R= V/I
As the length of the wire is increased the number 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 ( ) is also equal to the resistance of the wire.
The material and cross sectional area of the wire is constant throughout the experiment. The resistance should be directly proportional to the length.
Factors: in this experiment we will only change one factor, the length of the wire. This should effect the resistance of the wire in the ways stated above.
Fair test: in this experiment we are only changing one factor (the length of the wire) the factors that we are going to keep the same are:
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 affect on the resistance.
The cross sectional area of the wire must be kept constant throughout as well. 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.
Apparatus:
1. Wire SWG 28 (Standard Wire Gauge)
2. Powerpack
3. Two crocodile clips
4. 2 Multimeters (one to act as Voltmeter and one as Ammeter)
5. A metre rule
Plan:
1. Connect circuit as shown in the diagram.
2. Adjust until the ammeter reads 3 A.
3. Record voltage on voltmeter
4. Repeat the experiment with the following lengths of wire (cm), connected between the two crocodile clips: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, repeat this three times so that we can get an average to be more accurate.
5. Record results in a results table.
6. Use Ohm’s law to find the resistance of the wire and record in the table.
Diagram:
Safety: this is not a very dangerous experiment but despite this you must always be aware of electric currents and appliances, keep the current low, handle with dry hands.
Accuracy: to keep this experiment as accurate as possible we 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. We 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, also. The reading that we 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 we want to test it when heat is effecting it the least, like at the beginning.
Preliminary: upon testing to see if the experiment would work I found no problems with the plan I described earlier. I was able to get the following
Results:
Observations: we will observe the reading on the voltmeter change as we change the current to 3A we also observe a general increase in the voltage as the length of wire we use gets longer. The rheostat will also be set at different positions for the different lengths of wire that we use.
Anomalies: there was only one real anomaly in this experiment and it has been highlighted on the graph.
Conclusion: From the graph we can see one very clear trend, which is, as the length of the wire increases so does the resistance of it. Another, more significant thing is that it the increase is constant. This is indicating by the fact that the line drawn is a straight one. One may also note that the gradient of the line drawn is . I think that from my results I can say that my prediction was right. The resistance did change in proportion to the length. This is because as the length of the 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.
Evaluation: I feel that overall our results were quite accurate. This can be seen when we look at the graph, which shows a straight line with all of the points (exept 1) 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. The temperature of the wire was not always 20°C when we conducted the experiment and the material of wire may not be pure. The main reason for inaccuracy is 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 relationships that were predicted in the plan still were shown.
Most errors in our experiment were 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 rule 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 that errors occur.
I do not think that doing any more results in our experiment would have made it any more accurate.
