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Length vs Resistance

Extracts from this document...

Introduction

Physics Coursework

Contents                         Page Number

Introduction                                  2

Preliminary                                   5

Main Investigation                        8

Conclusion                                  12

Evaluation                                   14

Introduction

Resistance occurs in circuits because of components in the circuit and a larger amount of resistance causes the battery to have to exert more energy to push the charges through the circuit. This then causes the current to decrease as resistance is not allowing as much current to flow. Resistance can be useful in some circuits. Devices called resistors are used to control the flow of charged in a circuit. Variable resistors can be used to make varying the size of the current easier as the size of their resistance can be steadily changed using a dial or slider. Variable resistors are used in dimmer switches and volume controls like that of a CD player. Another type of resistor is an LDR, (Light dependent resistor) which is a variable resistor. This resistor's resistance increases, as light levels decrease as it gets darker and decreases, as light levels increase during daytime. This makes LDR useful for example in automatic security lights. One finally type of resistor is a thermistor, which has a higher resistance at low temperatures and a lower resistance at high temperatures. A thermistor can be used as a temperatures sensor in devices like fire alarms.

There are five factors which could potentially affect the resistance of the wire in a circuit. These factors are: Changing the voltage of the battery, changing the diameter of the wire, changing the temperature of the wire, changing the material of the wire and finally changing the length of the wire.

Changing the voltage, however will not affect the resistance of the wire. This

...read more.

Middle

Thickness of wire: 0.25mm (Power pack set to 2V)

Length

(cm)

Voltage

(V)

Current

(A)

Resistance (Ω)

10

0.70

0.58

1.21

20

1.15

0.50

2.30

30

1.23

0.36

3.42

40

1.29

0.28

4.61

50

1.32

0.23

5.74

60

1.33

0.19

7.00

70

1.34

0.16

8.38

80

1.33

0.14

9.50

90

1.38

0.13

10.62

100

1.40

0.12

11.66

Before I carry out my main investigation, I decided to carry out a ‘test'                            investigation, using the lengths, the thickness and the voltage that I have chosen for the investigation. I did this so that I can be sure that the diameter, lengths and voltage are the ones I want to be using in the main investigation and that at no point does the current exceed 1A. Using my test investigation, I was able to re-evaluate the measurements I was going to use in my main investigation and through this, I decided that I would use wire with the diameter of 0.25mm because I felt it would give me better results.

Main Investigation

        Key:  

image01.png

                         Variable resistor (the  image05.pngimage06.png

Constantan wire)

image07.png

Ammeter

image08.png

Voltmeter

image10.pngimage09.pngimage10.png

Power Pack

This is the method for my main investigation:

  • Set up a series circuit around the wire board, which is shown above, and connect the ammeter in series and voltage in parallel. The voltmeter has to be connected in parallel, or otherwise it will not work at all.
  • Connect the whole circuit to the power pack with the voltage I had chosen from my preliminary work, which is 2V.
  • Before I turn on my power source, measure at five points along the wire, its thickness with a micrometer and calculate an average thickness. This is so that my test is more precise, as I’m not assuming that the wire is the exact thickness stated by its manufacturer.
  • Connect the crocodile clip to the first length, which I will do by going along the metre ruler attached to the wire board, finding the length and connected the crocodile clip as near to it as possible. Then turn on the power source.
  • Measure the current and voltage and record it down. I will need these values to work out resistance using ohm’s law.
  • Turn of the power source, move the crocodile clip to the next length and turn it on again. This is so that the wire can cool down and temperature will have less of an effect on my results.
  • Measure current and voltage again.
  • Measure the voltage and current of the rest of the length, recording the figures as I go along.
  • Repeat measuring voltage and current for each length, three times so I can compile an average. This will increase the accuracy of the data because the more times I repeat the test, the more accurate the results will be.
  • When I finish measuring all the lengths, use my three sets of results to calculate the resistance of each length.

