Resistance also depends on the thickness of the wire. It is actually inversely proportional to the cross-sectional area of the wire: as the area doubles the resistance halves. This is demonstrated by using two wires to double the area (doubling the diameter of a circular cross-section wire would give four times the area and so one quarter of the resistance).
Equipment:
- Voltmeter
- Ammeter
- Power pack
- Different lengths of Nichrome wire
- Connecting Wires
- Metre rule
- Crocodile clip
Method:
1. Connect the battery, ammeter and voltmeter using the wires as in diagram.
The ammeter is set in series with the power supply while the voltmeter is set in parallel to the circuit.
2. Then I measured the length of nichrome wire to suit the experiment I was doing at the time e.g. 1st was 5 cm long and each time it increased by 5 cm more until 100cm.
3. I then set the power supply to 6 volts.
4. I then recorded the voltages and currents.
5. I found out the resistance using the two figures. Resistance is Voltage divided by Current.
6. I repeated the experiment two more times to make sure it is a fair test.
7. I found averages off all my results give an estimated view.
Hypothesis: I think that as the length of the nichrome wire increases, the resistance will increase. Each cm of wire has a particular resistance, if you double the length of wire; it is like having two of the shorter wires in series. If the small resistor represents a short wire and the large resistor is a long wire of double the length of a short one. One short wire has a resistance of 1ohm, 2 short wires have a resistance of 2ohms when connected in series. The long wire is just like two short wires put together. Therefore the resistance of the wire will increase as the length doubles or triples. The longer wire will therefore have a higher resistance. I expect my graph will look somewhat like this.
How I kept it a fair test:
Variables in the experiment: -
- In this experiment I varied the length of the wire each time using a range of lengths from 5-100cm.
- The temperature of the wires in the circuit needed to be kept constant to prevent the whole circuit from overheating. I managed to do this by quickly switching off the power pack supply every time I had recorded my set of readings. I then left the power pack to rest for a small interval of time before switching it on again to record the next set of readings.
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I also kept the diameter of the wire constant by using the same piece of wire throughout the whole experiment.
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I used the same voltage for all my lengths so the results would not be biased.
Safety Precautions taken: In doing this experiment, I took many safety precautions. I kept the current low enough to avoid the heating effect, which may burn us or cause damage to surrounding objects. I did not touch the wire once the power pack is turned on, because I did not know if the wire was hot or not. I did not put any liquid substances near the experiment, as water is a good conductor and if it was to come in contact with electricity, could be fatal.
How I made my results more reliable-
The following factors affect the resistance of a wire.
- Length
- Thickness (diameter of wire.)
- Material e.g. nichrome, copper
Therefore I had to control this to get a fair experiment. I used the same wire all through so therefore the material, length and thickness are the same for all the results. My results were also reliable as I tried to leave the clips on the wire for the same amount of time.
Results:
Results in Graphs
Conclusion: My resistance against length graph has a quite strong positive correlation, which shows there is a relationship between the resistance and the length of the Nichrome wire. It shows that as the length of the nichrome wire increased, the measure of resistance increased as a whole. There were a few exceptions because in some instances the resistance decreased. This might have been caused by a variety of factors e.g. inaccurate length, inaccurate time, coils in the wire etc. My graph also shows that after every 5cm of Nichrome wire, the resistance increases by 0.4~ estimated. Also to test my hypothesis, I will choose two lengths where one is double the other. I choose 50cm and 100cm.
Resistance of wire at 50cm: 1.99
Resistance of wire at 100cm: 3.45
- 100/50= 2
- 3.45/1.99 = 1.73 rounded up to 2.
Therefore the results allow me to come to the conclusion that my hypothesis was correct and that as the length of the nichrome wire increases, the measure of resistance increases as well. This is because each cm of wire has a particular resistance, if you double the length of wire; it is like having two of the shorter wires in series. If the small resistor represents a short wire and the large resistor is a long wire of double the length of a short one. the voltage increases as the length increases as my voltage graph shows a strong positive correlation. The current stayed more or less the same.
Evaluation: My experiment was quite accurate and I tried to make it a fair test by controlling some variable. There were a few anomalies as between the lengths 70cm and 75cm, the resistance decreases from 2.73 to 2.46. Also between lengths 85cm and 90cm, the resistance decreases from 2.80 to 2.28. Apart from these instances, my results were quite accurate.
I used the same piece of wire so the diameter of the wire was the same. I could have used different wires to also notice how the diameter of a wire affects its resistance. I could have had twenty different apparatuses at the same time so they would have been left for the same amount of time.
I could also have used more ranges of wire lengths e.g. fifty different set ups of differences of 2cm in lengths. This way, I might have been able to get a more accurate increase in every centimetre. I also could have checked the results using different voltages. Throughout the experiment, my power pack supplied 6 volts; I could have set it to supply 3 volts or 12 volts and also note down the results when the amount of volts have changed. Therefore I will have had a wider range of results and more information to come up with a more accurate conclusion. On the whole, I think my experiment was quite informative and I achieved the results I thought I would.