ω. This was as expected in my prediction. I said that the longer the wire the higher the resistance, and this was the case. Also, the wire with the lowest resistance was the 0.1m one. This had a resistance of 1.3Ω. The reason for this is that there are not so many particles in a short wire. This means that there were fewer collisions between the electrons and the wire particles. A low resistance translates as not many collisions, and therefore lower resistance.
Metals conduct electricity due to a large amount of “loose” or “free” electrons in their atoms. These free electrons can carry electricity easily. When there is no electric field in a wire the electrons stay still, but as soon as you put an electric field there the electrons will move from the negative to the positive. I have illustrated this below.
You could compare resistance to a high street. If you walk down one street and bump into a certain amount of people, in a street twice the length you are likely to bump into twice that certain amount of people. It is in this way that resistance works.
On my graph you can see that it is a steady, straight line of best fit. This suggests that length is directly proportional to resistance. This is not too surprising, because if you double the length of a wire, you would expect there to be double the amount of collisions and double the resistance. Likewise, if you triple the wire length there should be three times the amount of collisions and three times the resistance. It is logical. I have demonstrated this through illustrations below.
Ohm’s law states: “the current through a metal conductor is directly proportional to the voltage across its ends, as long as all other conditions are constant”, so I would expect that this would not be the case, as we changed length in this experiment. The formula for resistance is V/I, and if you change one of these things (out of potential difference and current) then you vary the resistance. V/I is a constant (when no other factors are affecting it) known as resistance. However, we changed current by varying the length, and for the longer wires this meant less current (more collisions, less electron flow), and changing current changes resistance.
So my results support my prediction well. They have shown clearly, and with no anomaly, that as the length of a wire increases, so does the resistance. There is a very clear, steady pattern visible both in the results table and in the graph. On the graph I have labelled several points that make this clear. You can see that for 0.2m the resistance is approximately 2.7Ω, and for 0.4m (twice 0.2) the resistance is approximately 5.4Ω. This is exactly double the resistance of a 0.2m wire. Also, at 0.3m the resistance is 4Ω, and at 0.9m the resistance is 3 times that at 12Ω. So again my results are solid proof for my prediction.
Evaluation
I think that the procedure used was fairly suitable, although not as much as I would have liked it to be, because we just used a crocodile clip to connect the wire at a certain length. Firstly, the crocodile clip is quite wide, and it is impossible to connect it at the exact length that you want. Secondly, the wire was not perfectly straight – it had several slight twists and bends in it, and this would have affected the accuracy of our results. We might not actually have been observing the results for the exact length we intended.
The only way we would be able to solve the problem of the bends and twists in the wire is to use a brand new piece of wire and look after it very carefully. We could solve the length problem by using a brand new piece of wire, which starts off at 1m in length, and we would cut it down to size for each result. This would make our observations closer to the exact length.
I think our results were accurate because we used multiple results, using one reading for 2V one for 3V and one for 4V on each. The reason we did this was to get a variety of results rather than from the same voltage (after all it is the resistance we are interested in rather than the current or potential difference). We took an average, which is more reliable and accurate than just using one reading.
Our results were also made more accurate by the fact that we used a fairly wide range. Using just one or two increments is not reliable enough to draw a valid conclusion, so we used 10 increments. This way we would have been able to cope with any anomalous results using a line of best fit.
There were no anomalies, which proves even further how reliable our results were. If we did have any anomalies though, it could have been because the temperature became too high, creating an extra variable to make the test unfair. If the temperature did get too high it would have decreased the current, increasing the resistance. Similar to this idea, the wire could have had some impurities in it, varying the resistivity and increasing/decreasing the resistance. Any of the remaining three (I say this because we have already used one in our experiment – length) factors affecting resistance could have been varied – temperature, resistivity and thickness, leading to unreliable readings. The other reason for an anomaly could simply be that we misread the voltmeter/ammeter.
We could use an even wider range of results to increase the reliability of out results, or we could repeat the results more times. For further work, we could think about which material, length, width and temperature wire has the highest/lowest resistance. We could also use different kinds of resistors in the circuit, for example thermistors, so we could see how resistance varied with heat and that resistor, or we could instead use a light dependant resistor, to see how resistance would vary with that. We could even use a motor instead of a bulb, to create a fan-like device.
2
How resistance is effected by changing the length of a Wire
How resistance is affected by changing the length of a Wire
Aim
To investigate how the resistance of a wire is affected by varying its length.
Prediction/Hypothesis
I predict that the longer the length of wire, the higher the resistance will be. Electrons colliding with atoms in the wire cause resistance. The longer the length of wire, the more atoms there will be, so the more chance there is of the electrons to collide. Thus causing higher resistance. The resistance and the length should be directly proportional to each other.
Independent variable
This is what I shall be changing in the experiment, and this shall be the length of the wire.
There are several variables in this experiment, they are:
- Length – The longer the length of the wire the further the electrons will have to travel along it, increasing the resistance. Because of this the length increase should be proportional to resistance increase.
- Cross section of the wire - If the wires width 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.
- Voltage passed through the wire – The higher the voltage the more electrons there will be passing through the wire, this should cause more collisions, causing higher resistance.
- Temperature – Resistance produces heat, but heat also increases resistance. This is because the atoms in the wire vibrate more due to their increased energy. This causes more collisions with the electrons as the atoms vibrate into the path of the electrons. Resistance is caused by collisions with atoms in the wire and
- Type of wire used - The type of material will affect the amount of free electrons, which 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.
My chosen variable in this experiment is going to be to see how altering the length of the wire affects the resistance.
