Wednesday, February 13, 2008

What Factors Affect the Resistance of a Wire? (1)







Nicola Tregay
Mill Chase School, Bordon, Hampshire, UK


http://www.sci-journal.org/index.php?template_type=report&id=26&htm=reports/vol3no1/v3n1k44.html&link=reports/home.php

Hypothesis

I am going to investigate what factors affect the resistance of a wire. There are three main factors which affect the resistance of a wire:
The material of the wire. (What the wire is made out of)
The length of the wire.
The thickness of the wire. (The diameter of the wire)

Resistance is a force which opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.
e.g. The filament across an electric light bulb.

Resistance is measured in ohms. The symbol for an ohm is Ω . A resistor has the resistance of one ohm if a voltage of one volt is required to push a current of one amp through it.

George Ohm discovered that the emf of a circuit is directly proportional to the current flowing through the circuit. This means that if you triple one, you triple the other He also discovered that a circuit sometimes resisted the flow of electricity. He called this resistance. He then came up with a rule for working out the resistance of a circuit:

V/I = R

V - volts
I - current
R - resistance

To begin with I am going to investigate which materials put up the highest resistance and I will combine it with the investigation about which length puts up the greatest resistance.

The type of material will make a difference because the electrons have to pass through the material. These electrons find it easier to pass through some materials than others. In this experiment I am going to use copper and nichrome wire. I predict that the nichrome wire will have a higher resistance than the copper wire. I can say this because I know that the electrons have to squeeze together more in order to be able to pass through nichrome wire than they do in order to pass through copper wire. (The more the electrons bump together, the higher the resistance.)

The length of the wire will make a difference. This is because when you have a long wire, the electrons have to squeeze together for longer to be able to pass through the wire than they do in order to be able to pass through a short wire. I predict that the longer the wire, the greater the resistance. If I had a 30 cm wire and a 60 cm wire, the 60 cm wire would have a resistance twice that of the 30 cm wire.

For this experiment I will be using a voltmeter, an ammeter, five wires, two crocodile clips, some nichrome and copper wire and a power pack. The wire will be: 26 swg of copper and 26 swg of nichrome. The diameter of the wire will be kept the same so that it is a fair test. The voltage will also be kept the same, although the readings may not be exactly the same each time. For the lengths I will be using 10, 20 and 40 cm wires, this is because I can then see if my prediction about doubling the length is correct.

To make this test fair I should take more than one result so that I can work out an average, this will help prevent any wayward results.

Method
Collect apparatus: a voltmeter, an ammeter, 5x wires, 2 crocodile clips, 10, 20 and 40 cm of both nichrome and copper wires and a power pack.
Set apparatus up as shown:

Set the power pack on as low a voltage as possible. (So that there is not too high a current passing through the circuit.)
Place the 10 cm of nichrome between the two crocodile clips to complete the circuit.
Turn on the power pack and record what the ammeter and voltmeter read.
Replace the 10 cm of wire with the 20 cm of nichrome remembering to keep the voltage the same. Turn on your power pack and record what the ammeter and voltmeter say.
Change the wire to the 40 cm of nichrome wire and repeat the experiment.
Do the same for the copper wires.
Work out the resistance for all the results using Ohm's law. V = I x R
Record your results in a graph.

Conclusion
My hypothesis was correct. The nichrome wire has a higher resistance than the copper wire and the longer the wire, the higher the resistance.

Ohm's law states that the current flowing through the circuit is directly proportional to the voltage applied. (If you double one, you double the other.)

I worked out the resistance of the wires by using the formula:
V/I = R

This happens because of the electrons that flow through the wire. These electrons travel at a steady pace, when they come to a different piece of wire, they have to slow down in order to be able to pass. (This is why the current differs). While moving through the wire, the electrons need to squeeze together. This is because there is not enough room/space for them to pass evenly through. The more the electrons have to bump together then the higher the resistance. This is because it will take longer for them to pass from one side of the wire to the other side. This is because the current is slowed down. (The longer the wire, the longer the electrons have to stay squashed together, and so the longer they take to pass through the wire and the higher the resistance. The material of the wire makes a difference because it is harder for electrons to pass through some materials than it is for them to pass through others. (Some wires cause the electrons to bump together more than others.)

I put my results into graphs although they are not on the same axes because the resistance for copper wire is so low. I had to draw a line of best fit on all the graphs because there were a few wayward results.

The graphs which compare the length of the wire to the resistance it gives travels in a straight line through the origin. This means that the size of the length is directly proportional to the resistance it gives. I can work out the gradient of this line by dividing the height of the line by the width. (Gradient = height/width)
The gradient of the graph for copper wire is: 0.01/9 = 0.001
The gradient of the graph for nichrome wire is: 1/16 = 0.0625
As the gradient of the second graph is higher. I can say that the resistance for the nichrome wire increases more rapidly as the length of the wire increases than the resistance for the copper wire. The results for the copper wire have a pattern to them. The result for 10 cm is 0.011, and the result for 20 cm is 0.022, so I can predict that the result for 50 cm will go 0.055 and so on.
The other graphs which compare the current of the wire to the length of the wire also travel in a straight line. These graphs however travel downwards, this means that they will have a negative gradient. As the current decreases, the length of the wire increases.
The gradient of the copper graph is: 30/-12 = -2.5
The gradient of the nichrome wire graph is: 0.2/-8 = -0.025
As the gradient of the second graph is lower I can say that as the length increases the flow of current decreases faster in copper wire than in nichrome wire.
There were a few wayward results. This could have been because the wires were not exactly the correct length or because I did not read the voltmeter and ammeter accurately. The wires might have been over-heated The temperature of the wire will affect the resistance. Hotter wires will have a higher resistance than cold wires. To improve my results I should have obtained more than one result for each wire and length. I could have then worked out an average for more accurate results.

The method I used was fine, although it was extremely hard to collect any results for copper wire. This is because copper has such low resistance. It has a low resistance there will be a large current. The ammeters we have in school cannot read such a high current. Without the current I cannot work out the resistance of the wire. I ended up using some results I had already collected the last time I did this experiment.

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