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Factors Affecting the Resistance of Wire.

Free essay example:

Ruth Rudd

Year 11 Physics Investigation

Factors Affecting the Resistance of Wire.

Aim of my investigation

The aim of my investigation is to see what effect changing the length of a piece of wire has on its resistance when the current in the circuit is kept at 0.2 amperes (amps).

Background information

An electrical current is a flow of charge around a circuit, and is measured in amperes (amps). Within the circuit there are conductors (metal) which are full of electrons that are mobile. Current cannot flow through an insulator like plastic because the electrons are fixed, hence why wire casing is made of plastic. Current measures the time rate at which charge passes through a circuit element or through a fixed place in a conducting wire. A complete circuit the battery will push charges through the wires and around the circuit, the charge flows all the way around the circuit and is never used up.

However the current depends on both the voltage and resistance in the circuit at the time. Firstly current will only flow through a component in the circuit if there is a voltage across it and is the force pushing the charge around. Voltage is measured in volts. Resistance in a circuit is anything which slows the charge down, ergo decreasing the current. Wire in an electrical circuit has such a small resistance that it is usually ignored. If a wire has a larger area then the resistance is greater because the electrons being pushed by voltage collide with more ions in the wire. Resistance is measured in ohms.

Prediction

From experiments that I have carried out previously and from the knowledge I have that I have included into my background information I predict that the longer the nichrome piece of wire the higher the resistance in the circuit, I also predict that when the length of wire doubles the resistance will also double.

Strategy

The factor I have chosen to investigate the effect on resistance after changing the length of nichrome wire, whilst the current is consistently 0.2 amps. I have chosen this factor because if I would have chosen rho (material) I would be limited as to the things I could investigate: the highest or lowest resistance out of all materials tested. This would mean that I could only put my results in a bar chart as it is not continuous data and would mean I would be limited as to the conclusions I could extract from my findings. Also if I did this depending on what material I used the experiment could be expensive to conduct, for example if we used metals such as silver. I did not choose the cross-sectional area of wire to be the variable because it would be extremely difficult to measure which could result in more unreliable results and because of it being more difficult to measure it would take longer to carry out the investigation meaning I could not repeat it a sufficient number of times to get more reliable results. If I chose to change the cross-sectional area it would also involve complicated calculations resulting again in a time lag and leaving a higher margin for human error resulting in less accurate results.

Equipment

  • 50 cm of nichrome wire
  • 1 power pack
  • 2 crocodile clips
  • 1 variable resistor
  •  wires
  • 1 ammeter
  • 1 voltmeter
  • 1 meter stick

Below is a labelled circuit diagram, it shows what my circuit will look like when I carry out my investigation and where certain items will be positioned.

I have chosen to use a range of length of wire from 5cm to 50, going up in 5cm. this has given me a total of 10 lengths of nichrome wire that will be in my circuit. I have used these lengths because it means my wire is reasonably spread out however, still within range of 45cm. if I would have used a lower or higher range in the length of wire then arguably if my prediction is correct I would come out with the same trend in any graph I draw. However, when deciding the lengths of wire to investigate I must also look at what is feasible. 50cm provided long enough to get a sufficient range however was not too challenging to measure working in a pair. After deciding the range of length of wire, I then decided the intervals at which I would measure the voltage. I knew that I wanted to have 10 different lengths of nichrome investigated to ensure maximum reliability and to help with spotting a trend in any graph I would draw. So I divided 50 (which I had previously decided was my maximum length) by 10 to get 5, this would mean that I could start at 5cm and measure the voltage and then continue to 10,15,20cm etc. up until 50cm which will give me 10 lengths. I also decided to repeat the experiment 3 times, not only would this give me more reliable results when working out averages, but it would also enable me to calculate error bars on my graph which are needed when drawing a line of best fit to spot a correlation or trend.

        The first thing we did when putting the circuit together was test it without the nichrome wire, this would make sure that all components were working correctly. Then we would use a meter ruler to measure the needed piece of nichrome wire, once one of us had attached the crocodile clips my partner would check it to make sure the measurement was correct. Then my partner adjusted the power pack and variable resistor so the current was at 0.2 amps, and I checked to ensure minimum margin for error.  I then firstly read the voltmeter and wrote down the readings, my partner would then also read the voltmeter.  I then turned off the power pack and extended the piece of wire, with again my partner checking. We repeated this process for the 10 lengths of wire and recorded the findings in a table in our books. We then repeated the whole experiment 2 more times so we had 3 sets of results for the 10 individual lengths. To ensure as accurate results as possible I worked in a pair, so that whenever we had to change something or read the voltmeter ourselves the other person could double check we had done it correctly. We also used a low current at 0.2 amps, we did this so that the wire didn’t become too hot which could not only be a safety hazard but the heat could also affect our results. We also carried out the test 3 times in total to ensure maximum reliability and to help us spot anomalies or any pieces of freak data.

