Investigating the Resistance of a Wire

Different wires have different resistances so there is a difference in slope. The steeper the slope the lower the resistance therefore the flatter the slope the greater the resistance.

To calculate resistance you use ohm's law V=IxR or

R=V

I

Different materials have many different properties therefore have different resistances. In a less dense material there is not a generous supply of electrons so that there is less collision so the current is low while the resistance is high. In a highly dense material there is a generous supply of electrons so that there is more collisions so the current is greater causing the resistance to be less. Density states that "resistance is proportional to the potential difference divided by the current"

The density of the wire is obtained using the equation

Density = MASS

Volume

I predict that for the resistance of a wire I will find that if I increase the length, the resistance will increase. This is because the longer the wire the greater the number of collisions between the positive ions and negative electrons, which create a greater number of obstacles so the current finds it harder to get through therefore the resistance will increase. Also I would be interested to find if the resistance does in fact double when you double the length of the wire, if this does happen it will show me that the number of electrons and ions also doubles which increases the number of collisions.

The prediction I have made has taken into account ohm's law and the structure of a wire, density and cross-sectional area all influences how the current makes it through the wire.

Method:

) Collect the equipment that is needed these are:

. A Power pack- make sure it is set on dc

. An ammeter- connected in series

. A voltmeter- which is in parallel to the device

. Nichrome wire- at 35cm in length and is 24 swg

. A variable resistor- to ensure that the same levels are of current is running through the circuit.

These are to be set up like so:

Diagram to show how to investigate the resistance of

a wire

2) Once the equipment is set up and the variable resistor is in place, measure the wire to 30cm and attach both ends to the circuit by crocodile clips.

3) Turn on the power pack remembering to switch it on to dc and not ac

4) Take a reading off the voltmeter and take a reading off the ammeter

5) Switch off the power pack and allow an appropriate time for the wire to cool down I will do 15secs (this time must stay constant after each run)

6) Turn on the power pack after the wire has cooled down...

7) And repeat steps 4,5 and 6, twice more so you have collected three results for the wire at 30cm.

8) Change the length of the wire reducing it by 5cm after 3 runs at each length

9) Repeat steps 3-7 for each length.

A fair test is when only one element of the experiment is changed, which is the length of the wire; the other elements stay the same these are:

The time between each run (15 seconds)

The set number of runs for each length (3 runs)

Same type and gauge of wire (Nichrome 24 swg wire)

Experiment is done at room temperature.

This experiment has to be carried out safely to do so I will use a safety mat this is because the wire heats up so becomes dangerous to touch and can also burn work surfaces. The wire can also burn out sending sparks flying so safety glasses will be worn throughout the experiment.

The Trial Experiment:

The trial experiment results

Length of a wire

(cm)

Voltage

(v)

Current

(A)

Resistance V

( ) I

30

0.65

0.48

.35

25

0.55

0.48

.14

20

0.45

0.49

0.91

5

0.40

0.50

0.80

0

0.30

0.50

0.60

5

0.15

0.52

0.28

I think that my actual experiment will go according to plan as the trial experiment went fine without any major problems. The results (above) show what I have predicted that when you increase the length resistance increases. I decided to only do one run through at each length as it was a trial but it was still a fair test as the potential difference stayed at 7 volts constantly with 15 seconds to cool after each attempt.

The following resources I have used to use as quote and information:

Encyclopedia title

Publisher

The Hutchinson educational

Helicon

Encarta 98

Microsoft

Britannica- standard edition

Britannica

Observing:

I feel that I used the equipment as safely as possible and I accomplished this by: wearing safety glasses from the start, using a heat proof mat and tying my hair back.

I didn't feel it was necessary to repeat any of my results as all seem pretty close to the other examples at the same length.

Results for the Experiment:

Length cm

Current A

Voltage V

Resistance

30

- 0.66

2- 0.66

3- 0.66

A- 0.66

- 0.8

2- 0.8

3- 0.8

A- 0.8

.21

25

- 0.68

2- 0.68

3- 0.68

A- 0.68

- 0.7

2- 0.7

3- 0.7

A- 0.7

.03

20

- 0.69

2- 0.70

3- 0.69

A- 0.69

- 0.6

2- 0.6

3- 0.6

A- 0.6

0.87

5

- 0.70

2- 0.71

3- 0.70

A- 0.70

- 0.4

2- 0.4

3- 0.4

A- 0.4

0.57

0

- 0.72

2- 0.72

3- 0.72

A- 0.72

- 0.3

2- 0.3

3- 0.3

A- 0.3

0.42

5

- 0.73

2- 0.73

3- 0.73

A- 0.73

- 0.2

2- 0.2

3- 0.2

A- 0.2

0.27

Calculations:

My results show clearly that by increasing the length the resistance has increased. This proves that I have been successful in finding the resistance of a wire using Ohm's law to help calculate the resistance in the wire. The results also show that the current decreases as the potential difference increases.

