Variables
· Length of wire
· Voltage
· Current flowing through wire
· Width of wire
· Equipment in use (leads rheostat)
To make the experiment a fair test I will vary only one thing- the length of the wire. The current and voltage stay the same. The wire will stay the same except for length,
I am going to measure the voltage from 5 cm to 50 cm in 5 cm increments.
I will repeat the experiment 3 times in order to gain an average result and use Ohm's law to calculate the resistance.
Predictions
I predict that the longer the wire, the higher the resistance. This is because the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions (as heat).
Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions.
Apparatus
- Rheostat
- Power Pack
- Volt Meter
- Ammeter
- Crocodile Clips
- 50cm Rule
- wire board - the wood with the wire on it
Method
First a length of wire over a metre long is screwed to a metre rule. The positive crocodile clip is attached at 0cm. And the negative is moved up and down the wire, stopping at 5, 10, 15, up to 50cm. Each time reading the ammeter and voltmeter to work out resistance R = V/I. Other variables, voltage, thickness, and temperature will be kept constant, although the temperature will rise once current is passing through it, which will cause the atoms in the wire to vibrate, and so obstruct the flow of electrons, so the resistance will increase, creating an error.
Safety
- Handle the power supply carefully.
- I am going to only use a voltage of four volts so the wire will not burn.
- Be careful when touching the wire, as it may be hot.
- Start on the lowest current, so the wire then will not melt or burn instantly.
- Be careful when the wire is connected, as it will get hot.
- Make sure the mains to the power supply are switched off when removing the wire from the circuit to be measured.
Results
Resistance and Averages
Conclusion
Having performed the investigation, the following conclusions were drawn:
As predicted, an increase in length resulted in an increased resistance. This can be clearly said for both wires tested.
Both wires show a strong trend of a straight line, i.e. the length of the wire is shown to be directly proportional to the resistance - double the length and the resistance doubles.
The overall resistance of the wires seems to differ considerably. Due to the strong correlation of the results, the explanation of this is unlikely to be the method used to obtain the results. The more likely explanation would be that the last (1.22mm) wire was actually of a larger diameter than the other three wires. Obviously this is a rather important oversight and this will be discussed more in the Evaluation section. The reason why this is the likely explanation is because resistance is known to be inversely proportional to the cross-sectional area.
Evaluation
The results of this experiment would be difficult to improve on because they are reasonably accurate, and there were no anomalous results. If I were to do this experiment again I would take a much wider range of readings and more readings so that a more accurate average can be taken. I would also take the reading from the ammeter more than once so I could get an average. This would make the experiment more accurate. I would use a longer length of wire to see if the graph of resistance changes from a straight line with a much longer length of wire.
I would also investigate another factor, such as diameter of the wire. I predict that if I did investigate this, when I increased the diameter the resistance would decrease. I would also do the experiments under different conditions such as temperature to see the difference this makes to resistance.