Method,
- Setup all equipment accurately, making sure there is a complete circuit.
- Draw up a table with appropriate headings for the ability to record results quickly
- Attach one crocodile clip to the wire on the metre stick at the point marked 0cm.
- Place the second crocodile clip on the 10cm mark on the wire.
- Point 5 will increase by 10cm after each recorded result until the experiment has reached 100cm.
- Turn on the power pack.
- The power pack should be set to 4V (4 volts)
- The readings on the ammeter may be fluttering, if this is so record the modal result that you can see on the ammeter.
- Place these results into your drawn out graph
- Carry on the experiment with the next 10cm.
- When this has been completed, repeat steps 3- 10, twice more in order to get very accurate results and be able to find a respectable average to draw the graph by.
Fair test:
In this experiment I am only changing one factor – the length of the wire, the factors that we are going to keep the same are as follows:
* We must keep the surrounding room temperature the same or the particles in the wire will move faster (if the temperature is increased) and this will therefore have an effect on the resistance.
* The cross sectional area of the wire must be kept constant throughout as well. This is shown in equation (2) where the cross sectional area is a factor that effects the resistance.
* The material of the wire must also be kept the same as different materials have different conductivity. The last two factors will be kept the same by using the same wire all of the way through the experiment.
* The current that we pass through the wire is to be kept the same, also. If this is changed the temperature of the wire might change in a way that is not constant making the results more confusing.
Safety,
This is not a very dangerous experiment but despite this you must always handle electricity with care. The following is the two main factors to ensure safety in this experiment:
Handling the Current – I will ensure that I have full control of the current and keep it quite low. I will ensure this by keeping my concentration levels to a maximum.
Dry Hands – It is essential when dealing with electricity I keep and stick to the correct precautions, and keeping my hands dry is common sense when dealing with electricity.
Reliability,
To keep this experiment as accurate as possible I need to make sure, firstly, 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 I do this. I must also make sure that the wire is straight when we conduct the experiment. If it is not, short circuits may occur and bends and kinks in the wire may effect the resistance. The reading that I take of the voltage should be done fairly promptly after the circuit is connected. This is because as soon as a current is put through the wire it will get hotter and I want to test it when heat is effecting it the least, i.e. at the beginning.
Results,
The tables below are showing the three repeated results which I carried out during this experiment:
Experiment 1,
Experiment 2,
Experiment 3,
Averages of all 3 Repeated Experiments
Analysis,
What I have noticed:
From the graph we can see that the line of best fit fits very accurately but above all else it is clear that as the length of the wire increases so does the resistance of it. Another perhaps more significant observation is that the increase is constant.
We can see now that the rate at which the resistance of the wire increases was directly proportional to the length. With electricity, the property that transforms electrical energy into heat energy, in opposing electrical current, is resistance. A property of the atoms of all conductors is that they have free electrons in the outer shell of their structure. All metals are conductors and have an arrangement in similar form to each other.
As a result of the structure of all conductive atoms, the outer electrons are able to move about freely even in a solid. When there is a potential difference across a conductive material all of the free electrons arrange themselves in lines moving in the same direction. This forms an electrical current. Resistance is encountered when the charged particles that make up the current collide with other fixed particles in the material. As the resistance of a material increases, so to must the force required to drive the same amount of current.
So this in fact proves that resistance, in ohms(R) is equal to the electromotive force or potential difference, in volts (V) divided by the current, in amperes (I) – Ohm´s law. As the length of the wire is increased the numbers of collisions the current carrying charged particles make with fixed particles also increases and therefore the value for the resistance of the wire becomes higher. Resistance, in ohms (R) is also equal to the resistively of the wire, in ohm-meters (ñ) multiplied by the length, in meters (l) divided by the cross sectional area, in square meters (A).
Overall, the resistance can be worked out using that particular formula:
Resistance = resistively of the wire x length
Cross sectional area
Resistance= electromotive force
Current
Symbolized Formulae:
R= ñ x l
A
R= V
I
Evaluation,
I feel that overall our results were quite accurate. This is can be seen when we look at the graph, which shows a straight line with all of the points apart from one being very close to or on that line. The one point that was not that close to the line was a slight anomaly, but it was only slight and did not effect the final gradient of the graph. I have found out that for the wire I was using, the resistively at 20C is 4.9 X 10-7 ohm-meter. From this we can then work out the percentage error of our results:
The accuracy for this experiment is, theoretically, ± 15.7%, but as one can see this does not seem to be the case from looking at the graph. The reason for this could have been due to a number of different factors. Firstly the temperature of the wire was not necessarily 20C when we conducted the experiment and the material of wire may not be as pure as it should have been. The main reason for this was probably due to the equipment that we used being inaccurate. This did not stop us from seeing the trend, though, because the equipment would have been out by a constant amount each time therefore there was a constant error. So the trends that were predicted in the plan still were shown.
Most errors in our experiment were encountered in the measuring of the wire. This is because it simply was not very practical to hold a piece of wire straight, whilst holding it next to a ruler and then trying to accurately fix crocodile clips to the right part on the wire. Also I do not feel that the crocodile clips were always fixed securely to the wire with a good connection. This also meant that they were easy to move about on the wire changing the length of it. Errors rarely occurred in the setting of the current and the reading of the voltage. It was just in the preparation area that they did occur. Another example of this is the wire was never totally straight when we started the experiment, which may also, as said earlier on, effect the resistance of it.
I do not think that doing any more results in our experiment would have made it any more accurate. I feel that the only way to make it more accurate would be to use a different method – perhaps were we had a bar that did not bend in place of the wire. We could even use a rheostat in place of the wire, because it is essentially a long coiled wire that is connected at different lengths to change the resistance of the circuit