I realise that the resistance increased as the length increased due to the increase in collisions caused by a growth in number of both electrons and positive metal ions thus stopping the current getting through easily. As the current couldn't get through easily I knew that there was resistance present but using ohm's law " resistance equals potential difference divided by current" I could calculate just how much resistance was present at a certain length.

The trends I noticed in my results are doubling the length to a certain degree doubled the resistance. From my graph I can see that at 5cm the resistance equals 0.27 but at 10cm the resistance equals 0.42 therefore doubling the length in this example did not double the resistance. At 10cm the resistance equals 0.42 but at 20cm the resistance equals 0.87 therefore this example shows that doubling the length from 10cm to 20cm does double the resistance. While at 15cm the resistance equals 0.57 but at 30cm the resistance equals 1.21 therefore this example does double the resistance to some extent but isn't as accurate as the previous example.

I think that doubling the length does double the resistance, this would be due to the positive ions and electrons doubling in number as the length doubled. Which would cause an increase in the number of collisions that led to the resistance doubling which strengthens the fact my results are correctly calculated.

Another trend I have noticed is that the results have a mathematical sequence, this means that if u take the answer away from the previous number you are left with a pattern and this happens to a certain degree. Using my graph the difference between the resistance at 30cm and at 25cm equals to 0.18 , the difference between 25cm and 20cm equals 0.16 , while the difference between 20cm and 15cm equals 0.30 , the difference between 15cm and 10cm equals 0.15 , the difference between 10cm and 5cm equals 0.15 , and finally the difference between 5cm and 0cm equals 0.27 . This shows that the results 5cm, 15cm and 20cm seem to be wrong as the results all would be close together i.e. within 0.05 which they are not for these results but are for 10cm, 25cm and 30cm. This is also shown on the graph using the line of best fit.

Apart from this I am content with my prediction I made before carrying out the experiment it shows that if you increase the length you will certainly increase the resistance I have also learnt that doubling the length does not always double the resistance. Now I've realised what results do not abide all the trends and they are 5cm, 15cm and 20cm I would have liked if I had more time to carry out re runs on all three lengths. So I could compare my new results to my previous and plot them on my graph to see if the would fit more closely on the line of best fit.

I am happy with my experiment in total it has been a success I followed my method correctly, which led to accurate results although they were completely different to the theoretical resistance results for nichrome. This I imagine is because the theoretical resistance didn't calculate in a gain in resistance due to a rise in temperature. This did certainly happen in my experiment as temperature was the only element that could change resistance that I couldn't stop from changing, I could only add a cooling period of 15 seconds between each run this obviously was not a long enough time. So if doing this experiment I would certainly increase that cooling period so as to stop an increase in temperature (which causes the electrons to vibrate more causing more collisions which causes more resistance).

If I were to do this experiment again I would make the following improvement:

Placing the wire in water this is so that I could take the temperature of the water to measure a difference in temperature against the length of the wire. Also it would cool the wire far quicker after each run. The experiment would be set up like below.

This change/ improvement would increase fairness but an increase in safety would be needed so that I wouldn't get a shock if the crocodile clips touched the water so that would be one thing which I certainly would need to stop.

The extension experiment that I investigated was "does the resistance increase as the cross-sectional area increased"

To do this experiment the equipment is set up as before with the only thing changing instead of the length is the wire thickness using 12swg, 24swg (used in previous experiment) and 36swg all have the same nichrome wire. To keep this a fair test the temperature had to remain at room temp, the length of the wires and the voltage stayed the same while the wire thickness is the only thing that changed.

The results that I gathered:

Density

Current

Voltage

Resistance

2 swg

0.72

0.20

0.27

24 swg

0.70

0.40

0.57

36 swg

0.50

0.30

0.60

These results showed me that as the cross-sectional area increased the resistance became less which proves the collision theory which states that the more collisions the current has the less the resistance will be.

By sherry goodchild x