Equipment:
- Leads x 8
- Wire (1 metre)
- Ampmeter
- Voltmeter
- Cell
- Bulb
- Meter Stick
- Cellotape
- Crocodile Cilps x 2
Preliminary Method:
I set up the circuit using only one cell, an ammeter and a voltmeter all in series. Between the ammeter and the voltmeter I placed a bulb and the component, the wire. The wire was taped to a meter stick so to make sure that all the measurments were as accurate as possible to the nearest millmeter. I attached the wire to the leads by using two crocodile clips, one at each end. I then took readings from the ammeter and the voltmeter at 1000mm, 800mm, 600mm, 400mm and 200mm. I took each reading three times then averaged them out to make sure my results were more reliable and accurate. From this I then worked out the current.
Preliminary Results:
Problems:
However many problems did occur with this first experiment. First of all the ammeter and voltmeter kept comming up with negative readings or none at all. The wires kept comming loose and the wire was not straight so the measurments would not of been accurate to the nearest millimeter. I then also realised that the bulb and maybe the cellotape its self was a resistor so it would of made it an unfair test as I would of then of been testing the resistance of two resistors. It also was hard to get the wire to 1000mm as it kept comming unattached to the meter stcik and the wire its self was only just over a meter.
Solutions:
I found soloutions to the majoraty of my problems.I replaced the ammeter and voltmeter to see if that made a difference but it didn't so I checked to see that I had out the correct leads into the correct places so opposites were together (positive to negative) and the same charges wern't (not positive to positive). I did have this problem at the ammeter and once I changed I was gettin positive readings. The voltmeter however was still showing negative readings even after it was connected properly. I then found out that "The Voltmeter must be placed in parallel around the component under test" - page 86, AQA Modular Science Early Modules, Edited by Richard Parsons. I replaced several of the wires to make sure that they were not loose, and tried as best as possible to straighten the wire. I did this using the side of a ruler running it along it. I also tested the cellotape to see if my readings changed while having it on there which they did slightly so I only taped it down on the side of the wire that wasn't connected in the circuit to make it a fair test. I decided to make the maximum measurment of 800mm so the measurments of the wire can be made more accurate.
Improved Experiment:
I carried the experiment again taking into consideration how my soloutions could make it a fairer test and much more accurate. I placed the cell, ammeter and the wire in series the placed the voltmeter in parallel to the wire. I placed the cellotape on the two sides of the wire that were not connected to the circuit so not to affect the results at all. I then took the readings from 800mm to 50mm going down by 50mm each time. I decided to take more results to make it as accurate and reliable as possible. I took three readings of the voltmeter and the ammeter at each measurement and then averaged these out to work out the amount resistance that there was.As we took the results we observed that as the current went down the potential difference went up.
Problems & Solutions:
There were still problems that came up during this experiment. First of all I had to change the attery half way through as it seemed to of stopped working so I had to replace it, therefore having to do take the reading all over again incase the voltage was slightly higher or lower and it woiuld of made the whole experiment unreliable and inaccurate aswell as an unfair test. Some of the leads kept comming out again so we had to make sure they wouldn't by changing them therfore having to redo the results again incase the wires inside the leads were different and affected the current. The wire also kept becoming unattached to the meter stich therefore making it unstraight, this was simply solved by just applying more cellotape to the parts of the wire that wern't connected in the circuit.
Results:
Conclusion:
Looking at my results and my graph my predication was correct. As I increased the length of the wire the resistance also increased and as I decreased the length of wire the resistance also decreased. I can see this clearly from my table of results and graph. Looking at my graph you can see that how the resistance of a wire varies with length has a positive gradient and there is also a positive correlation. It looks quite accurate as no anomalies appear on it. As the length of the wire increases, so does the resistance in the circuit this is also said to be proportional.
I have researched this and have evidence to back up that my results are correct. “The resistance and length of a wire are directly proportional. The longer the wire the greater the resistance. If you double the length of the wire then you double the resistance across it. This is because s the wire becomes longer, the 'electrical slope' (potential difference) across a given length becomes less steep. As the potential difference becomes smaller, so does the current, as the changes have less of a 'potential gradient' to 'push' them along in the wire. A smaller current flows, therefore the resistance must be higher.” (www.cyberphysics.pwp.blueyonder.co.uk/topics/physics/electric/resistance/LENGTH/Physics.htm)
This is all beacause of what happens inside of the wire, the atoms in the wire and how the negativly charged electrons are colliding with the atoms and are then being caused to vibrate and therefore slow down. this then takes longer for the electrons to go through the circuit therfore causing resistance and making the rate of resistance higher as the lenght of the wire is increased.
"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." - http://www.sci-journal.org/reports/vol3no1/v3n1k44.html
Evaluation:
Despite the problems that occured during this experiment I think that overall it was quite succesfull. There were some factors that could not of not prevented in the classroom, like that of the temperature but that had to be overlooked. My teacher said " The temperature may affect the resistance in your circuit and may threrfore affect you results slightly." Inside the wire the temperature increases as the resistance does because the current begins to use alot more energy to get around the circuit and go against the resistance. This uses more electrical energy and then from that creates more heat energy.
I don't think that my graph of results was directly proportional, this could of been due to the flucuations that occured during some of our readings from the inaccuracy of the voltmeter and ammeter or just some of the leads or the wire not being secure enough. Although none of our results relly stande out as being "wrong" or not in line with the pattern of the rest of the results.
If I could redesign or of improved this experiment I would pay more attention to the fact that temperature is another factor which affects resistance within the circuit. I would try to counteract that by maybe leaving the wire to cool down between each reading, but making sure it for the same amount of time each time so to keep it a fair test. I would also use more accurate equipment such as a power pack to keep the voltage the same throughout to make the readings more reliable. I would also use a piece of wire that has the same cross-section throughout as that also affects the rate of resistance. I would then use a more accurate device to measure the wire and a more stable way to keep it all in place, as sometimes the crocodile clips kept comming out of place or someone might of accidently of knocked them without realising. I would also use a more accurate ammeter and voltmeter as some of my results flactuated from time to time so I would use ones which go further than to just two decimal places.