We are going to do two experiments: a preliminary experiment and an actual experiment. Both experiments will be testing the resistance, voltage and current of six different length wires. We will carry out the preliminary experiment as a tester to make sure it works and if anything doesn’t, we will know what to change for the actual experiment.
The six different lengths on the wire board that we will test are:
- -10cm
- -20cm
- -30cm
- -40cm
- -50cm
- -60cm
The Independent Variable is the length of the wire. The independent variable is the variable that is being changed.
The Dependent Variable is the variable that changes in result of the independent variable being changed. The dependent variable is the resistance.
The Controlled Variables are the temperature of the room, the thickness of the wire and the material of the wire. The controlled variables are the variables we are going to keep the same, to keep it a fair test.
Why are we keeping the current the same?
We must keep the current the same because if we change the current as well as the resistance it is changing two variables, making it an unfair test. Changing the current may also affect the heat of the wire, which will also affect the resistance, making my results unreliable. The temperature changes the resistance because the temperature is another independent variable and it changes the dependent variable which, in this case is the resistance. The current is a control variable, this means that we will keep it the same.
Prediction
I predict that the resistance will increase as the length of the wires increase because, from previous research and the science behind resistance, I have learnt that the longer the wire is, the more atoms there are. This means that there are more atoms for the electrons to collide with. So the more the electrons are colliding, the higher the resistance is.
Method
This is a circuit diagram for the preliminary experiment:
Battery Pack Ammeter
Voltmeter
Rheostat
Method for Preliminary and Actual experiment:
To do this investigation we made a circuit , I will tell you how we constructed the circuit in a few simple steps.
Firstly we collected all of our equipment, Ammeter, Battery pack, Wireboard, Wires, and Rheostat etc. (RHEOSTAT IS NOT INCLUDED IN PRELIMINARY, ONLY IN ACTUAL EXPERIMENT)
Next we drew up a table to record our results.
Thirdly, we up the circuit with the wire length 10 cm and then increasing the length eventually to 60 cm.
We repeated this for all the lengths and another two times for the actual experiment.
After recording all our results we worked out the resistance of each length by using the triangle shown below.
If our preliminary experiment goes as planned then the actual experiment will have the same method, but we will repeat the experiment three times so we get three sets of results to get more accurate results. We will also draw a graph from our actual results.
Our preliminary experiment was successful, however, we want to keep the current the same throughout so we will add a rheostat to our circuit. We are keeping the current the same to stop the temperature of the wire changing, because if the current changes as well as the wire length, then we will be changing two controlled variables which I mentioned earlier on, making the test unfair. This means our method for our actual experiment will be the same as for the preliminary, but our circuit diagram will change.
This is a circuit diagram for the actual experiment:
Results
A graph showing the average results for my Actual experiment
Conclusion
As predicted the resistance of the wire increased as the length of the wires increased.
As I predicted earlier, an increase in length resulted in an increased resistance. The resistance increases because there is more wire for the electrons to pass through, which means that the electrons can collide with more metal ions in a longer wire than shorter. This can be clearly be seen in the graph. The line of best fit on the graph travels in a straight line directly through the origin. This means that the length is directly proportional to the resistance it gives – when you double the length the resistance also doubles (double length means double number of collisions). I can work out the gradient of this line by dividing the height of the line by the width. (Gradient = height/width).
I think my results are very accurate because they follow a strong positive correlation and mainly fit the line of best fit. The line of best fit is a line that is drawn on a scatter graph as close to all the points as possible, which clearly shows the trend between two sets of data. The line of best fit is touching all of my points except one, showing that my results are very accurate and that the point not touching the line must be an outlier. The only anomalous point that I would re-do is my experiment for Test 2 with a 40cm wire because it doesn’t follow the line of best fit as well as the rest of the points do, re-doing this would give me even more accurate results. As you can see from my graph with the average results, the results for length 40cm doesn’t follow the line of best fit as well, this is because one of my results seemed faulty.
On my graph to show how the resistance of a wire varies with the length of wire for my actual experiment, it looks like I have only collected one or two results; this is because my results for all three experiments are so similar they overlap each other.
As long as you make sure your test is fair then collecting reliable data is very simple, but you must repeat your experiment at least three times to make sure there aren’t any outliers in your results.
The gradient of a line is the change in the y-axis divided by the change in the x-axis:
I have found out by using scientific knowledge and my experiment to back it up that the resistance of a wire increases as the length of the wire increases, this is what makes the graph a positive correlation. Looking at scientific knowledge that the longer the wire length the higher resistance shows that my results are very accurate and reliable.
The gradient of a line is the slope of the line, the higher the gradient then the steeper the line. The gradient of my graph for my actual experiment is 0.1884. This means that the resistance is 0.1884 Ω per centimeter of the wire. The resistance per cm stays constant throughout the experiment; so if you want to work out the resistance per 10centimetres of the wire you would times 0.1884 by 10. Also all of the error bars were so small (when not including the anomaly) and didn’t show up on the graph – this shows the results were reliable as they had only very small range.
Evaluation
From the start of our experiment we had to successfully achieve out our aim. This was our investigation on how the length of a wire affects the resistance which I think was a success. I was successful because I was well prepared for my experiment and I was prepared for all of the three tests. I think that all of the experiments we carried out went really well and I was able to find clear and accurate results from the repeated tests.
At first we didn’t include a rheostat which made the current change as well as the resistance, so changing two variables made it a fair test. To solve this we made the results another set of preliminary results. We added a rheostat to our circuit and did another three experiments; the only outlier in our experiment was in our second experiment when we measured the resistance with a 40cm wire. I would repeat the experiment again next time to get a more accurate result. I found that our wire board seemed very worn so next time I would use a new wire board as I think this would make my results more accurate and reliable. The reason why I think this would affect it is because where the wires had been used they seemed loose and at the end of the wires it had unravelled a bit; this could slightly affect the length of our wires. My results did not change significantly from my preliminary experiment although I found that keeping the current at a smaller value makes the resistance lower.
In conclusion I think that I have successfully managed to investigate how the length of a wire affects the resistance of a wire. I have gained accurate and a clear range of reliable data, and I have also presented the data in scatter graphs and discussed the anomalous result, and the trends and correlation of the data. I enjoyed this task because we were allowed to choose our way of investigating our experiment.
