Investigate how the resistance of a wire changes.

Fair test:

* Let wire cool down after each reading for two minutes

* Use same type of wire (because if we use different wires then the results will not have the same consistency e.g. if we use a thick wire and a thin wire for the voltage then we would get different results. It would not be directly proportional (length and resistance).

* Cut the wire carefully, get the exact length. (cannot be even 1mm longer) if possible.

* Power supply must always stay at 4v.

* Surrounding temperature must be the same.

* The same equipment should always be used.

Safety:

* Use low voltage so that overheating does not happen or wire does not melt. (no more than 4v)

* Make sure not to touch the wire as it may be hot and hands could get burnt.

Are there any variables?

* The length of the wire is the only variable.

Readings I shall take & how many?

* Length: I will take 10 readings of length. 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100cm on a table.

* Voltage: I will take 10 readings of voltage. To measure to amount of push and put it on a table

* Current: I will take 10 readings of current in amps and put results on a table.

* Resistance: I will calculate resistance using the equation: R=V/I.

Is my evidence reliable?

Yes. Because I will test everything three times and most of the information in the prediction was my own knowledge. Also my preliminary results below shows the evidence is reliable. I will use results so I will get an idea of what I will get in real results.

Length (cm)

Voltage (V)

Current (A)

Resistance (?)

00

.37

0.45

3.04

90

.30

0.49

2.65

80

.28

0.55

2.32

70

.28

0.57

2.24

60

.11

0.75

.48

50

0.87

0.89

0.97

40

0.70

.20

0.58

30

0.50

.38

0.36

20

0.17

.50

0.11

0

0.03

2.44

0.01

Other sources of information I used:

* My school exercise book

* CGP revision book

Results table:

Length (cm)

Voltage (V)

Current (A)

Resistance (?)

00

90

80

70

60

50

40

30

20

0

Observations

Did I make some measurements?

Yes I made measurements of current, voltage and length of wire.

Are my measurements appropriate?

Yes my measurements are appropriate because we are testing how the resistance of a wire changes depending on the length, this indicates that length is an appropriate measurement. We need voltage and current to work out resistance. I put length in cm, volts in V(volts) and current in A (amps).

How will I record my observations?

I will record my observations on a table and plot the average results of resistance on a table.

Did I measure correctly?

I measured correctly because I used an ammeter to measure current, a voltmeter to measure voltage. I used these so I could take down the readings and calculate the resistance.

Did I take enough measurements?

I took enough measurements because I can work out resistance, with the two measurements of current and voltage. I also took measurements of length so I could compare whether or not, when the length increases the resistance decreases.

Will I repeat my measurements?

Yes, I will repeat my measurements three times.

Is my range correct?

My range is correct because I took readings every 10cm and that is what I am supposed to do & also there is an equal amount of difference between each length. E.g. 10 20 30 etc.

10 10

Did I use my apparatus with skill so that the readings will be accurate?

I used my apparatus with skill by putting the voltmeter in parallel with the circuit (produce too much resistance in series). And by putting the ammeter in series with the circuit.

Do my results contain the correct number of digits?

My results contain the correct number of digits because they are to 2 d.p.

Results:

Observation results one:

Length (cm)

Voltage (V)

Current (A)

Resistance (?)

00

.35

0.49

2.75

90

.33

0.53

2.50

80

.32

0.61

2.16

70

.29

0.71

.81

60

.13

0.93

.21

50

.07

0.95

.12

40

0.98

.15

0.85

30

0.83

.18

0.70

20

0.49

.66

0.29

0

0.03

2.47

0.01

Observation results two:

Length (cm)

Voltage (V)

Current (A)

Resistance (?)

00

.36

0.49

2.77

90

.35

0.55

2.45

80

.34

0.63

2.12

70

.29

0.73

.76

60

.15

0.90

.27

50

.08

0.96

.12

40

0.97

.16

0.83

30

0.80

.18

0.67

20

0.50

.65

0.30

0

0.03

2.44

0.01

Observation results three:

Length (cm)

Voltage (V)

Current (A)

Resistance (?)

100

.34

0.48

2.79

90

.30

0.50

2.60

80

.27

0.64

.98

70

.24

0.76

.63

60

.20

0.91

.31

50

.13

0.96

.17

40

.02

.14

0.89

30

0.86

.18

0.72

20

0.55

.63

0.33

0

0.03

2.49

0.01

Analysis

What do my results mean?

My results mean that as wire length increases the voltage increases and current decreases. Also the resistance increases as wire length increases.

