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# Find out (through an experiment) how much resistance a piece of copper wire will produce when it is used in an electric circuit.

Extracts from this document...

Introduction

Science coursework.

Aim;

In this investigation I am going to find out (through an experiment) how much resistance a piece of copper wire will produce when it is used in an electric circuit.

Theory;

Inside a piece of copper wire there are millions of fixed protons. Electrons are attracted by these protons. These protons may give away one, two or three of the electrons attracted to them as long as they will get some from somewhere else (another set of protons), to make up for them given away. However, whilst an electron is missing from a set of protons it makes a positive ion. In an electric circuit, the power source supplies a “push” of electrons through the wires. The protons layout throughout the wire supplies a “resistance” to the “push” from the power source. This resistance does not allow as many positive ions to be made.

To measure the resistance within my circuit I shall be using the formula R=V ÷ I

Significant factors;

Length of wire;

The length of the wire would affect the resistance due to the length of the protons the electrons would have to push through. This will make the time for positive ions to be used longer.

Width of wire;

The width of the wire would change the number of moving electrons that could pass through the wire at any time. This would change the number of positive ions that could be made at any time.

Temperature of wire;

The temperature of the wire would affect the resistance due to the energy inside the wire.

Material of wire;

The material of the wire would change the resistance due to the structure of the atoms inside the wire; this would alter how easily the electrons would pass through.

What I will change;

Middle

0.91

0.65

1.40

40

1.02

0.55

1.85

50

1.08

0.48

2.25

60

1.15

0.42

2.74

70

1.19

0.37

3.22

80

1.22

0.33

3.70

90

1.26

0.30

4.20

100

1.27

0.28

4.54

Having completed two sets of results for one wire, it was noticed that these was a large black mark towards one end of the wire, where it appeared that it had been melted to some degree at some point. It was therefore decided to conduct experiments on an additional piece of wire that was checked for integrity prior to investigation:

Wire 2, Set 1:

 Length (cm) Voltage (V) Current (A) Resistance (W) (to 2 d.p.) 10 0.95 1.06 0.90 20 1.19 0.67 1.78 30 1.28 0.48 2.67 40 1.35 0.37 3.65 50 1.38 0.32 4.31 60 1.42 0.27 5.26 70 1.45 0.24 6.04 80 1.46 0.21 6.95 90 1.48 0.19 7.79 100 1.50 0.17 8.82

Wire 2, Set 2:

 Length (cm) Voltage (V) Current (A) Resistance (W) (to 2 d.p.) 10 0.92 1.05 0.88 20 1.16 0.66 1.76 30 1.28 0.47 2.72 40 1.34 0.39 3.44 50 1.38 0.32 4.31 60 1.42 0.27 5.26 70 1.45 0.23 6.30 80 1.47 0.21 7.00 90 1.47 0.17 8.65 100 1.48 0.16 9.25

Averages for each wire were then calculated to give these results, which were then graphed:

 Length (cm) Resistance (W) (to 2 d.p.) Wire 1 Wire 2 10 0.52 0.89 20 0.96 1.77 30 1.40 2.70 40 1.86 3.55 50 2.29 4.31 60 2.76 5.26 70 3.26 6.17 80 3.67 6.98 90 4.36 8.22 100 4.57 9.04

## Conclusions

Having performed the investigation, the following conclusions were drawn:

• As predicted, an increase in length resulted in an increased resistance. This can be clearly said for both wires tested.
• Both wires show a strong trend of a straight line, i.e. the length of the wire is shown to be directly proportional to the resistance - double the length and the resistance doubles.
• The overall resistance of the two wires seems to differ considerably. Due to the strong correlation of the results, the explanation of this is unlikely to be the method used to obtain the results. The more likely explanation would be that the first wire was actually of a larger diameter than the second one. Obviously this is a rather important oversight and this will be discussed more in the Evaluation section. The reason why this is the likely explanation is because resistance is known to be inversely proportional to the cross-sectional area, i.e. if you increase the cross-sectional area (by increasing the diameter) then you decrease the resistance. This is because a wider wire means less likelihood of the free electrons having collisions and losing energy.

It is important to realise, however, that despite the fact that it would appear that the resistance of wire 2 is double that of wire 1, that does not mean that the diameter is half that of the wire 1. That is because if you halve the diameter then you decrease the area by a factor of about 3 (A = πr2)

