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Determination of the resistivity of nichrome wire

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Introduction

Determination of the resistivity of nichrome wire

The experiment

The aim of the experiment was to investigate the resistivity of a piece of nichrome wire. The equipment was set up as below.

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             The current and voltage were measured and then the results manipulated to produce resistance values.

             The results table and methods for the calculations are attached.

             Cellotape is used to secure the wire to the ruler and so ease measurement of the length of wire.

             The jockey is used to make good contact with the wire and so improves reliability. The jockey is better than a crocodile clip because it has better contact and so eliminates any problems with loose connections between the crocodile clip and wire.

             The current is kept below 0.5 A to avoid over heating and melting of the wire.

About Resistivity

            The resistivity of a piece of wire depends upon three things, the resistance, the length of the wire and the cross sectional area. The relationship between the four is shown below.

ρ = RA

      l

             Resistivity is therefore measured in ohmmeters. The resistivity of nichrome is 100 x 10–8Ωm according to Hutching’s ‘Physics’ book.

The resistivity of a metal depends upon the number of free electrons and how easily they can travel through the metal. In a metal, all the available free electrons that will ever be free are free so this does not affect the resistivity.

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Middle

An increase in resistance will lead to an increase in resistivity.

Conclusion from results

The following conclusions can be drawn from my results: -

  • The resistance is proportional to the length of the wire. This is known because a straight-line graph is achieved. This was expected in accordance with the theory of resistivity. A straight line is produced since metals are ohmic devices.  
  • As the voltage, doubles the resistance also doubles. This is because as the voltage increases (doubles) so does the number of collisions between the electrons and atoms inside the wire and hence the resistance increases (doubles).
  • The resistivity value of nichrome wire that I found is 108.7 *10 –8 ± 6.3*10 –8 Ωm

Evaluation

The experiment was found to be relatively safe. The only hazard was if a current in excess of 0.50A was used, as the wire would over heat and the pupil could possibly suffer from burns. I avoided this type of accident by keeping the current at 0.45A, I did not use 0.50A because there is a risk of a current in excess of 0.50A flowing through the circuit when trying to gain 0.50A exactly, I did not want to take this risk.

If I was going to repeat the experiment myself or if other pupils were to do this experiment I would recommend that the current is kept reasonably below 0.50A, perhaps between 0.40 A and 0.45A.

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Conclusion

Another problem that I identified was keeping the wire straight. This was due to the kinks. The main problem area was round the middle of the area. I attempted to solve this problem by using cellotape to secure the wire to the ruler in the middle. Although this was effective, it too may have affected the results. A more successful way to secure the wire would be to perhaps use clips; they should lightly hold the wire in place without exerting a force of pressure on the wire, as that would affect the results. Therefore, a more successful way would be to use pointers instead of crocodile clips. This is because the pointers would be more accurate because the tips have a much smaller area than the crocodile clips giving a more accurate measurement of the length of wire.

As well as making the above modifications, I would also improve my experiment by testing different widths of the nichrome wire. I would do this to expand on my experiment to see how width affected the resistance and resistivity.

AS Physics Coursework- Determination of the resistivity of nichrome wire.

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