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An Investigation into the factors that affect the electrical resistance of a wire

Extracts from this document...

Introduction

Physics Practical Assessment

Rachel Jones

Candidate Number:

Farmors School Centre Number:

An Investigation into the factors that affect the electrical resistance of a wire

Aim:  To explore and explain why specific changes to a wire can increase or decrease the resistance of that wire.

Background:What is resistance?

  An electrical conductor is a substance through which electrical current flows with little resistance. Electricity is passed through a conductor, in this case a length of wire, by means of free electrons. In metals the atoms are arranged in a regular crystal lattice, and so the electrons in the outer shells of the atoms are free to move through the metal, even if it is a solid. It is this ‘sea’ of mobile electrons that allow the conduction of electricity. The number of free electrons depends on the material due to the varying amounts of electrons in the outer shell. The more free electrons the better the conductor, i.e. it has less resistance.

(An example of a possible metal crystal lattice with a ‘sea’ of mobile electrons)

  For example, by using the periodic table we can work out that Copper will be a better conductor of electricity than Iron. Copper has more electrons in the outer shell of its atoms, its electronic structure being 2,8,8,11 meaning that there are 11 electrons in the outer shell of each atom in the metal. Iron’s electronic structure is 2,8,8,8 meaning that there are only 8 free electrons compared to copper’s 11.

  When the free electrons in a wire are given energy, they have the ability to move and collide with neighbouring free electrons. This happens across the length of the wire, and so electricity is conducted.

...read more.

Middle

Results:

Length of wire

(cm)

Average

Voltage (V)

(Volts)

Average

Current (I)

(Amps)

Resistance (R)

(to 2d.p)

(Ω)

20

3.12

0.45

6.93

30

3.78

0.33

11.45

40

4.14

0.27

15.33

50

4.48

0.24

18.67

60

4.65

0.21

22.14

70

4.77

0.18

26.50

80

4.87

0.17

28.65

90

4.96

0.15

33.07

100

5.02

0.14

35.86

Observations:

  I observed that both the readings on the voltmeter or ammeter were not precise measurements, and kept flicking from one number to another, so I used the first reading, as I thought that this would be when the wire would be least affected by temperature, and then worked out the average (as shown above) from the three tests.

  I was not able to obtain accurate results from the 10cm length of wire as the wire became very hot very quickly, so, as this would invalidate the results, I decided not to include it in my findings.

Anomalies:

  There were no obvious errors in my experiment.

Conclusions:

  In view of my results and the graph I was able to construct from them, I have made the following conclusions:

  1. As predicted, when the length of the wire increased, the resistance also increased.
  2. The graph shows a strong trend forming a straight line, showing that the length is directly proportional to the resistance; as the length doubles the resistance doubles as well.

  From my analysis, I can confidently say that my prediction was correct. From studying the manner in which electricity is conducted, one can explain why the length of a wire is directly proportional to the resistance.

  The number of freely mobile electrons in a given length of wire will produce a specific amount of resistance. This is because as a current is passed through the conductor the electrons in the outer shells of the particles are given energy so that they can move. As they travel through the wire, they come into contact with impurities in the conductor, other particles and any immobile material with which they ‘bump’ into.

...read more.

Conclusion


Evaluation:

  There were a few mistakes in my experiment, mainly caused by carelessness in the storage of the materials used:

  1. I included a rheostat in my circuit, but I did not adjust it for each reading (to control the current) as I believed that leaving the circuit connected for a long period of time would considerably alter the temperature of the wire, as collisions occurred and electrical energy was converted into heat, so the current readings vary slightly, which I believe will produce varying temperatures, but none so dramatically as to render my results totally inaccurate.
  2. However, I attempted to take the readings for the ammeter and voltmeter quickly so that any temperature change would be very slight and would not affect the results.
  3. There were some problems with the equipment I used throughout this experiment. The apparatus was not totally accurate as it was built and maintained to a poor standard due to lack of funds, for example the crocodile clips were coated in rust, and so the contact between it and the wire was not perfect. The wires were labelled incorrectly so the results were confusing.
  4.  Some of the anomalies will have been caused by human error in the measuring of the wire. This is because it is not very practical to hold a piece of wire straight and cut it perfectly at the designated length. The crocodile clips will also have not been placed exactly at the specific length, and so the actual length of the wire in the circuits will vary from the length recorded.

  Although there were a few mistakes in this experiment, on the whole I do believe that they provided me with a good basis for my conclusions.

...read more.

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