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This coursework will show how the resistance of a wire, (depending on size of diameter, the thickness and the length) changes. I shall then determine the resistivity of the material (of the wire).

Extracts from this document...

Introduction

Planning

This coursework will show how the resistance of a wire, (depending on size of diameter, the thickness and the length) changes. I shall then determine the resistivity of the material (of the wire).

This experiment will be carried out using lab equipment.

Question:How will resistance of a length of wire change with the diameter?

                 How do we determine the resistivity of a material?

Hypothesis-I expect the resistance of a length of wire to increase with a small diameter, and vice verse.  

I expect to find the resistivity of a material by using a formula.

Equipment

  • Variable resistor-this will be used to vary the resistance in the circuit, also trying to keep the voltage constant.
  • Wires-with the least amount of resistance, so they don’t affect the experiment as they can add to the resistance.
  • Voltmeter-to measure the amount of volts going round the circuit, and must have a very high resistance so the electrical current does not go through it (connected in parallel).
  • Ammeter-to measure the current going round the circuit, must have very little resistance so the current may pass through (connected in series).
  • Micrometer-to measure the diameter of the wires very accurately.
  • Crocodile clips-to hold on to the free wires conducting the current through them.
  • Ruler-to measure the length of the wire.
  • Power pack with D.C. current-to supply energy to the circuit and its components.

What will changing my variables do to 1) The resistance of the wire? 2) The resistivity of the wire?

Variables + Constants

  • Changing the wires to test, each having a different diameter.
  • Length-constant at first, but then a variable to test resistance as length changes, at that time diameter will have to be constant.
  • Voltage will have to be a constant using the variable resistor.
...read more.

Middle

1.02

What this table should show that as diameter decreases then resistance show increase, this table does not show, that as length of wire increases then so should resistance.

The reason for this is that I only controlled the length of wire, though it does show nichrome has more resistance than constantan.  

Method

I will first gather my apparatus together carefully, and then I shall set them up ready for experimentation. I will connect together the components in an arrangement like this:

Fair test

To make this a fair test I will need to measure accurately the length of the wire, the diameter of the wire. I will measure the diameter of the wire using the micrometer, which is accurate to 0.05mm. I shall the measure the length of the wire by using a metre rule, which accurate to 0.5mm.

I will need an extremely high resistance voltmeter and an extremely low resistant ammeter because if the voltmeter had a low resistance connected in parallel then this would create an extra path for the current to flow into. If the ammeter had a high resistance then the current would find it hard to come round again.

Background knowledge

I know that if the length of wire were long there would be more resistance, as there would be a longer line of electrons to go past and not to collide with.

Wire type

Diameter(mm)

Length(mm)

volts

amps

Ohms

Constantan          

0.40

500

5.2

3

1.73

Constantan          

0.40

700

5.2

2.10

2.48

This would be my evidence because the wire type, the diameter and voltage are the same. The only differences are the length, amperes and the ohms, as you can see the amps in the 500mm wire is 3, however the amps in the 700mm wire is 2.10. This shows a larger current can get through the wire more easily.

Therefore when there is a smaller current there would be a larger resistance measured in ohms.

I also know that if the wire were thicker, then this would allow more current to pass freely, therefore less collisions would take place and an easier route would be available.

Wire type

Diameter(mm)

Length(mm)

volts

amps

Ohms

Constantan

0.56

400

4.5

5.3

0.85

Constantan        

0.43

400

5

3.45

1.45

...read more.

Conclusion

The internal part of the metal has a regular array of positive ions (+ve); this is an ion (metal atom), which has lost its free electrons. The free electrons can randomly swim about in the space between the ions like gas molecules. When voltage is applied across the ends of a wire the negative ions (-ve) electrons are attached towards the positive end of the wire and current flows.

Measuring current

Current (Amps) can be measured by dividing charge (Q) by time (t) because 1 amp is the flow of current, where the amount charge on a point per second.

1 amp is the flow of 6x10 18 electrons each second, so the charge on 1 electron is about 1.6x10-19. This means this equation is correct: I=Q/t

This diagram shows the coulombs passing a point each second containing the same amount of electrons per coulomb.

The whole point of measuring amperes is to work out the resistance.

Current and drift velocity

A=Cross sectional area

I =current

n=free electrons per metre (cubed)

e= charge on each electron

v=drift velocity

Formula: I=nAve

Fair test

I expect there to be an internal resistance, this does therefore affect the experiment because less voltage can be applied through the circuit, hence a lower amount of coulombs per second.

Kirchhoff’s second law states ‘Around any closed loop in a circuit, the sum of the E.M.F.’s (Electromotive force) is equal to the sum of the p.d.‘s (Voltage).

Є = IR + Ir

...read more.

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