Resistance - My aim in this piece of investigative work is to take one aspect of a material and see how varying it affects the resistance of the piece of material.

* Power supply

* Wires- To join the components

* Ammeter

* Voltmeter

* Crocodile clips

* Calculator

* Variable length of wire

* Wood block

Using this equipment I will make this circuit:

With this circuit set up I am ready to take my measurements to see how much resistance.

The equation to measure resistance is R=V/I (Ohm's law)

Resistance= voltage divided by current

Preliminary tests:

From my preliminary tests I know that constantan is a good material to use because its resistance doesn't vary much with temperature.

I also found out that for safety I should use low amps because otherwise the wire becomes very hot to touch and could burn.

What do I need to measure and how?

So I need, from my circuit to take the voltage and the current. Using them I can then go on to work out the resistance. I can measure voltage using a voltmeter and I can measure the current using an ammeter.

I will also need to measure my variable- length. I will do this using a ruler. I will measure each of these every 10cms. I will do this because this is a big enough gap to see a difference but not to big so I can come up with a clear conclusion.

To make sure that my measurements are correct I will be doing the experiment twice. Once using 2amps and then again at 4 amps.

Is it a fair test?

To make my experiments fair I need to keep the material and length of the wire the same and also the temperature. I will keep the diameter and material the same by having a piece of wire 1 m long and instead of cutting it or having various pieces I will just move the crocodile clips along.

The temperature is much more difficult to keep the same. It will be impossible to make it completely accurate but I can try and control it. I will use a low voltage because this will mean the temperature doesn't change as much. When I did my preliminary tests I started on 6 amps this was far to big and my wire became very hot! I can also control the temperature by using the temperature coefficient of resistance. Different materials resistance change in different amounts when temperature changes. Temperature effect resistance because if it is hotter the vibrations of the atoms increase. This makes it harder for the free electrons to travel through the wire. Again, in my preliminary results I found that constantan is a good metal to use because the resistance doesn't change depending on what temperature it is. I tried copper but it was not nearly as good. I then found out that copper has a temperature coefficient of 43.00 where as constantan has one of 30.00.

So my experiment will...

. Set up equipment.

2. On the 1m board underneath the wire, mark out intervals at every 10 cm

3. Connect the crocodile clips from the voltmeter to the first 10 cm point.

4. Turn the power pack firstly to 4 volts.

5. Record results

6. Do this for every 10 cm

7. After you have done the length of the wire, change the voltage on the power pack to 2 amps and record the results again.

I will then work out the resistance of each measurement using the equation in my theory.

Prediction:

My prediction is that the longer the length the more resistance. I have many reasons and theory's to back up this prediction. The longer the piece of wire the more atoms the electrons have to be pushed through. The electrons need more energy to get to the positive side so the wire is giving out more resistance. I also predict that length and resistance are proportional.

My second is resistivity. Resistivity is the electrical resistance of a conductor with a unit of cross sectional area and length. The letter 'rho' (p) which is Greek shows resistivity. It is quantitatively equal to the resistance R of something such as a wire, multiplied by the wires cross sectional area A and then divided by the wires length I. It is shown in this equation,

P=RA

I

Resistivity= resistance* area this could be: Resistance=Resistivity*length

Length Area

From this I know that resistance depends on Resistivity, length and area..

Resistivity = 0.44 × ? × (0.1875 × 10­³) ² / 0.10

The 0.44 is the average resistance we came across at 0.10m. The 0.1875 is the cross sectional area of constantan and the 0.10 is obviously the length we initially tested.

Constantan- 28 standard wire gauge = 4.32 resistance per meter (ohm/m). I predict our results will be similar to this.

RESULTS:

Length/cm

Voltage/V

Current/A

Resistance/

Average/

Resistivity/

0

0.30

0.70

0.44

0.44

4.86

0

0.13

0.30

0.43

20

0.62

0.70

0.90

0.90

4.97

20

0.27

0.30

0.89

30

0.95

0.70

.37

.37

5.04

30

0.41

0.30

.36

40

.27

0.70

.83

.82

5.03

40

0.55

0.30

.81

50

.58

0.70

2.30

2.28

5.03

50

0.69

0.30

2.26

60

.91

0.70

2.73

2.73

5.03

60

0.82

0.30

2.73

70

2.23

0.70

3.23

3.20

5.05

70

0.97

0.30

3.16

80

2.55

0.70

3.70

3.67

5.07

80

.11

0.30

3.64

90

2.85

0.70

4.17

4.12

5.06

90

.25

0.30

4.07

00

3.20

0.70

4.60

4.55

5.03

00

.35

0.30

4.57

Analysis:

From this graph I can see that the resistance is directionally proportional to the length. This is what I predicted. If the length of the wire doubles then the resistance will double also double because resistance will increase the length. It is a positive correlation.

As we can see the resistance gets higher, this is due to the electrons in the outer shells become "free" or "delocalised" due to the low attraction between it and the nucleus. These electrons then move in an erratic fashion. The more electrons there are free the more voltage is needed to push the electrons round the circuit.

For example when my wire was 0.10m the resistance is 0.43 and the voltage is 0.31. When the wire is 0.20m, the voltage needed is doubled, 0.62, the resistance has also doubled to 0.90. This proves that the graph is proportional.

On the table I can also see that I was right in my prediction saying that resistance and length are proportional because all the Resistivity results are very similar.

Evaluation

I feel for this experiment my results are near to perfect as I can make them. I have no anomalous results and the two separate tests I did were very similar. I am very confident that they are reliable I can take a clear conclusion from them.

I have read frp, the correct scale on the metes and I have made sure I took more than 1 result for each length for accuracy. An average is much more accurate than 1 single measurement. I can tell my experiment was a success because my graph made a straight line which I s what I predicted due to Resistivity.

I think my results gained from this experiment are enough to prove my hypothesis. Because I only went up to 1m this was a slight limitation. It would have been better to use bigger lengths and got more results to be more thorough.

Again I think my results are conclusive because they follow a pattern and I can relate them back to my theory's of resistance and Resistivity.

There are some slight ways I could change my experiment to make it more accurate. I could take three reading rather than two just for the extra security that my results are completely accurate. I could use longer lengths of wire for a larger range of results.

To extend me experiment I could take a different variable of the ones I discussed in my plan. Of course if I did this I would have to keep length the same. I could look into Resistivity and temperature coefficient in more detail to deepen my thought and understanding in the subject.

I have thoroughly enjoyed this experiment and have learnt a lot from it. It has been very beneficial.

Lucy Smith Resistance 07/05/2007

Mr Pompfrett GCSE Physics Coursework