Is there a pattern in my results?

Yes. There is a pattern in my results, it is as wire length increases so does resistance in the wire. Also in my graphs I can see positive correlation because the points are going uphill.

Can I draw a graph to show the pattern?

Yes. I can draw a graph and it will show positive correlation (uphill) and I can put a line of best fit.

Does my graph prove my prediction?

My graph does prove my prediction that as wire length increases so does resistance. I have plotted points and labelled the axes and drawn a line of best fit.

Conclusion:

My conclusion is that as wire length increases so does resistance and current decreases, this shows my prediction is true. It is proportional because if you double the length it will be double the resistance e.g. if 50cm has a resistance of 1.55 ?, then 100cm should have a resistance of 3.10 ? or close to it. If the length is longer the electrons and atoms have more chances to collide into each other and this makes more resistance in the wire.

Is there an equation for the graph?

There is an equation for the graph R (resistance)? L (length). I can also work out the gradient by dividing resistance by length of two points, if the graph is a straight line. (The further apart the points are the more accurate the gradient is).

Why didn't all the points on the graph make a straight line?

No, the points from 30cm to 70cm were not. A reason for this could have been that when I connected the crocodile clips to the wire it was longer or shorter than the required length. The power pack might have been faulty. I set the power pack at 4v, but the power might have not been 4v it could have been fluctuating from 4v to either higher or lower, there might have been a faulty connection so the ammeters and voltmeters did not give accurate readings. The ammeters and voltmeters I used were old and could have been damaged in the past therefore giving false readings.

Is there any scientific information to explain my conclusion?

If you double length then there is double amount of atoms and therefore double resistance.

Is my prediction true?

Yes. As wire length increases so does resistance. This is also shown in my graph and results table.

Evaluation

How well did the experiment work?

The experiment worked out quite well, but it could have been improved.

Were the results accurate?

Yes the results were accurate but the voltmeter and ammeter did not stay at the same value, they kept on flicking from one value to another (only by one or two e.g. 1.37, 1.36, 1.38,1.36,1.37 etc.).

Could the experiment be improved for better results?

The experiment could be improved for better results by having more accurate equipment and if we were using batteries instead of a power pack. If we use a battery there will be the same amount of volts flowing through at one time, but with a power pack we never know how much voltage is going into the wire, we know it is 4 volts but it might be 4.01 or higher or 3.99 or lower, it is not as accurate as a new battery (it is an unlikely problem but still a factor). I would use pointers instead of crocodile clips because pointers have a smaller clip and this would make the measurement of length more accurate and I would use a digital voltmeter and ammeter.

Were the results reliable?

The results were reliable enough for a classroom experiment. We used fairly good voltmeters and ammeters, which showed fairly accurate results (they were not always on one value they flickered from one value to another e.g. 1.37, 1.36, 1.38,1.36,1.37 etc.) Also we let the wire cool down before we tested it again. The wire length was no exactly 90cm, 80cm etc. it was only as accurate as a ruler, the human eye and the way it was cut. The equipment was setup correctly so this did make the results reliable and the same precautions were taken for each test. The temperature of the room could have changed and this might have had a slight effect on the results. Because the results for each length were very close this would also make the results more reliable. I would use pointers instead of crocodile clips because pointers are more accurate; I would use pointers because the tips have a much smaller area giving a more accurate measurement of the length of the wire. I would use a new meter rule because the ruler I used at school was rounded at the edges and I couldn't see the 0cm mark and this means I might have placed the wire at the wrong place.

Did the results follow a scientific theory?

Yes. The results followed the theory that resistance is directly proportional to length.

Can any additional work be done?

I can make more graphs. I could make a graph for each observations set of results. After making the graphs I could compare the three graphs and see if they look fairly the same. If they do I will know that the experiment gave out fairly the same type of results each time and that each experiment was tested fairly and correctly each time. We could test the same type of wire but thicker in diameter. It would be the same test conditions but the only part I will be changing is the thickness of the wire. I would test the thicker wire three times and get three sets of results again and make graphs for each set of results. I would compare the tables and graphs of the thicker and thinner wires. I would test this to see if you increase the thickness of the thinner wire then will the resistance increase or decrease for each type of length e.g. thin wire (20cm): 0.50 volts, thick wire (20 cm): ?. The thick wire resistance might be double that of the thin wire. If it is double, I could then work out how much resistance there is for a wire 10 times as thick as the thin wire and create an equation for it as well.

Kamlesh Vadukul 10 palmer 5/4/2007