My prediction is that as the wire gets shorter the resistance become smaller e.g. if it is half the length it should have half the resistance. I think this because the wire is full of atoms and the electrons have to try and get through it. So if the wire is short there is less atoms so the electrons will travel easier. If the wire is long there are a lot more atoms the electrons have to travel through more atoms making it harder to travel through the wire.
Resistance is caused by the internal structure of the metal getting in the way of moving electrons. The more difficult it is for the electrons to move the higher the resistance.
Equipment
Power pack
Digital voltmeter – 10 Amp Maximum. Measured to nearest hundredth of an amp.
Digital Ammeter – 20 Volts Maximum. Measured to nearest hundredth of a volt.
One meter of copper wire
Five connecting insulated wires
Two crocodile clips
Method
Collect the above apparatus and assemble as shown above. Then keeping the voltage the same for every reading, measure the current with the wire at different lengths. Start at 100 centimetre and move down in increments of 10. When you get to 10 go down in increments of 1. Take 3 readings for each length. Record your results to work out the resistance using this formula:
V/I = resistance then make an average of the 3 readings.
I am going to take 3 readings for each length from 100cm to 1 cm.
Trial Experiments
First of all I needed to see how much voltage it takes to heat up the wire. I needed a high voltage but not so much that the heat would affect my results because it’s a factor that affects the rate of resistance.
These were the first set of results we got:
From this I realised the wire was already getting too hot. I also tried a much higher voltage of 4.5 with a longer length wire. This also got warm so I decided to have a voltage of 5 when taking readings from 100cm to 10 cm. I decided to lower the voltage to 0.5 when taking readings under 10 cm.
The reason that from 100 down to 20 cm the readings are only in 10cm increments is because in between there was not much difference but under 10cm there was a large difference between each one centimetre.
I also found out from my trial experiments that to make a good contact I had to wrap the wire around the crocodile clips several times.
Fair testing
I will make my experiment a fair test by keeping the cross sectional area, the material of the wire and temperature of the wire the same. I will do this by using the same wire each time and making sure the wire doesn’t heat up too much, but I have no control over the temperature so I’ll just have to take it into account.
I will make sure that the same apparatus was used (i.e. crocodile clips and leads) as they can affect the resistance in the circuit. Also to make it a fair test, I will make sure only one of the factors listed in my aim will be changed. I will measure the length of the wire to the nearest millimetre. I will use the same material throughout the experiment. I will also make sure that no magnetic materials are placed anywhere near the experiment as it too can affect the resistance.
Analysis
My graph tells me that when the length increases the resistance does too. My graph has a strong positive correlation. This means that the line is sloping up and all the results are near the line of best fit. This shows that my results are steady and I think fairly accurate.
I predicted that doubling the length of the wire doubles the resistance. For the results between 100 and 20. This roughly applies but should not be used to find accurate results. It doesn’t work between 1 and 10. I think this is because the gaps between each measurement are too small.
The graph of the experiement is a straight line through the origin, which means that resistance is directly proportional to the length. This is because if you double length, you double the number of atoms in it, so doubling the number of electron ‘jumps’, which causes resistance: The results support my predictions well, the results turned out the way I had expected.
Evaluating
My results were obtained with care but I had a slight problem when trying to obtain my results one time. Some of the equipment was faulty so I replaced all of it. I did this because I thought the experiment would be more accurate this way.
There was one anomalous result. This was the average for the 40 cm length. I don’t know why this happened. The resistance here was the same as for 30 cm. I think I would have to do more experiments and repeats to discover why this happened. It could be that the wire had heated then and so there was less resistance then it should have been so it matched up to the 30 cm even though it is 10 cm longer. It could also have been because of a large kink in the wire.
I think my method was good. This is because I got constant results with only one anomalous. There was a steady correlation and the results matched up to what I had predicted.
To get more reliable and accurate results I could use ammeters and voltmeters that go to more decimal places i.e. further then to the nearest hundredth. I would need to use new wire each time because throughout the experiment we got kinks in the wire. This would have changed my results because it alters the length of the wire.
To help back up what I have already done I could investigate the other factors that affect resistance. I could try the same experiment again but get a wider range of results. I would do this buy using wire longer then 100cm and I could measure in increments smaller then 10 cm. I could also repeat my experiment more times, for example do each length 5 times.