To Investigate How the Resistance of a Constantan Wire Changes When Length Changes.
Physics coursework plan
Aim- to investigate how the resistance of a constantan wire changes when length changes.
Key factors-
* Thickness of wire: if the wire is thin, the electrons are forced to travel through a smaller area. This results in them colliding more often so they give up more of their energy to surrounding particles.
* Temperature: resistance increases as the temperature increases. This is because atoms in the wire are oscillating faster. When an electron collides with an atom, it loses energy. If the wire cools, resistance will decrease.
* Length of wire: the longer the wire, the larger the resistance. More particles are in the way, so electrons find it hard to flow. Also, a longer wire means the electricity is forced to travel further. Electrons bang into each other so resistance increases. More energy is used.
* Conducting material of the wire: if the metal is a good conductor, there is less resistance due to the fact that more current can flow. If the material isn't a good conductor, less current flows through, so there is a larger resistance.
Another way in which the conducting material of a wire affects the resistance is that some metals have a lot of electrons that are mobile (able to move). In this situation, there is less resistance. It is more difficult for current to flow through when there are few electrons able to move. This results in a higher resistance.
* Wire density: If the wire has a higher density, the resistance will be higher. This is because the wire contains more atoms in a smaller space, creating smaller and less gaps for the electrons to flow through. Because there is a lack of space, there should be more collisions of atoms and electrons.
I have chosen to investigate the length of wire
Prediction:
In electricity, the property that changes electrical energy into heat energy, in opposing electrical current, is resistance. Something that atoms of all conductors have in common is that they have free electrons in the outer shell of their structure. As a result of the structure, in all conductive atoms, the outer electrons are able to move about freely, even in a solid (in this case, the constantan wire). Electrical current is formed when there is a voltage across a conductive material and all of the free electrons arrange themselves in lines, moving in the same direction. Resistance comes across when the charged particles that make up current collide with other particles in the material. As the resistance of a material increases, so does the force required to drive the same amount of current.
I predict that every time I increase the length of wire, resistance will also increase. This is because the longer the conductor, the more particles there are in the way. Therefore, electrons find it hard to flow. Another reason why I think the resistance increases as length of wire does is that the electricity has to travel further. The electrons bang into each other, increasing the resistance. More energy is being used because electrons have to travel further. Also, the number of atoms increases as the length of wire does, so resistance increases. More atoms collide with electrons.
I predict that because the metal wire has a lot of mobile electrons, it is easier for current to flow through, so this will cause a low resistance.
The constantan wire is very thin; therefore there is a smaller area for electrons to travel through. On their journey from the negative terminal to the positive terminal, they collide more often so they give up more of their energy to surrounding particles.
It is unlikely, but if I choose to increase the voltage on the power pack, the resistance will decrease because there is more push.
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I predict that because the metal wire has a lot of mobile electrons, it is easier for current to flow through, so this will cause a low resistance.
The constantan wire is very thin; therefore there is a smaller area for electrons to travel through. On their journey from the negative terminal to the positive terminal, they collide more often so they give up more of their energy to surrounding particles.
It is unlikely, but if I choose to increase the voltage on the power pack, the resistance will decrease because there is more push.
I know that as resistance increases, the temperature in the wire will go up. This is because the atoms in the wire are oscillating faster. When electrons collide with an atom, it loses energy. So, I predict that as the wire cools, the resistance will decrease. I believe that the rate at which the resistance of the wire increases will be directly proportional to the length of wire. Therefore, R?L. I predict that the graph to illustrate this, should look like this:
I think there will be super conductivity. This is because the metal wire loses resistance when the wire is at a low temperature.
I will draw a graph showing length of wire against resistance. I predict that the equation for the graph will be R= ?L
A
This formula would apply to prove that the resistance is proportional to he length of wire. The terms in the formula are-
R represents resistance
P represents resistivity
L represents length of wire
A represents the cross- sectional area of the wire
Another thing I predict is that as the length of constantan wire doubles so does the resistance.
Apparatus:
The apparatus I am using are:
* 100cm ruler: to measure the wire accurately.
* 100cm wire: to experiment on.
* 1 ammeter: to measure the current in the circuit.
* 1 voltmeter: to measure the voltage in the circuit.
* Connecting wires: to join up the devices and make a complete circuit.
* Crocodile clips: to connect the wires to the devices.
* 1 power pack: to supply AC current and control the voltage in the circuit.
Diagram:
Method:
. Collect all of the equipment. Set it up like in the diagram. Make sure the ammeter is inserted in series and the voltmeter is placed in parallel. Switch on the power pack. It should be set to 2 volts.
2. Set the 100cm wire to the right length. The first measurement is 0- 10cm, so two conducting wires with crocodile clips should be connected to 0cm, and 10cm.
3. Record the current in amps, and the voltage in volts into the results table.
4. Repeat the process from step 2. Remember that one crocodile clip should always be on 0cm, and the other one moves up 10cm each time, so the length of wire does increase.
5. Repeat the process from step 4 two more times.
6. Using your results, work out the averages of the current and the voltage.
7. Using the formula R= V/I, use a calculator to work out the resistance in ? ohms.
Fair test- in this experiment, we are only changing one factor- the length of the wire. There are particular factors that we have to keep the same in order not to alter the correct results.
- Do not alter the positions of the devices during the experiment.
- Make sure the total length of wire is exactly 1m.
- Record the current and voltage accurately, using the correct units.
- Always place the crocodile clips on the right measurement.
- Before you start the experiment, test the devices being used. If any are faulty, change them.
- Leave the power pack set at the same voltage for the whole of the experiment.
- The surrounding room temperature must be kept, otherwise the particles in the wire will move faster (if the temperature increases). Therefore, this will have an effect on the resistance.
