Find out what affects the resistance of a wire.

Plan

The aim of this investigation is to find out what and how affects the resistance of a wire. To carry out the experiment I will need to build a circuit. In it I will measure the current it the circuit and voltage across the wire. I will need the know the current and voltage because:

Resistance (Ω ) =Voltage (V)

Current (A)

So to find out resistance of a component you have to know the voltage and the current. Therefor I will need to measure them both in my circuit with an analogue meter to measure current and an analogue meter to measure voltage. Analogue meter can give more accurate results than an ammeter or an voltmeter, so I will use it instead. The list of apparatus I will need, to do the practical must include all of this:

Lab pack (will supply the energy in the circuit).
5 leads (will be needed to connect the circuit together).
2 crocodile clips (to clip on the on both sides of the wire).
2 analogue meter+two shunts (1 to measure current and 1 to measure voltage).
Heat prove mat[s] (to protect the table if the wire overheats).
Wire[s] will be a component who's resistance will be recorded.
1 meter ruler (to be able to measure to wire).
Wire cutters (to cut the wire into different pieces i.e. 5cm--40cm).

There are different variables, which could change the resistance of a wire. I think that there is four variables which could be investigated:

Thickness of the wire,
Length of the wire,
Material the wire is made out of and
Temperature of the wire.

I have decided to investigated how length affects the resistance of a wire. So, to make this experiment fair I will keep thickness, material and temperature the same at all times in the experiment. The only variable that will change is going to be length of the wire. Also to make this test fair I will keep the supply at the same voltage and will make sure that this voltage is not to high so the wire could not overheat. This is because temperature can affect resistance if it gets too high and the results will be effected.

Diagram (circuit)

Method

I will start my practical with getting all the eight apparatus out and ready to be used. After this I will use the 1 meter ruler and the wire cutters to measure and cut of the needed pieces. This are going to be 5cm, 10cm, 15cm, 20cm, 25cm, 30cm, 35cm, 40cm. Each length will be measured out three times, so that we could do repeats and get more reliable results. Then the circuit above could be assembled together, before switched in the supply. First I will connect the analogue ammeter with the wire by using some leads and crocodile clips, then I will place the analogue voltmeter across the wire which will be placed on the heat prove mat[s]. Only then I will connect it to the supply (the lab pack) and switch it on. The lab pack will always give the same voltage supply which I will pick after doing my preliminary work. For safety it is important to switch the supply off every time the wire tested is removed and replaced. I will do a rough table for the results beforehand so the recording of results should not tack a lot of time.

Preliminary Work

I have done a little investigation before doing an actual practical, in which I have used the circuit above to test a number of things. The aim of this is to finally decide on some details of the method and learn to use new equipment. The first problem I faced was which wire should I use in my practical. So in my preliminary work I tested two different wires and afterwards decided on which one was the most suitable. But before this I took both wires and measured the current and voltage to see which the ...

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Preliminary Work

Nickel chrome 0.5cm, and
Bare copper 0.8cm.

I have used a micrometer to measure the thickness of the wires. After selecting this wires I had placed them into the circuit and measured the current and voltage at different supplies. I nave only tested these wires at length 5cm and 40cm because these are the two extreams and if they are alright the middle values will too, be acceptable. The the following are the results I got:

Where it says “more then 10” , the current measured more then ten but because I had an analogue meter which measured up to ten, I was not able to get an exact answer. The state of wire column shows if the voltage supplied was too big or not. After doing this preliminary work I can not add more details to my plan. First of all I decides to look at how resistance changes in a nickel chrome wire that is 0.5cm thick. This is because bare copper's voltage results are very similar when I recorded it at the different length and because I cannot measure more accurately then to on decimal place my results will not be accurate. So after deciding to use nickel chrome I thought that 3 volts of supply would be sensible to use. This is because at 5 volts the wire heats up to the condition when temperature could effect the results. I also tested nickel chrome piece of wire 5cm and 40 cm long and it did and it was fine at 3 volts so the middle length will not cause any problem. The final desigeon I had to make was which shunts I will use with the analogue meter. I thinkt that the best shunts to use would be: an 0-5 (V) to measure voltage and 1-10 (A) to measure current.

Prediction

I think that there will be correlation or a pattern between length of a wire and resistance of a wire. I also think that this will be positive correlation i.e. As the length of a wire will increase the resistance will increase as well. This is because the leads which are placed at ether sides of the wire are thicker and made out of copper, so they have much less resistance and if the electrons travel less distance over a high resistance area the current will be bigger and that will make the resistance will be smaller. However if the wire is long the current will measure smaller and this will make resistance higher. This is similar to an real live situation where a big corridor narrows down to a a part where it is much smaller and then opens up again. The chortler is the part were it is narrow the more people will be able to go through. To add to my prediction I will to add some research. Keith Johnson in his “Physics for you” states on page 252 that length of a wire is one of the factors which change the resistance: “As the length increases the resistance increases.” My preliminary work supports this idea because the results I got chow this exactly. In the nickel chrome table for 5cm and 40cm when the supply was at 5 volts the resistance was higher when the wire was 40cm because:

Safety

It is very important to work safely at all times. The following procortions are to make shur our practical will be safe:

No one will dismantle the circuit or even touch it when the supply is switch on.
Before changing the wires I will make sure that they are not hot by giving them time to cool.
I have chosen low voltage at the supply because it will not overheat the wire and it would be safe to touch it.
I will very accurately cut the wire because the wire cutters will be very sharp.

