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# Identification of an unknown test wire through the experimental determination of it's resistivity.

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Introduction

## Physics Coursework

Identification of an unknown test wire through the experimental determination of it’s resistivity.

## Planning

The aim of this coursework is to find out the resistivity of an unknown piece of wire. Resistivity is a constant that depends on the resistance, length and the cross-sectional area of the wire at a certain temperature. I am going to cover these four factors in details later on in my coursework. You can have several different types of wires made out of different material. Each of these materials has a different resistivity value. Resistivity is measured in ohmmeters as shown below: -

### ρ=RA⇒Ωm²=Ωm

l                   m

• ρ = Resistivity (Ωm)
• A = Cross-sectional area (m²)
• R = Resistance (Ω)
•   l = Length (m)

From the equation above, we know that resistance is directly proportional to length. We know this because doubling the length would double the chance of the collisions, i.e. the longer the wire the higher the resistance because the electrons have to travel further so the hit more ions on their way. This will lower the current because the electrons hit the ions more often therefore they slow down more often, hence the resistance increases. We also know that the cross-sectional area is inversely proportional to the resistance.

Middle

#### Safety

• Don’t put your note pads or any kind of paper in contact with the wire because it might catch fire when the wire really hot up in the preliminary results.
• The taps must be turned off from the classroom mains because the experiment is about electricity. Therefore the reaction of electricity in water is very vigorous.
• Make sure that no one touches the wire when the current is switched on because at shorter lengths the wire might hot up and someone might burn their finger.
• Make sure that everyone dries their hands thoroughly so their hands are not wet.

## Variables

1. The cross-sectional area of the metal have to be the same because then there will be the same amount of free electrons and the structure of the ions will be the same. If you increase the cross-sectional area there will be more free electrons and the structure and number of the ions will be different.
2. I am changing the length every 5cm up, starting from 60cm. This means when the length measured is greater the resistance will be greater because as the electrons have to travel longer they will collide with the ions more often therefore the current will decrease.

Conclusion

The temperature could destroy the structure of the atoms in the wire during the preliminary results. Therefore the damaged wire might not give accurate results. I think we should be given 2-meters of wire, 1 meter for preliminary experiment and 1meter for the real experiment. We should be allowed to use a thermistor to control the resistance which will control the temperatureI should repeat the experiment 5 times to get more accurate average. I would also like to take more readings on the wire to find an accurate average of the diameter.The crocodile clips must be tested before being used, because some of them might have a poor contact with the wire and not allow current flow through the wire.I would try to use a bigger range on my graph so I could plot error bars more accurately.I can also try to let the wire cool down for 50 seconds between every length.From my preliminary results I have found out that the wire starts hotting up at 0.5m. This means the wire could start warming up before 0.5m from inside. Therefore even that would affect my results. So I would have to find a way to overcome the internal heat of the wire.I could use mass spectrometer to determine what the wire is made out of.

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