- The wire along the metre ruler must be straight and exactly 1m long. Bends in the wire may affect the resistance.
- The reading of the voltage should be taken promptly after the circuit is connected. This because as soon as a current is put through the wire, it will get hotter. I want to test the constantan wire when heat is affecting it the least.
Safety-
- Do not set the power pack voltage to more than 2V. This is a safety hazard.
- If you smell burning, promptly switch off the power pack from the mains.
- Make sure that when the power pack is switched on, the near by taps are switched off. If there is any water spilt near by the sockets or surrounding areas, wipe it before you start the experiment.
- Make sure the power pack cable or the conducting wires aren't frayed.
Results table-
Length (cm)
Voltage (V, volts)
Current (I, amps)
Resistance (R, ohms)
2
3
Average
2
3
Average
0- 10
0- 20
0- 30
0- 40
0- 50
0- 60
0- 70
0- 80
0- 90
0- 100
Preliminary results-
Length (cm)
Voltage (V, volts)
Current (I, amps)
Resistance (R, ohms)
0- 10
0.83
3.43
0.24
0- 20
.26
2.34
0.54
0- 30
.52
.76
0.86
0- 40
.63
.45
.12
0- 50
.72
.22
.41
0- 60
.76
.08
.71
0- 70
.82
0.90
2.02
0- 80
.88
0.82
2.29
0- 90
.92
0.74
2.60
0- 100
.93
0.66
2.92
The reason for doing my preliminary results was to get an indication of the patterns that would occur in the results. Also, it was a practice so if I made any mistakes then, they could be fixed so that for the real experiment, minimal mistakes would be made. These preliminary results are very useful as from them, I can check to see if my prediction was correct.
Analysis
Now that I have completed the experimental side of my investigation, I can use the results to find patterns to explain if my prediction is correct or understand why it is incorrect.
Explanation of results-
As the length of wire increased, the current decreased.
As the length of constantan wire increased, so did voltage.
Resistance increased as the length of wire did.
As the aim is to investigate how the length of wire changes when resistance does, I can draw a more accurate graph to prove that resistance increases as length of wire does (see graph paper).
The most important thing I found out while doing my experiment was that resistance increased as the length of wire did. This was an important aspect of my prediction.
On my graph, it is clear to me that as the length of wire increases so does the resistance. The rate of increase is constant.
This is indicated by the fact that the line drawn is a straight one, showing that the rate of resistance is directly proportional to the length of wire. The reason for this is, as electrons pass through the wire, the electrons hit the atoms of the wire whilst making the journey from the negative terminal to the positive terminal, giving opposition or resistance to the electrons. When this happens electrons move an electromotive force such as voltage, and in hitting these atoms, also create heat using friction of the electrons and atoms. When the wire is lengthened, the journey is longer and the resistance changes in proportion.
Equation for graph- it is clear to me that the equation for the graph is R= ?L
A
This formula would apply to prove that the resistance is proportional to the length of wire. The terms in the formula are-
R represents resistance
? represents resistivity
L represents length of wire
A represents the cross- sectional area of the wire
Conclusion
Judging from my results, I can safely say that the majority of my prediction was right. The resistance did change in proportion to the length of wire. This is because as the length of 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 have already explained why a longer wire means more resistance. A thinner wire also means more resistance. Resistance is known to be inversely proportional to the cross- sectional area (diameter). I.e. if the diameter is increased, the resistance decreases. A wider wire means less chance of the free electrons having collisions into atoms and losing energy.
Another point of my prediction was that as the length of wire doubles so does the resistance. This proved to be true. I can show this in my graph. The straight line indicates it. I can also see these in my results. E.g. 0- 10= 0.32? 0- 20= 0.65.
Evaluation
Overall, my experiment worked very well. The method that I used in the practical was efficient and my results were reliable. After studying my results, I realise there is one anomalous result. It occurs in the voltage on the second try. When the length of wire is 90cm, the voltage is 1.61V. For 100cm, the voltage decreases to 1.61V. The pattern for voltage is that it increases as the length of wire does. This individual anomalous result didn't alter the increasing pattern in resistance.
I think all of my other results were accurate and followed the patterns I predicted previously. In my conclusion, I mentioned a theory about the resistance doubling as the length did. This would occur because if the area of the wire doubles, so does the number of possible routes for the current to flow down. Therefore, the energy is twice as spread out, so resistance might half.
The method I chose to use was very suitable. Following the fair test criteria ensured that my results were accurate, and the experiment was completed appropriately. However, minimal changes could have been made in order to improve the reliability of my evidence.
Measuring the lengths of the wire is an inaccuracy the rulers may not be exact. As some were old, some measurements may have been scratched off. It is difficult to get an accurate reading of length by eye, as the wire may not be completely straight.
If I were to do this experiment again, I cold use a more accurate voltmeter, as this was when the anomalous result occurred.
Another factor that perhaps reduced the degree of accuracy was the connection of crocodile clips. They may not have always been connected to the constantan wire securely. This meant they could have been free to move, altering the length of wire and therefore accuracy of readings (voltage and current).
I can prove that my experiment was successful because of the graph I drew. It showed length of wire against resistance.
I don't think that by doing any more results in my experiment would improve the accuracy at all. I took three readings each of voltage and current and also worked out an average, which I used to work out the resistance.
The only way to make the results more accurate is to use a different method. Perhaps I could use a metal bar in place of the wire. This way, I could still investigate the length of wire affecting resistance, but more accurately. A disadvantage of this would be that the attachment of the crocodile clips would be less secure depending on the width and thickness of the wire.
If I had the chance to do the experiment again, I would investigate other factors. Examples of these are temperature, voltage or current. I would see how these additional factors affect the resistance.
Observations
Results table-
