Monday, December 26, 2011

how the length of a wire affects the resistance

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How the length of a wire affects the resistance


Aim To investigate how the electrical resistance of a wire changes in relationship to its length.


Variables


1.Temperature When electrons pass through a wire, they give off some of their energy to the atoms, the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance.


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.Wire length If the length of the wire is increased then the resistance will also increase as the electrons will have a longer distance to travel and so more collisions will occur. Due to this the length increase should be proportional to the resistance increase.


.Cross Section As the cross-sectional area increases the resistance decreases.


4.Metal Type Copper is a good conductor and is used for connecting wires. Nichrome has more resistance and is used in the heating elements of electric fires.


Below are the type of graphs I would expect-


Hypothesis


I predict that if the length increases then the resistance will also increase in proportion to the length. I think this because the longer the wire the more atoms and so the more likely the electrons are going to collide with the atoms. Therefore, if the length is doubled the resistance should also double. This is because if the length is doubled the number of atoms will also double resulting in twice the number of collisions slowing the electrons down and increasing the resistance. My graph should show that the Length is directly proportional to the resistance. If the length of the wire is only half the length of the wire on the same type of wire, there should be half the number of collisions between the electrons and the atoms.


If the wire is twice as long, there should be twice the number of atoms, resulting in twice as many collisions and a predicted doubling of the resistance.


Apparatus


- Voltmeter


- Cell


- Nichrome wire


- Ammeter


- Crocodile clips


- Metre ruler


Diagram(method)


Below is a diagram of how we set up our circuit. The (V) in the circle is the voltmeter and the (A) in the circle is the ammeter, the line highlighted red is the wire of different lengths that we will be finding how the length of it affects the resistance. The lengths of wire I will be using are 0,40,50,60,70 and 80cm’s long. To get as accurate measurements as I can from the wire I will tape it to a metre ruler and put the first crocodile clip at 0cm and the second where needed, this is also a lot easier for me.


Fair Test


We must keep the surrounding room temperature the same or the particles in the wire will move faster and this may have an affect on the resistance.


The cross-sectional area of the wire must be kept the same as well.


The material of the wire must also be kept the same because different materials have different conductivity. Therefore I will use the same wire throughout the experiment.


Below is a table showing the lengths of wire increasing by 10cm each time starting from 0cm going up to 80cm. The input voltage is V, in the next column is the reading I took from the voltmeter, next is the current and then the resistance. To calculate the resistance of the wire I shall use the equation below-


RESISTANCE= VOLTS÷AMPS


evaluation!!!!


I think from looking at my results that I can say that my predictions were near enough correct. I predicted that the resistance would change in proportion to the length and this is what happened. This is because as the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire causing more collisions and therefore a higher resistance.


I feel that overall my results were quite accurate, this is made clear when we look at the graphs, which shows straight lines near enough, with the acceptation of one point which seems to be off the line a bit but not off enough to completely spoil the pattern. This may be explained by the fact that the wire started to give off a lot of heat therefore affected the result. Another thing that may have proven inaccurate is the lengths of the wires which may not have been straight when taped to the ruler, so I perhaps could have improved the way I measured the wire by maybe using a metal bar that does not bend. For my results to be more accurate I perhaps could have done the investigation a few times and then found the average result.


To expand my experiment I could have also change the widths of the wire, I would have done so if I was given more time.





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