I decided to change my lengths from 20cm-100cm, to 10-100cm because it gave me a slightly wider range of data.  When I tried the length 10cm again, the current did not go above 1A; the earlier result had been caused by systematic errors in the equipment because of the preliminary being carried out over several days, so the equipment I used would have changed. However, I will collect all the results for my main investigation in one day so that my results will not be affected due to equipment changes.

Equipment:

  • Wire board with 0.25mm Constantan wire attached
  • Voltmeter
  • Ammeter
  • Wires for the circuit
  • Power Pack
  • Micrometer for measuring wire thickness

The variables:

The independent variable is the wire lengths and the dependent variable material and temperature of the wire. I cannot keep the temperature of the wire the same exactly because the wire heats up anyway as the current run through it, due to the electrons colliding against the metal. But I can be able to control it as much as I was able to by keeping the current below 1A and by turning off the power pack after testing each length.

I have also tested each length of the wire three times so that I have a fair test and my results are more reliable as I will be taking an average from them. After I recorded the results in tables, I then found figures for the average resistance, which will be used in a graph.

As mentioned in my method, I decided to measure the thickness of my wire at five points using a micrometer. The reason for doing this is so that I could see if the wire was of a uniform width throughout it and also calculate an average width.

Point

Thickness (mm)

1

0.24

2

0.23

3

0.23

4

0.23

5

0.24

Average thickness

0.23

These results show that the thickness of a wire varies throughout the length of it and the diameters aren’t always the same. My average diameter was 0.23mm, which however is not 0.25mm, the diameter of wire the manufacturer states. This may have an effect on the results I record for my investigation, because as I have stated before, diameter does have an effect on resistance, with a wider diameter reduced resistance due to there being more space for the electron to move around. Since my diameter is smaller than it should be, the resistance values I record may be higher than they should be.

Diameter of wire 0.25mm

Length

(cm)

Voltage

(V)

Current

(A)

R (Ω)

Voltage

(V)

Current

(A)

R (Ω)

Voltage

(V)

Current

(A)

R  (Ω)

10

1.03

0.91

1.18

1.02

0.89

1.15

0.95

0.85

1.18

20

1.19

0.52

2.34

1.19

0.52

2.29

1.17

0.50

2.34

30

1.27

0.36

3.51

1.26

0.36

3.50

1.23

0.35

3.51

40

1.31

0.28

4.64

1.31

0.28

4.68

1.30

0.28

4.64

50

1.34

0.23

5.78

1.35

0.23

5.87

1.33

0.23

5.78

60

1.37

0.19

7.11

1.37

0.19

7.21

1.35

0.19

7.11

70

1.38

0.17

8.56

1.39

0.16

8.12

1.37

0.16

8.56

80

1.40

0.15

9.20

1.40

0.15

9.33

1.38

0.15

9.20

90

1.40

0.12

10.69

1.42

0.13

11.67

1.39

0.13

10.69

100

1.41

0.11

11.75

1.41

0.12

12.82

1.41

0.12

11.75

...read more.

Conclusion

Though I do not think my results were very accurate, they were very reliable and precise because of the reasons stated above.

I took readings from an ammeter and a voltmeter. I intended to wait for each reading to stop fluctuating so that my results could be made more accurate but I did not have enough time so I did not do that. This may have made my results more accurate as I might not have recorded the true values for current and voltage since I ended up recording the first number I saw. This could have lessened the credibility of my conclusion, if the values I ended up using were not correct then the theory of length being proportional to resistance that I sought to prove with my data may be wrong. This decreases my confidence with my conclusion, however because most of my data followed a clear trend and I was able to reach my conclusion using a variety of methods to check it, I cannot readily discount my the validity of my conclusion.

Another reason why I’m less confident with my conclusion than I should be is due to the ranges of lengths I have chosen. Though going up in tens is a reasonable amount, I feel that my results could have gotten a much better accuracy by using smaller lengths. For example, I could have gone up in fives instead and ended up using twenty lengths instead of ten. To improve my conclusion next time, I must first increase the precision of the way I collect my data so that my results will be more accurate. I would also have to research more into resistivity and resistance, so that I could better use science to evaluate my data and write a better conclusion.

...read more.

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