## Evaluation

• As mentioned previously, the biggest downfall of the investigation was the apparent mistakes when choosing the wire, in that they would appear to be of differing diameters. This did not, in this case, cause a big problem as the same wire was used for each set of results so it is known that the results for each wire are correct.
• Generally speaking, wire 1 would appear to contain the most accurate results due to the fact that all of its points bar one sit on the line of best fit for that wire. The only one that does not is the point at 90cm, which was exactly at the point that the black mark (mentioned previously) was found to be.
• Wire 2, on the other hand, had three main anomalous results: at 50, 80 and 90cm. They are by no means that far off but in an experiment such as this, which is generally a very accurate one anyway, such anomalous results should not be quite so common. Possible explanations for these anomalies are as follows:
• The length of wire for that particular measurement was not correct. At 50 and 80cm it is possible that the length was shorter, causing a lower resistance, and at 90cm it is possible that it was longer, causing a higher resistance. The solution to this is to measure the lengths more carefully and ensure that the wire is pulled tight against the metre rule.
• For a particular result, one or more of the connections could have been faulty, causing extra resistance at the connections. A solution to this would be to, before each experiment, connect the connections together without the wire in place and measure the resistance then. If it is higher than it should be then the connections could be cleaned.
• Whilst extremely unlikely, it is conceivable that the power supply was providing a different voltage for some of the results. This is unlikely to be a problem in this investigation but it might have been an issue had we used batteries instead.

NB: If one were to assume that Ohm's Law applies, then another possible explanation could be that at some points (more likely in the lower lengths), the wire was not allowed to cool completely so that the temperature was higher for that measurement. Whilst unlikely (due to the two sets of results), this would cause a higher resistance as explained previously. However, it is now known, after researching the metal alloy "constantan," that the resistivity (the electrical resistance of a conductor of particular area and length) of this alloy is not affected by temperature. Therefore, in these experiments Ohm's Law does not apply.

## Variables

### Length of wire:

The length of a wire will affect the resistance because wires have a net of atoms and the electric current has to pass through this net. Every time a charged atom hits the net it loses some of its charge, therefore the longer the wire the larger the chance of the charged electrons colliding with the net and losing some of their charge therefore increasing the resistance.

### Width of wire:

The width of a wire will affect the resistance because the wider a wire the more space in the net for the charged electrons to get through without colliding with the net and therefore decreasing the resistance. It is like letting a lot of water out of a small hole; it would go slowly, whereas with a bigger hole it flows out faster.

### Temperature of wire:

The temperature of the wire would affect the resistance because as the wire gets hotter the net begins to vibrate because they are given more energy; this therefore makes it harder for the charged electrons to get through the net without colliding with it and causing resistance. Therefore the higher the temperature the higher the resistance.

### Material of wire:

The material of a wire would affect it resistance because different materials have different resistivity because the have different nets. Some of the nets may have smaller holes that others and this would increase the resistance because there would be a larger chance of the atoms colliding. Similarly some materials may have larger nets, which would mean that there would be a lesser chance of the atoms colliding.

The variable that I am going to change is going to be the resistance because I think it will be the easiest to change with the apparatus I have available to me and I think it will give fair results.

## Research

So far I have taken a lot of information from my G.C.S.E. textbook but some of my work has come from the Internet. I used many different search engines but the two main engines, which I used, were www.ask.com and www.google.com and here is some of the information, which I got by using these engines:

Conclusion

1. Whilst repeating the experiment I would make sure that I did it in the same place as before because if I repeated it next to a radiator the resistance would go up.

## Results

Length Of Wire (mm)

(Ohms)

Results 2

(Ohms)

Average (1dp)

(Ohms)

50

1.3

1.2

1.3

100

2.7

2.1

2.4

150

3.3

3.1

3.2

200

4.2

4.1

4.2

250

5.2

5.1

5.2

300

6.2

6.1

6.2

350

7.2

7.0

7.1

400

8.2

8.0

8.1

450

9.2

9.0

9.1

500

10.1

9.9

10.0

550

11.1

10.9

11.0

600

12.2

11.9

12.1

650

12.9

12.8

12.9

700

13.9

13.8

13.9

750

14.8

14.8

14.8

800

15.8

15.8

15.8

850

16.8

16.7

16.8

900

17.8

17.7

17.8

## Explanation Of Results

###### My first set of results has an anomalous result, which is circled on the first graph. This could be because I did not wait for the ohmmeter to settle and I took the reading too quickly. My second and average set of results are good with no anomalous results and they show a definite relation between the resistance and the length of the wir they show that as the length goes up so does the resistance.

In my prediction said that if the length doubles so should the resistance and my first set of results show that at a length of 250mm the resistance was 5.2W and at a length of 500mm the resistance was 10.1W which is very close to my predicted answer.

## Conclusion

I think that my experiment went quite well because all my graphs had straight lines showing a strong relationship between the length and the resistance.

But I could have improved this method in a couple of ways. For example I should have used pointers instead of crocodile clips because they are far more accurate, this is because they have a far smaller tip than crocodile clips and they would give a more accurate measurement of the wire. Also pointers would not have compressed the wire like crocodile clips. Crocodile clips have a spring in the to keep them shut and this could have compressed the wire therefore increasing the resistance.

I also should have been more careful about making sure the wire was taught when I took the readings because if it was loose it would have been a longer length that the one I thought I was reading.

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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