Electric current is a measure of the charge that flows per second. As the charge on each electron is the same, the current is the number of electrons flowing per second. If the resistance is high, the flow of electrons is slowed down, thus the number of electrons per second is reduced. i.e. the current is reduced.
If the metal is heated, the metal ions will vibrate more. This means that there is a higher chance of the electrons bumping into a metal ion so the flow of electrons is reduced. i.e. the resistance is increased.
This information is from a web page by Roger Downs.
The current is inversely proportional to the length of the wire. This is expressed the formula:
Resistivity → temperature coefficient α = increase in resistance in °C
Resistance at 0°C
This table below I found on the school network. I will use it to decide on what wire I will use.
PRELIMINARY WORK
For my preliminary work, I used a computer program on the school network to recreate the experiment. These results were obtained from a circuit using 26 SWG constantan wire, the circuit will be powered by a power pack:
These are my results I acquired from my preliminary results. I have used 26 SWG constantan wire. Constantan has the lowest resistance; this can be seen in the first table with the resistances of four wires. In my actual investigation, I will use constantan at a SWG weight of 28. This is because the wire will have a lower resistance than say if the SWG of the constantan was 32.
KEY FACTORS
Heat
An increase in heat increases the resistance. This is due to the increased vibration of atoms and the higher chance of the electrons colliding with them.
Width
An increase in the width decreases the resistance. This is because the electrons have a wider course to flow and has less chance to collide.
Length
An increase in the length increases the resistance. This is because the electrons have further to travel in one second.
Density
A higher density will increase the resistance. This is down to the increased number of atoms in the same amount of space. This creates a much higher chance of collisions.
Material
Different conductors materials will have different resistances. This is down to the atom size and the number of free electrons
VARIABLES
I intend to keep the following the same this will maintain a fair test:
- Temperature
- Width
- Material
- Density of the material
Varying the width is complicated and would need the school to have the correct widths of wire. Varying the temperature will be complicated to get the correct temperature and to sustain that temperature would be too problematic to accomplish with accurate results. Varying the material could be done with the school equipment, but I think that varying the length of the wire will be the easiest to carry out and will be more accurate. Nothing will be varied apart from the length to make this a fair test!
PREDICTION
I predict that resistance will be increased as the length is also increased. I came to this deduction based on my background information, which tells me that electrons would have to travel further through a longer wire than a shorter one.
Also, since the resistance is measured by how many electrons pass through the material in one second, this would increase the resistance and this will be directly proportional to the length.
APPARATUS CHOSEN
I have chosen the following apparatus for this experiment:
Voltmeter
Ammeter
Power pack
Length of copper wire
2 Metre Rulers
2 Crocodile clips
PRECISION
My results will be to a hundredth degree of accuracy, that is to 2 decimal places. That is, for example 2.46 volts. I think this will be precise enough for my experiment, as it is not crucial for my results need to be very precise, but not as vague as a tenth, or as exact as a thousandth degree of accuracy, so, I will be using a hundredth degree of accuracy. To keep this experiment as accurate as possible I need to make sure that the length of the wire is measured precisely from the inside edge of the crocodile clips making sure that the wire is straight when we do this. I must also make sure that the wire is straight when I conduct the experiment, if the wire is not, short circuits may occur and bends and kinks in the wire may affect the resistance and I will get inaccurate results. The reading that I will take of the voltage should be done as soon as I have set up the circuit. This is because as soon as a current is put through the wire, it will become hot and I want to test it when heat is effecting it the least, i.e. at the beginning.
DIAGRAM
METHOD
- Set up the apparatus as shown in the diagram on the previous page.
- It is important that the voltmeter is set up in a parallel circuit and the ammeter in a series.
- Take readings at equal intervals of 10cms.
These lengths will be:
2 metres
1.9 metres
1.8 metres
1.7 metres
1.6 metres
1.5 metres
1.4 metres
1.3 metres
1.2 metres
1.1 metres
1 metre
0.9 metres
0.8 metres
0.7 metres
0.6 metres
0.5 metres
0.4 metres
0.3 metres
0.2 metres
0.1 metres
- Accuracy is paramount so readings will be taken to two decimal places.
- The reading from the ammeter will be used to work out the resistance. This will be done using Ohms Law: (V=IR). This can be rearranged to,
R = V
I
FAIR TEST
To ensure that the experiment is carried out in a fair way and that the results will be accurate. I will only vary the length of the wire, this will be the only thing I will vary. The wire must be pulled tight against the ruler and taped in place to ensure the length is accurately measured and that there are no kinks or bends in the wire to avoid an incorrect result. The same circuit and power pack must be used throughout as different batteries may have different voltages if they are old. The experiment should be repeated more than once and an average taken to make sure that the results are reliable. I will take twenty results to make sure that they are wide spread. I will repeat the experiment twice, and take both the average of