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Factors that affect the current in a wire.

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Introduction

Planning – physics coursework

Factors that affect the current in a wire.

Aim:

To see how the change in the size of the cross-sectional area of a wire affects the current flowing through it. image00.png

Method:

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I will set up my apparatus as above. In this investigation I am going to change the cross-sectional area of copper wire measured in mm². Then I will be measuring the current through different thicknesses of wire. I will be using eight different sizes of cross-sectional area, from 26 SWG, 0.164mm² to 36 SWG, 0.029mm².

SWG(Standard Width Gauge)

Diameter of wire (mm)

Area of wire (mm²)

26

0.457

0.164

28

0.376

0.111

30

0.315

0.077

32

0.274

0.058

34

0.233

0.043

36

0.193

0.029

I am going to repeat each reading three times so I can obtain an average and therefore more accurate results. I will be using a Rheostat to control the voltage and a voltmeter to check that the voltage is kept constant. I will also be using an ammeter to measure the size of the current each time. I am using a transformer in this circuit as I need to convert the 240 volts from the mains to 12 volts with which I can run my circuit.

        I will keep the rest of the variable factors of the experiment the same to ensure accurate results. These factors are the length (to be kept at 1 metre), the temperature of the wire (room temperature), the voltage (0.25 volts) and the material that the wire is made of (nichrome).

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Middle

Prediction:

I think that the current will become greater as the cross-sectional area of the wire gets bigger. I think that this will happen because when the cross-sectional area is greater then there is more space between the atoms so therefore it is easier for the electrons to pass through the wire as there is less resistance for the electrons to fight against. So therefore there is a higher current flowing through a wider wire as there is less opposing resistance. The Resistivity of copper wire is 1.7×10-8 Ωm. From this I can work out the resistance of copper, using this equation:

Resistance = (Resistivity × Length) ÷ Area

Therefore using this formula I can find the resistance of each cross-sectional area that I used;

Resistance = (1.7×10-8 × 1)  ÷ each different area

Therefore I can draw a table up of all of the different resistances of all the different cross-sectional area;

Cross-sectional area (mm²)

Resistance (Ω)

0.164

0.103

0.111

0.15

0.077

0.21

0.058

0.28

0.043

0.39

0.029

0.58

Then from these resistances I can work out a set of predicted results for the investigation. To do this I must use the formula;

Current = Voltage ÷ Resistance (I = V ÷ R)

For instance for the area of 0.164 the current would be;

0.25 ÷ 0.103 = 2.43 (3 s.f.)

Therefore I can draw up a table of my predicted results;

Cross-sectional area (mm²)

Current (amps)

0.164

2.43

0.111

1.67

0.077

1.19

0.058

0.89

0.043

0.64

0.029

0.43

Preliminary investigations:

I decided to carry out a set of preliminary investigations as I felt it would be very useful for me to have had a practice using the equipment.

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Conclusion

Cross-sectional area (mm²)

Percentage error in (%) to (3.s.f)

0.164

9.09

0.111

5.88

0.077

2.22

0.058

4.72

0.043

4.65

0.029

2.69

 Therefore the overall average, percentage error = 4.88% (3.s.f)

Therefore I can say that as the percentage error of my evidence was only 4.88, overall my results were consistently quite accurate and reliable.

        For my investigation I feel that it would be a good to do some extension work to create a more broad variety of evidence with which to back up my investigation. If I investigated the effect of lengthening the wire, on the current then I would be able to draw up a further conclusion for this factor. I know that if I increased the length of the wire then resistance in the wire would be greater, also if I double the length of the wire then the resistance would also double. This is because there would be twice the amount of atoms for the electrons to get past so it would be twice as hard to do this. Therefore as the resistance increases it causes the amount of current flowing through the wire to decrease. This would cause a negative coloration on a graph showing the length of the wire against the current flowing through it.

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