As I am going to need to use several different lengths of wire in this experiment, it would be more economical to use something called a sliding contact. This is where a long length of wire, that is to be tested for resistance, is placed in the circuit, held in place at either end by two crocodile clips. To test the resistance of the different lengths of wire, the crocodile clips are placed along the wire, so that the desired length is between the clips. The wire in between the clips will be live when a current is passed through it, but any other wire that is outside of these clips will be neutral. The reason for this is because electricity is lazy, and it will take the quickest and easiest route.
The lengths that I have chosen to test are 200mm, 400mm, 600mm and 1000mm. The reason I have chosen this particular lengths is because they are ion a logical sequence and are well spaced out and easily measurable. This will hopefully give me good averages, and a good graph.
Dependent variable
This is what I am going to be measuring. This shall be the resistance of the wire. I shall do this by using an ammeter and a voltmeter in the circuit. I have chosen to do this because it is more accurate than using a separate resistance meter. I have decided to rely on these instruments, because I know that the power-packs indication of voltage is highly inaccurate. I shall test each length of wire three times, to make sure that I get a good average. The way I shall calculate the resistance shall be using the following formulae:
Resistance I = potential difference (p.d) across resistor in volts (v)
__________________________________________
Current flowing through it in amps (I)
Fair Test
To make sure that this experiment is conducted as fairly as possible I have come up with a number of things that need to be kept constant. The reason I need to keep conditions on the experiment as regular as possible is so that I can get accurate results and graph, otherwise there is little point of conducting the experiment.
The voltage, all the apparatus and measuring equipment, the wire that is being measured for resistance, and accurately measuring the lengths to be tested all must be kept the same.
I shall keep the length the same by using the same meter rule every time I measure the length of wire that is to be tested, and making sure that the measurements are exact.
For each individual experiment I am going to make sure that the voltage is the same, by checking the voltmeter and adjusting the dial accordingly, because the voltage is known to fluctuate as the experiments go on.
Resistance is affected by heat, and one of the products of resistance is heat. So I will make sure that I let the apparatus cool down for about a minute in between each individual experiment. The heat causes the atoms in the wire to vibrate quicker causing higher resistance.
Safety
In this experiment we are using a bare electric wire, which could e very dangerous if the voltage is not kept low. As we are using electricity we must be ever vigilant of the dangers that it can cause, especially looking out for water, which might be on the lab benches from previous experiments that day.
All of the usual lab rules to this experiment, as they would do with any other. Such as no running etc.
Equipment
- 1 x power pack
- 6 x power leads
- 1 x ammeter
- 1 x voltmeter
- 1 x un-insulated wire
- 2 x crocodile clips
- 1 x meter rule
Diagram
How to put the apparatus together
-
Connect the power pack to the mains and switch it on at the plug socket. However make sure that the power pack is turned of at the power switch.
- Take 2 wires and connect them to the negative and positive terminals of the A.C supply.
- To one of the wires connect one side of the ammeter.
- Take another wire and connect it to the other side of the ammeter.
- Assemble the voltmeter component, with a wire either side of the voltmeter.
- Your apparatus should now look like this:
- Connect your voltmeter th from studentcentral.co.uk e rest of the apparatus in the way shown below:
- Attach the crocodile clips to the end of the wires still left bare.
- Your completed apparatus should now look like this:
Preliminary method
- I have decided to keep the voltage to 2 volts. This is because it is a fairly low voltage and is quite safe, bearing in mind there are bare wires in this experiment. Also I have chosen this low voltage because it means from the findings of my preliminary tests I can raise or lower the voltage if need be.
- The apparatus will be set up as described I the diagram. And the wire will be placed in the crocodile clips.
- 20cm of the wire will be in the clips.
- I will repeat each test for resistance on each length 3 times to make sure that I get a good average.
- I will allow the wire to cool down for at least 30 seconds in between each test.
- I am going to draw up a results table so that I can record my findings accurately.
Results Table
As my graph to the preliminary testing shows, everything seems to have gone according to plan, and there were no anomalous results.
This proves that my methods and predictions are accurate and correct.
I have conducted these preliminary tests in exactly the same way I had planned to conduct my main experiment. The purpose for these preliminary tests was to see if there was anyway in which I could improve my method, and to see what I would have to change to the experiment to make it work for me as best as possible. However, I came across no problems and there was nothing, which I could see that needed changing. Therefore I am going to adopt my preliminary tests as my main experiment, because if I repeat the experiment, I will get exactly the same results.
Analysis of graph
From my graph I have proved my prediction that resistance is proportional to length because of the line of best fit that I have got on the graph.
Conclusion
In my prediction I said that resistance is directly proportional to length, and I have proven this to be correct, by following by plan and a well worked out method that is both fair and accurate. The length of the wire affects the resistance of the wire because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion. The resistance of a wire depends on the number of collisions the electrons have with the atoms of the material, so if there is a larger number of atoms there will be a larger number of collisions, which will increase the resistance of the wire. If a length of a wire contains a certain number of atoms when that length is increased the number of atoms will also increase. From my results table and graph I can see that my results that I collected are very reliable. I know this because my results table does not show any individual anomalous results this means that I did not have to leave any results out of my averages because they were anomalous. Also on the graph I can see that none of the averages plotted are anomalous because all the averages lie along the same straight line.
Evaluation
If I were to make any changes to my experiment I would have put the voltmeter apparatus on the wire that was being tested. By using this way it would of measured the voltage of that wire, instead of the wore that was being tested and the rest of the circuit.
Also I would of tested the same wire, as I had done in the experiment described above, but used different thickness of the wire, to see how this affected the resistance to add to my investigation.
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