Resistance of a Wire

First a length of wire 34cm long is measured. The positive crocodile clip is attached at 0cm and the negative is attached at 20 cm then at 34 cm. Each time reading the ammeter and voltmeter to work out resistance (R=V/I). This is done using 0.234mm wire. In both experiments nichrome wire is used because it does not heat up as much as copper, so the resistance is not affected as much.

Experiment 2: The relationship between a wires thickness and resistance.

The circuit is set up the same as in the first experiment, as is the method apart from the fact that I am keeping the length constant at 34cm and the thickness is changed between 0.234 mm and 0.274 mm. For both experiments the voltage will be kept the same at 6.5-volts/ 2 volts dc from the power pack. Both experiments will be done twice to gain more accurate results.

To make sure this is a fair test I am going to:

) Keep the voltage of the wire the same each time.

2) Keep the power pack at a constant Voltage.

3) Repeat the experiment with the same amount of wire twice.

Predictions

Metals conduct electricity because the atoms in them do not hold onto their electrons very well and so they create free electrons, which carry a negative charge to jump along the line of atoms in a wire. Resistance is caused when these electrons flowing towards the positive terminal have to jump the atoms. If we were to double the length of a wire, the number of atoms in the wire also multiplies as do the number of jumps. Therefore, the energy is twice as spread out and in theory the resistance should halve. In other words the resistance of a wire is inversely proportional to its area.

Results

Experiment 1- Increase in length

Length (cm)

Distance apart (clips)

Volts

Amps

Resistance (Ohms)

34

6.5

3.48

.87

34

6.5

0.85

7.65

32.5

6.5

3.61

.80

32.5

6.5

.05

6.19

Experiment 2- Cross sectional area

Thickness (mm)

Length (mm)

Area (mm2)

Amps

Volts

Resistance (Ohms)

0.274

3.25

0.19

0.98

6.5

6.63

0.234

3.4

0.15

.87

6.5

3.48

0.234

3.25

0.14

3.16

6.5

2.06

0.274

3.4

0.2

.35

6.5

4.81

The results of experiment one supported my predictions well and matched scientific theory. The results turned out how I expected. I had expected that the length would increase as well as the resistance and it did. This showed that my results were reliable and that if the length were to increase from 34 to 68 then the resistance would increase from 1.87? to 3.74?.

The results of experiment two on the other hand did not support my predictions and the cross-sectional area increased and the resistance decreased. This is shown as you compare the readings of the first column and the third column where it clearly shows that as the cross sectional area increased so did the resistance. This could be due to a failure in taking down the results correctly or an error in the experiment. Unfortunately I was unable to carry out the experiment again in order to gain accurate results.

Conclusion

If I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings so that a more accurate average can be taken. I would also think about using at least three more different lengths of wire so that I can check how reliable and accurate my results were. Measuring the lengths of the wire is also an inaccuracy as the wire might not be completely straight and it may be of different thickness throughout the length. It would be hard to improve these results as they are quite accurate and there were no anomalous results.

The result of experiment two were not as accurate as experiment one so given the chance I would carry out experiment two again to gain correct results. Scientific theory states that the resistance should halve as area doubles, which it does; however this is not what was illustrated in my results.

My results suggest that the inaccuracies were either due to poor recording of the experiment or that the inaccuracies were due to permanent errors due to problems with the measuring equipment. The ammeters and voltmeters could have been damaged and reading falsely on both the meters used.

I would also investigate other factors, such as temperature, voltage and current, and see how these effect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance. As these results had a range of only four readings and were only repeated twice, and that the results are not 100%, accurate due to the errors discussed earlier.