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# Identification of an unknown test wireThrough the experimental determination of it's Resistivity, p.

Extracts from this document...

Introduction

Nishit Gopal        Physics Coursework

AS PHYSICS COURSEWORK:

“Identification of an unknown test wire

Through the experimental determination of it’s Resistivity, p”

NISHIT GOPAL

Mr Toor/ Mrs Poole

Identification of an unknown test wire through the experimental determination of it’s Resistivity, p

##### Introduction

The purpose of this experiment is to identify an unknown wire that has been used in the experiment.  I will determine the unknown wire by working out the resistivity value of the wire.

In order for me to work out the resistivity, I will first need to determine the following:

• The resistance of the wire;
• The cross-sectional area of the wire;
• The length of the wire.

I am going to use resistivity of the wire to identify it because the resistivity value of a wire only depends on the material of the wire, the area and the length of the wire will not affect the resistivity value of the unknown wire.

Before I start the experiment I already know that as the wire gets thicker, the resistance of the wire decreases, we say that the resistance is inversely proportional to the cross-sectional area of the wire (R  1/A).  The reason for this is because as the wire gets thicker, the free electrons have more room to vibrate

Middle

• After recording the resistance, plot the RA/L graph, and work out the gradient, which is also equal to resistivity.
• Match the value of resistivity of the wire with one from the data book, and hence identify the unknown wire.
###### Preliminary
 Length/mm Voltage/V Current/I Resistance/Ω 900 0.63 0.18 3.5 800 0.60 0.22 2.73 700 0.57 0.27 2.11 600 0.56 0.29 1.93 500 0.53 0.31 1.71 400 0.51 0.40 1.28 300 0.48 0.49 0.97

I carried out preliminary work to see how the temperature of the wire was affected at lengths between 300mm to 900mm; I kept the voltage constant at 2V.  From the results table above, we can see that the resistance at 300mm was very low, hence the wire was getting too much current through it, and because it was getting excess current the temperature increased causing the wire to burn. At 900mm the resistance was too high simply because the electrons had a longer distance to travel.  For these reason I have chosen to investigate the wire using lengths 400mm, 500mm, 600mm, 700mm and 800mm.

###### Diameter of wire

For this I measured the wire’s diameter with micrometer six times to get a mean (average) as this increased the precision.

 Length/mm Diameter/mm 300mm 0.39 400mm 0.39 500mm 0.39 600mm 0.38 700mm 0.39 800mm 0.39 Average diameter 0.39 (3 s.f.)

Conclusion

• Though I took three readings of the voltage and the current, I could improve my    average by taking about five separate readings for both the voltage and the current for each length.
• Instead of using crocodile clips to hold the wire in place, I could use pointers, this would improve my investigation because the area that is in contact with the wire would be much smaller, and therefore allow me to measure the wire more accurately.
• I could minimise the temperature error by allocating an interval time in between each reading this would allow the wire to cool back down to room temperature.  The temperature of the wire increases slightly every time current is passed thorough it.
• Use a wider variety of lengths to get a more spread of findings, this would help me achieve more accurate average and a better understanding of the behaviour of resistance on the wire.
• Repeating the same investigation, but this time I would vary the width of the wire instead of the length, this would also enhance my understanding of how the resistance is affected in different situations.

Overall my from my investigation I can say with confidence that my hypothesis was correct.  As the length of the wire increased, the resistance also increased.  This is due to the current having a longer distance to travel, passing an increased number of ions in the lattice arrangement of the wire.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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