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# Resistance of wire.

Extracts from this document...

Introduction

Background Information

The resistance of an object is how much it opposes or resists the flow of an electrical current through it.  Something with a high resistance will only allow a small current to run through it whereas a smaller resistance will allow a larger current to flow.  Therefore, a good electrical conductor will have a low resistance while a poor conductor/insulator will have a comparatively high one.  Resistance is present in all electrical components to some extent.  In a circuit, resistance is the factor which makes the current slow down.

Several factors affect the resistance of a wire.  These include:

The material the wire is made of.

The length of the wire.

The diameter of the wire

The temperature of the wire.

The Material of the Wire

This factor affects resistance because, in different materials, the atoms are arranged differently.  Metals are good conductors of electricity because the electrons contained in their atoms are loosely connected to their nuclei, therefore it is relatively easy for them to move around and allow electricity to flow through them.  On the other hand, non-metals are generally insulators because the electrons and nuclei are set much nearer together, thus it is harder for an electrical current to pass through them.

N.B – Carbon is an exception to this, as it is an electrical conductor although it is a non-metal.

Atomic structure of a metal                  Atomic structure of a non-metal

The Length of the Wire

The longer a wire is, the more it will resist electricity.  This is easy to understand, as the electricity will have to pass through a longer space, and it will therefore take it longer to reach the other end of the wire.

Middle

6.14

7.3

1.2

6.08

9.3

1.5

6.2

11.5

1.8

6.39

Average:                                  6.175

SWG 28 – diameter 0.375 mm

 Voltage across wire (V) Current in Amps Resistance in Ohms   (Ω) 1.6 0.38 4.21 2.45 0.57 4.3 3.35 0.77 4.35 4.25 0.97 4.38 5.25 1.2 4.375 7 1.6 4.375 8 2 4 11 2.45 4.4 Average:                                      4.3 SWG 26 – diameter 0.45 mm Voltage across wire (V) Current in Amps Resistance in Ohms   (Ω) 1.6 0.52 3.08 2.4 0.8 3 3.3 1.1 3 4.2 1.35 3.11 5 1.65 2.88 6.75 2.25 3 8.75 2.85 3.07 10.5 3.45 3.04 Average:                                    3.02

SWG 24 – diameter 0.56 mm

 Voltage across wire (V) Current in Amps Resistance in Ohms   (Ω) 1.45 0.685 2.12 2.25 1.15 1.96 3 1.55 1.94 3.85 1.95 1.97 4.75 2.4 1.98 6.25 3.25 1.92 8 4.15 1.93 N/A (see below) Average:                                    1.97

Conclusion for Experiment 1

As predicted, those wires with larger diameters had less resistance than the thinner wires.  The following table is a summary of the resistance of a wire compared to its diameter.

 Diameter of wire in mm 0.25 0.3 0.375 0.45 0.56 Average resistance in Ω 8.09 6.175 4.3 3.02 1.97

This is shown on the graphs on the following pages.

Some of my results are printed in italics.  This is because these results are anomalous.  For example, in my experiment of SWG 32 wire, my first result is 7.73 Ω.  This is very low compared to my other results, and may have been so because the wire was cold and therefore had less resistance.   On the next set of results, there is one that reads 6.39, which seems abnormally high.  This is probably because the wire was now so hot that the resistance increased.  I found that although my graph 1/D2 was a straight line, there was one anomalous result, namely 8.09.  The average resistance for this diameter of wire should have been higher.  It wasn’t because, as I explained above, one of the results was 7.73 Ω.  This brought the average down.  I also noticed another anomalous result for the SWG 24 wire – the average went up by 0.02 because one of the results was higher than the rest (2.12 Ω).  I cannot explain this result, as it was the first in a series of results, and therefore, if anything, should have been lower than the rest, not higher.

I explained in my preliminary experiment that for the SWG 24 sized wire, I would only be able to measure the resistance up to 10 V on the power pack because for 12 V, the current would be so high that I would not be able to measure it on my ammeter – therefore, I would not be able to find out its resistance.

Experiment 2

Testing how the length of a wire affects its resistance

Again I used Constantan wire for this experiment.  However, since I was testing how the length of the wire affected its resistance, that was the only factor I changed for this experiment.  I used SWG 28 wire (d = 0.375 mm) for this experiment, and used wire ranging from 20 cm to 1 m.  As opposed to the micrometer which I used to measure the diameters in Experiment 1, I used a metre ruler to measure the length of the wires in this experiment since the micrometer was not long enough to measure them.  This probably introduced some error into the measurements.

20 cm wire

 Voltage across wire (V) Current in Amps Resistance in Ohms   (Ω) 1.1 1.3 0.85 1.75 2 0.875 2.3 2.7 0.85 2.9 3.35 0.87 3.6 4.1 0.88 N/A (see below) Average:                                0.865

40 cm wire

 Voltage across wire (V) Current in Amps Resistance in Ohms   (Ω) 1.3 0.78 1.67 2.1 1.2 1.75 2.896 1.6 1.81 3.6 2.05 1.76 4.45 2.5 1.78 6 3.4 1.76 7.5 4.25 1.76 N/A (see below) Average:                                   1.76

Conclusion

Between the recording of each result, I turned the power pack off in order to try and keep the temperature constant and my results fair.  However, it was inevitable that the wires would heat up a little while the power packs were on, especially as it took some time to read the results off the ammeter and voltmeter.  One way to avoid this problem would have been to change the wire between each test.  I did not do so because it would have been far too time consuming and wasteful.

Possible Extension

I mentioned at the beginning of my coursework that there are four factors which affect resistance – namely material, length, diameter and temperature.  Of these, I only tested to see how length and diameter affected resistance.  I could have extended my investigation in two ways:

• By testing wires made of different kinds of metals to see which metals are more resistant and which allow current to flow through them easily.
• By changing the temperature of the wires I used to see how temperature would affect resistance.  The problem with this would have been finding a way to vary the temperature of the wire.  However, had I done this experiment, I would probablyhave found that hot wires offered more resistance than cooler ones.

Bibliography

Balanced Science 2 – Cambridge University Press – Jones, Jones, Marchington & Acaster

Science Through Diagrams – Oxford Revision Guides – George Bethell

Encyclopaedia Britannica (CD-ROM version)

A New Physics – D Bryant & D G Kershaw

Encyclopaedia Encarta

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