I predict that with a wire length of 0.1m, the resistance will be lowest. I say this because in such a short wire there are not so many particles for passing electrons to collide with. You could compare it to a high street that you are walking down. In the time that you walk down a longer high street you would encounter more obstacles (in the form of people in this case) than you would in a shorter high street (this is assuming the streets are as busy as each other). It is the same with electrons. If they must travel further round the circuit, they will have to avoid a lot more wire particles. They will obviously collide more, producing less electron flow (otherwise known as current).
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 13.27?. 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 1.3?. 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 2.7?, and for 0.4m (twice 0.2) the resistance is approximately 5.4?. This is exactly double the resistance of a 0.2m wire. Also, at 0.3m the resistance is 4?, and at 0.9m the resistance is 3 times that at 12?. 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 would of been more accurate if we had used multiple volts: 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.
