The length of the wire will also have an effect on the resistance of the wire. The longer the wire, the more particles the current has to find a route through. This is the variable that I will be changing and investigating. Any change in the resistance of the wire in my experiment should be due to the change in length.
Method
Apparatus:
- Power pack at 6V
- Metre ruler
- Connection leads
- Protective thermistor
- Nichrome wire
- Crocodile clips
- Ammeter
- Voltmeter
- Micrometer
Protective
thermistor
Nichrome wire
I am going to set up my circuit as above, using nichrome wire as the variable. To work out the resistance, I shall rearrange the equation V = IR to R = V
I
I am going to measure the current and voltage of the wire at different lengths. I have chosen to conduct my experiment using seven different lengths between 10 and 70cm at 10cm intervals. I have decided to use this wide range of lengths to make my results clear. I will do three readings at each length and produce an average to be as accurate as possible. If I have any anomalous results then I will repeat them to be more precise.
I am going to measure the different lengths using the metre ruler. The first crocodile clip will be clipped to the wire at the 0 cm position on the metre rule, the second to the place on the wire which I will be measuring. I will measure the current and voltage once at each length then repeat this three times.
I will measure the length of my wire to the nearest millimetre. My voltage and amp readings will be taken digitally by the same person and will be recorded to two decimal places. Also, the experiment will all be done on one day to keep the room temperature constant. This will make the experiment as fair as possible.
Safety
There are a number of safety issues that I need to take into consideration when carrying out my experiment:
- Over heating – Lengths lower than 10cm will not be tested and the power will be turned off between each reading
- The experiment will not be undertaken in any wet areas as electricity could be conducted through the water to someone.
- The protective resistor must not be touched as it will heat up significantly during the experiment.
Pilot Test
Before the main method was carried out, a preliminary experiment was carried out to determine whether a thick or thin wire should be used and to decide what the range of lengths should be for the nichrome wire.
After doing this preliminary experiment, it was decided that nichrome wire lengths fewer than 10cm should not be used as the wire overheated considerably and almost melted. Also, it was decided that the thicker of two wires should be used as this would present me with clearer results.
Prediction
As the length of the wire increases, the number of collisions between electrons and the molecules of the wire will increase because the current has further to travel. These collisions will result in an electrical energy loss (the electrical energy will become heat). The current will decrease because fewer electrons will be able to travel through as the wire gets longer. The voltage will increase because more push will be needed to get the current round the circuit. As the voltage increases and the current decreases, the resistance will increase. I therefore predict that as the length of the nichrome wire increases, the resistance will increase in direct proportion. For example, if the length of the wire doubles, so will the resistance. This means that my graph of my results should be a straight ascending line.
Results
Analysis
From my graph I can see that an increase in length resulted in an increase in resistance. The straight ascending line of the graph shows that the length of the nichrome wire is directly proportional to the resistance. This is because the longer the wire becomes, the more particles the electrons collide with and loose energy to, increasing the resistance.
I can see from the graph that nichrome has quite a low resistivity, or ability to resist current, because it is a conductor. I can tell this because the gradient of the graph is not very steep. This confirms the resistivity of nichrome, 130 x 10-8.
The equation for resistivity backs up my results:
R= l The in the equation stands for resistivity.
A
This equation shows that resistance is directly proportional to length, as the length is part of the numerator of the equation.
The points of my graph follow my line of best fit very well. This shows that the width of the wire did not dramatically affect the resistance. If some points of the wire had of had a much larger cross sectional area, then the graph would not be a straight line and the resistance would decrease. The resistance would decrease because the current would be able to find a path through the wire more easily. These diagrams show this theory:
As my results run in quite a smooth line, I can see that temperature has not affected my experiment. If the wire had of heated up too much then the resistance of the measurements taken of the hot wire would have increased dramatically. The current would have found it difficult to make a route through the vibrating particles and so the p.d. would have increased as the current decreased, leading to a much higher resistance. These diagrams demonstrate this theory:
Conclusion
My results prove my prediction right. I envisaged that the resistance would double if the length of the wire doubled, and it did. For example, when the nichrome wire was 30cm long, the average resistance was 2.13 Ω and when the wire was 60cm long, double 30, the average resistance was 4.21 Ω, which is roughly correct.
This means that whatever you do to the length of the wire: halve it, quadruple it or double it; the resistance will always do the same, as it is directly proportional to the length.
Evaluation
The current and voltage in the experiment were measured using an ammeter and a voltmeter respectively. These two devices were digital and so provided very accurate results. This is better than using ones with dials as digital readings eliminate possibility of human error.
The graph results sit or very nearly sit on my line of best fit. There are no anomalous results, proving that my method was successful. Any results that do not entirely follow the line of best fit may have been affected by some of the variables mentioned in the planning. These slight dissimilarities in the results may be due to the measurement of the wire. The measurements were only done by sight and so may not be as accurate as they could be. The wire was also not completely straight. If I were to repeat this experiment, I would use a straighter wire.
Also, the diameter of the wire was not uniform all the way along the wire. As shown in the table of results, the thickness varied by 0.02mm. This could have affected the resistance due to the facts stated earlier in the planning.
Another variable that could have altered was the temperature of the wire. As I never measured the temperature of the wire or of the room temperature, I cannot be sure that they stayed constant. This did not obviously affect the experiment severely because the results followed my prediction and scientific evidence very well.
More experiments could be done to investigate the other variables that affect the resistance of the wire:
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Thickness of wire. Wires with different thicknesses could be tested to find the effect on the resistance. In this experiment, the length, wire material and temperature should all be kept constant. I predict that the resistance would decrease as the wire thickness increases showing that the resistance is inversely proportional to the diameter of the wire.
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Material of wire. Different types of wires, such as nichrome, copper and aluminium could be used in place of the nichrome wire on the circuit I made. The wire length, thickness and temperature should all be kept constant. In this experiment, the resistivity of different materials will affect the resistance. Using the equation = resistance x CSA/ length, the resistivity of each material of wire could be calculated. (CSA = cross sectional area)
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Temperature of wire. Resistance could be tested in the same circuit at different temperatures. The length, width and material of the wire should all be kept the same. I predict that as the temperature increases, the resistance will decrease because current does not pass through hot wires as easily.