Collectingdata

Before carrying out my investigation for real, I carried out a preliminary experiment with my partner to help us spot any problems we could encounter and change them previous to the investigation to ensure everything ran as smoothly as possible. In the preliminary test we repeated the test only once however, we noticed that this didn’t make our experiment reliable so we ensured that in the real experiment we would repeat everything a further 2 times to give us maximum reliable results. Another change we made from our preliminary test was that we ensured the table to record results was previously drawn up in our books before the experiment began. This meant that we could get the experiment completed in a shorter amount of time to minimise the effects that could come hand in hand with time i.e. a change in room temperature which could affect our results. Due to prior knowledge we knew there was a chance of the wire heating up if the current was too high and making a huge safety hazard however, the wire never became hot so for the real experiment we kept the current at 0.2 amps.

        We had to ensure that we controlled certain factors to make it a fair test. Firstly we controlled the cross-sectional area of the wire, meaning it had to be the same thickness. If the cross-sectional area of the wire was not controlled then I believe it would have effected out results. This is because; when there is an increase in the number of ions in the resistor (wire) the chances of an electron colliding with one will increase. However, there is also an increased amount of electrons getting through an increased amount of gaps, so resistance will decrease. Another factor we had to try and control was room temperature. To ensure we would minimise the effect the room temperature had on our results we ensured we carried out our whole experiment in one lesson. Trying to speed up proceedings to limit the chances of a plummet or fluctuation in temperature meant that it could not effect our results and I also felt that although the time pressure I did not let that compromise the accuracy of readings.  

        I am going to try and work to the highest level of precision possible; I am doing this by working in a pair. This will benefit precision because it means that when one of us measures the wire or reads the voltmeter the other one will be able to double check. We will also be using a meter ruler stick that has millimetres on it, this means that I we will be able to measure the nichrome wire as accurately as humanly possible. However it is also reasonable to add, that there is as ever a margin for human error. When handling metal wire, it can be awkward meaning that some results may differ from others, but due to me carrying out my investigation 3 times it should be easy to spot any anomalies. I will also be adding error bars to my graph, so when drawing a line of best fit it enables me to count for any human error. As soon as we got a reading we put it into our tables worked out averages and resistance and then after convert the results into a graph. We would then also add error bars to our graph to show our findings clearly and to help us spot any observations.

Results

Current (amps)

Length of nichrome wire (cm)

1st attempt (volts)

2nd attempt (volts)

3rd attempt (volts)

Average of 3 results (volts)

Average resistance (ohms) R=V/I

Highest resistance (ohms)

R=V/I

Lowest resistance (ohms)

R=V/I

0.2

5

0.5

0.5

0.5

0.5

2.5

2.5

2.5

0.2

10

1

1

1

1

5

5

5

0.2

15

1.2

1.2

1.2

1.2

6

6

6

0.2

20

1.9

1.9

1.9

1.9

9.5

9.5

9.5

0.2

25

2.4

2.4

2.4

2.4

12

12

12

0.2

30

2.9

2.9

2.8

2.86

14.3

14.5

14

0.2

35

3.2

3.2

3.2

3.2

16

16

16

0.2

40

3.8

3.8

3.8

3.8

19

19

19

0.2

45

4.2

4.2

4.2

4.2

21

21

21

0.2

50

4.8

4.6

4.6

4.66

23.3

24

23

Interpreting data

From carrying out my experiment I can see that the longer the piece of nichrome wire the higher the voltage needs to be to continue to push electrons round the circuit at a rate of 0.2 amps. My graph supports this by showing a clear positive correlation, meaning that the longer the piece of wire the more resistance and due to the current staying at 0.2 amps then the voltage must have been increasing. The original aim of my investigation was to see what effect changing the length of wire had on the resistance in the circuit. From drawing a line of best fit on my graph I can see that by increasing the length of wire there is an increase in resistance.  Apart from one anomaly that I have circled on my graph, my line of best fit goes through all the points. This shows that the points on my graph do support the relationship very well, I will go on to explain why the anomaly could have occurred in my evaluation.