Obtaining Evidence

This is a table to show how resistance varies in a nickel chrome 0.5cm wire as its length changes. I highlighted in red the result which stands out from the reading on the same length, some thing wad gone wrong.

My results definitely show a clear pattern but as I found out later, after the practical, not all of the equipment was the most accurate from what was available. The shunts I have used were an 0-5 (V) to measure voltage and 1-10 (A) to measure current, however the voltage never rose higher then 1 and current did not become bigger then 5. So the results would have been much accurate if I had used 0-1 (V) and 1-5 (A) instead. This would make me able to record readings of voltage to 2 decimal places and the different in the current values would have been remark and the values would have been more exact. Different wire was used to measure each of the repeats and we did three tests for each length. The results were very similar for each length so this made me thing that this results are reliable. With this number of repeats being the same any mistake could be easily sported, and I did see a value which was different from others so I ignored it when was calculating the average. I think I would get the same voltage and current if I was to repeat the practical once again and if I used the same equipment.

Analysing Evidence

After doing my practical I have found out that resistance of a wire changes at a steady rate. There is a clear pattern in my results: as the length of the wire increases the resistance increases too at a constant rate. On the graph it could be seen that the points are going up and diagonally. This is called positive correlation, and I think my results have quit a strong positive correlation because the points are arranged almost in a straight line. Because the points make almost a line I was able to draw the line of best fit very accurately. This makes my able to predict were the points should have been if my results were perfect. So the last point, which is away from the line other points make, should have read--0.660 but the averages divided by each other made--0.714. The resistance grew as the length increased this could be seen from the results table and the graph. This tells means that length has a big impact on the resistance and my results roughly show that if you get a wire twice as long the resistance will be twice as big. This is what my result should have shown they were very accurate but my results do not always show this very vaguely. There is a formula, which could have been used to work out the resistance exactly. This formula connects resistance with length and thickness of wire using P as a set value. The formula is the following:

Resistance =(P*Length of wire)/The area of wire’s cross section

In this formula P stands for the resistance of the wire being investigated which is 1 meter squared in its cross section and 1 meter long. If this resistance is known the resistance of a wire with any dimensions could worked out using the formula above. Also from the formula could be seen that if the length is doubled the resistance will grow to become twice as big if no other variables are changed, and the area of cross section stayed the same because the thickness did not change.
Evaluation

To conclude this peace of work I will evaluate my result. I think my result could have more accurate if I would have used more suitable equipment. I could have used the 0-1 (V) shunt to measure voltage because my voltmeter always read between 0 and 1. Also if I used the 1-5 (A) to measure current the results would be more accurate. The calculated resistance could have been really affected by the inaccuracy in the current and voltage because the inaccuracies together could have lead to a mistake. The is mistake could not be big because the line of best fit looks straight except the last point which possibly is out of the pattern because the equipment used was not the best. However my results are quit reliable because I did three repeats for each length and then by taking the average I illuminated the possible mistakes. Also the results for repeats were quit the same so this makes it really improbable that the mistake repeated three times. Also when one of the repeats was different from other two I did not use it to calculate the average. The result, which was odd I highlighted in red and the average that was affected I highlighted as well. I do not know for sure why this result dose not fit with rest but I think that it could have been my fault because I read the value wrongly. I think this because no one in our group got this 0.8 there. Also it is possible that something was wrong with the wire of the analogue meter but this problem must have disappeared later on.

I had measured eight different lengths and all of the results follow the same pattern, more or less. I think that this is sufficient evidence to make me able to draw a suitable and firm conclusion. I can even use this results to predict what the resistance of a longer or a thicker wire is going to be. It it enough to work out P for each length, average it out and use in the formula above to calculate resistance for a wire with any length or thicker which is made from the same material. I also think that if I did the same experiment again I would get very similar results except the highlighted one. This is because my result seem to be the same in most repeats and I can trust my results.

I think that there are a couple of important imruvments I could do in my practical so that my results become even better then before. First I think that a better shunts will have to be used, this would make my results more accurate. Then I think a method of keeping the wire at the same temperature all the time will be needed to get reliable evidence. I think that a termomitor could be used too control the wire’s temperature and if it changes a fan could be switched on to spin faster and cool the wire down more. This would make sure that temperature is not affecting the result and this could improve the results a lot. I think that a better measuring equipment to measure the length of the wire, because it is hard to measure out distance on a flecsible wire using a 1meter ruler.

I think there is many ways in which this investigation could be expanded. However one of the most promising is if the extent ions which could be carried out is the study of all other factors that influence resistance. This are all the variables I had listed at the beginning. Thicker or area of cross section will be a good one to start of with because the results for that will be the opposite and the graph would show negative correlation, if the results gathered would be correct. It will be important to use wire out of the same material. After doing this it would be suitable to investigate the affects of temperature on resistance, this will much harder and will reqier more time time and equipment. After this the final part of the investigate could be carried out. I would be possible to test wires maid out of different material but with the same dimensions. Some of the elements could be used in the tests and then an order of resistance could be assembled, from lest resistant to most resistant.