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Investigation into the factors affecting the resistance of wire

Extracts from this document...

Introduction

Investigation into the factors affecting the resistance of wire

Part 1 (Basic investigation)

Aim:

The aim of this first experiment is to look into the factors affecting the resistance of a length of wire when a current is passed through it. Resistance is caused by electrons colliding with ions in the wire. When this happens energy is wasted because it is converted into heat and light etc.

The less resistance there is the more useful the wire is as a component to transfer electricity, although obviously in some cases resistance is wanted in certain components such as heating elements.

The factors I am going to focus on will be length, diameter and material of the wire, two properties that can be changed affecting any simple circuitry.

Plan:

This part of the investigation will involve collecting resistance readings firstly for varying lengths, then varying width, and finally varying metals.

The majority of the experiment setup will be kept the same for these three tests and will be set up as shown in fig.1

The equipment list is as follows:

DC power supply

Leads

Ammeter

Voltmeter

Crocodile clips

Test wire:

16, 18, 26, 30, 36swg copper wire, 26swg constantan, 26swg manganin

Meter rule

With the equipment set up as shown I will first start by testing the resistance over different lengths of wire. Using the 36swg copper wire because it should be expected to have the highest resistance due to being thinner, and so should show the greatest change in resistance in relation to length; the wire is placed on the rule and the crocodile slips are attached to each end using a meter of the wire.

I will then put a low voltage of 0.1 volts through the circuit.

...read more.

Middle

950

0.09

0.1

0.1

0.17

0.16

0.16

0.593137255

0.2

0.031415927

1.96147E-05

Copper

36swg

1000

0.1

0.1

0.1

0.16

0.16

0.15

0.638888889

0.2

0.031415927

2.00713E-05

Copper

30swg

1000

0.1

0.1

0.1

0.38

0.39

0.38

0.260908682

0.315

0.077931133

2.03329E-05

Copper

26swg

1000

0.1

0.1

0.1

0.76

0.78

0.76

0.130454341

0.45

0.159043128

2.07479E-05

Copper

18swg

1000

0.09

0.1

0.1

4

4.03

4.02

0.024063173

1.2

1.130973355

2.72148E-05

Copper

16swg

1000

0.1

0.1

0.11

5.2

5.15

5.16

0.019988691

1.6

2.010619298

4.01896E-05

Constantan

26swg

1000

0.1

0.1

0.1

0.04

0.05

0.05

2.166666667

0.45

0.159043128

0.000344593

Manganin

26swg

1000

0.1

0.1

0.1

0.05

0.06

0.06

1.777777778

0.45

0.159043128

0.000282743

Copper

26swg

1000

0.1

0.1

0.1

0.76

0.78

0.76

0.130454341

0.45

0.159043128

2.07479E-05


image00.png

image01.png

image03.png

Conclusion:

The results for the test of length matched my prediction. The graph shows a strong positive correlation with the resistance roughly doubling when the length of wire is halved

The test on diameter gave some patchy results. Although the overall trend matched my prediction, that the resistance decreases with increased cross sectional area, or to be more precise the resistance halves when the area doubles, as the graph shows.

With these two sets of results we can find the resistivity of the material in ohm mm. If you times the resistance of the wire by the cross sectional area you get the resistance of the same wire if the area = 1. Then divide this by the length of the wire to get the ohm/mm resistivity.

image04.pngP=RA/l

Where

ρ= resistance of the material

R= resistance of wire

A= cross section area of wire

l=length

I have then used this formula to test the accuracy of my test results as shown in the table. If the results are accurate then all the calculations done on the copper wires should be the same because the material is the same. As is shown, the majority of the copper wires have a material resistivity of around 2x10-5.

There are a few outliers to this, the main one being the result for 16 swg copper. I think that because the wire is so thick it has less resistance than the leads that make up the rest of the circuit, and the current flowing through it doesn’t take up its “capacity”. This would make its resistance seem higher than it actually is, because R= V/A.

...read more.

Conclusion

I would therefore consider using batteries to power the circuit, giving a try DC current, although I would then have to take into consideration the drop in power as the battery drains.

As is shown in my results, the one major failure in my test was with using the iron core with 25 coils. I found during testing that the amount of weight it could hold became unpractical to measure and also made it possibly dangerous should something break. I also found that with it being so awkward to attach that amount of weight to the core all I was getting was random anomalous results rather than something that had any logic two it. I think not only did I reach the maximum capabilities of my apparatus there, but I also may have been at the very limits of the magnets strength too.

As gauss’ formula shows, the power of the magnetic field is not so much governed by the number of coils as the density of them. This means there is more power available from many layers of coil rather than one big long one. This explains why electrical motors have lots of layers of copper wire. In future tests my magnet could be improved by using these methods.

Overall I feel I did the best I could in regard to the limitations of the equipment I had, although a true DC setup with flawless magnet cores and a larger number of tests would hopefully lead to a better conclusion should I repeat this experiment in the future.

...read more.

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