         At the beginning of the investigation I made a prediction was that the longer the wire the more resistance would occur. I have proven my prediction to be true. The reason for this is because: in a longer piece of wire the electrons have further to travel meaning more collisions with atoms in the wire. The more collisions that occur the greater the resistance. Also now that the collisions are taking place it requires more voltage to drive the electrons round the circuit. Due to us making sure that the current was always 0.2 amps, and current is basically recording how long it takes for electrons to travel round the circuit then it is going to take an increased amount of voltage to ensure the amp can stay at 0.2 because it is being resisted by the nichrome wire. These collisions also convert kinetic energy into heat energy, which is not useful but dangerous. We also used relatively thin pieces of nichrome wire, meaning a small cross-sectional area. This would result in electrons having to be pushed through a small area, meaning the chances of an electron and an atom of the wire colliding is high. However it must be added that although the latter is a relevant point as to why the resistance measured is rather high for the length of wire used, the wire gauge was the same for every reading so it did not affect the results from one reading to the next.

Evaluation

If I was going to re-do my investigation to improve the quality of the apparatus and means of collecting data I would have used better and more accurate equipment. I would do this by firstly to replace the crocodile clips for something more practical. The reason for this is because they were difficult to attach to the wire as they were not originally intended for that purpose. Also, with the slightest of interference the clips could be knocked out of place, altering the length of wire. This is human error however implementing something that would grip the wire more powerfully the risk of error would me reduced.

        We used a meter ruler stick to measure the wire and because it went into millimetre we were happy with the degree of accuracy. However with hindsight, I can see that there could have been bends in the wire before it was previously delivered to us meaning it was harder to measure. The fact that we re-used the same piece of wire could have also caused some damage to the wire. I would suggest that in future the wire should be measured using a Vernier calliper, this would offer a much more precise reading of the wire despite still being controlled by humans. As for the bends in the wire, there are little ways of overcoming this hurdle. The only obvious solution is to ensure the store of the wire and transportation is handled with care to ensure damage limitation. Also we should use a separate piece of wire for each time we take a reading. On the other hand however, if that was implemented then there would also be another factor we would have to try to control, just in case the cross-sectional area of one piece of wire was bigger than another.

Another change I would make if I was to carry out my investigation again is to check more thoroughly my components in the circuit. I already did make sure it worked before adding my nichrome wire however, there could have still been something wrong with my voltmeter or ammeter concerning their accuracy. In future I would conduct a more thorough check before the experiment to make sure all of my equipment was in good health and giving accurate results.

        All though my partner and I both checked the reading on the voltmeter, there is still margin for human error. Which could be due to eyesight etc. to stop this problem from occurring again I would use a digital voltmeter, although a test would have to be made to ensure it was accurate, it at least delivers the correct reading practically eliminating human error in this part of the investigation?

        Finally the last and most important way I think the investigation could be improved is to try and control the temperature in separate areas of the investigation. Firstly if we had a thermometer in the room we would be able to monitor the temperature of the room. So we could postpone the experiment if the room’s temperature changed. Although the temperature of the room may not make a huge difference on the results the temperature of the why certainly would. Although I used a low current at 0.2 amps to ensure that my wire was not heating up, I wouldn’t be able to tell for sure unless I used a piece of apparatus to measure the heat of the wire. I would then be able to wait the necessary amount of time to get the wire back to its normal temperature.

The only anomaly I found was when we used a 15cm piece of wire. For this result there was no error bar for resistance, this means that the 3 times I carried the experiment out for this length of wire it came back with the same answer. I think that after weighing up what I would do to change this experiment I have realised the factor that could have caused this anomaly is the crocodile clips. All though we were extremely careful it is reasonable to believe that there would be at least one anomaly with the rather large chance of human error that there was. Also, the anomaly could of resulted from the meter stick’s measurements being worn down in that area of length making it harder for us to judge 20cm. the fact that the meter stick were not new when we used them supports that this could have been a contributory factor to the anomaly. Apart from this one error, the data was extremely reliable with every error bar intercepted with my line of best fit.

If I wanted to make my conclusions more secure I should repeat my experiment more times, instead of doing the experiment 3 times in total I should do it 5 times to makes my overall results more reliable. I could also use more lengths of wire, ranging for example up to 100cm of wire to see if the pattern of the longer the wire the more resistance continues. I could also use smaller intercepts, by changing them to 3cm for example it would give me more results and I could look more closely at working out a percentage increase you will expect the resistance to increase per 3cm of extra wire used. To extend my experiment I could also use different types of electrical conductors. Other types of metal may naturally have less of a resistance e.g. silver or copper.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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