Diagram:
Apparatus
I will need the apparatus stated below;
1 Power Pack,
6 lengths of connecting plastic coated wires
2 Crocodile clips,
1 Single stranded piece of wire (1.0 metres long),
1 Ammeter,
1 Voltmeter,
1 1.0 meter ruler.
Safety
This experiment is not too dangerous but it could be if not handled with caution and care.
§ Make sure that the electricity is off at the plug socket when connecting and altering the circuit.
§ Be careful of any sharp edges on the crocodile clips or on the wire cutters or on the wire once cut. If you cut your self make sure you wash the wound and see a medical nurse if needed.
§ In this experiment I will use a low current, this is because to high of a current will make the wire very hot and it may melt the plastic on the plastic coated wires or it may melt the resistance wire or it may burn you during or after the experiment. It may also get hot and drop onto something (e.g. a piece of paper, plastic) and it could start a fire.
§ If a fire breaks out switch off the power and use a near by fire extinguisher that is not H2O (because of the electricity) to safely stop the fire.
§ Make sure the coils in the resistance wire don’t touch and short circuit because this will ruin the experiment and may heat up the wire and catch fire.
Method
1. Set up apparatus as shown in the Diagram.
2. Make sure that the electricity is turned off when setting up the apparatus.
3. Cut wire to specific lengths (Starting with 10cm and going up to 100cm).
4. Coil wire to prevent short-circuiting.
5. Switch on the power onto setting number 6. Adjust the Power Packs variable controls until the ammeter reads 1 Amp.
7. Record the voltage and current in a table.
8. Turn off the power.
9. Repeat the points 2-8 with 1.5 Amps instead of 1 Amp.
10. Repeat points 2-8 except with 2 Amps instead of 1.5 Amps.
11. Repeat the entire experiment with the following lengths of wire,
-100 cm
-90cm
-80cm
-70cm
-60cm
-50cm
-40cm
-30cm
-20cm
-10cm
12. Pack away experiment safely
13. Use Ohms law to work out the resistance
14. Take and record an average of the three different values of resistance for each length of wire.
Prediction:
I predict that when the length of the wire increases then the resistance will also increase proportionally to the length of the wire. I think this is because the longer the wire is, the more atoms to push through and so the more electrons should collide with the atoms. Therefore, if the length is doubled the resistance should also be doubled. This is because if the length is doubled the number of atoms and free electrons will also be doubled resulting in twice the number of collisions and doubling the energy and time lost. This will therefore increase the resistance of the wire by two (twice).
For example, a school hall holds a maximum of 200 pupils, this could represent the wire. Fill the hall with 150 students these represent your atoms. Make 20 pupils start at one end of the hall and try to walk through to the other end of the hall. This would be difficult, I then doubled the length of the hall and the amount of students representing atoms and free electrons there should be twice as many collisions between the electrons at atoms so it would take twice as long. This means the resistance will also be doubled.
From the prediction above I believe that if the length of the wire is increased by two times the resistance of that piece of wire will also be doubled. e.g. a piece of wire 4cm long has a resistance of 6Ω , when doubled I predict that the 8cm piece of wire will now have a resistance of 12Ω.
Fair Testing
To make sure it’s a fair test the only variable I will change will be the voltage of the wire. Although I will also change the current with each length of wire to make sure that I don’t make a mistake and then take an average for more accurate results.
The variables that I will be keeping the same are:
The material that the wire is made from because different metals have more or less free electrons than others making different metals better or worse conductors than others. I will do this by using the same piece of wire through the experiment. This means the material the wire is made of will not alter.
The surrounding temperature because the particles in the wire will move faster when it is hotter and this will therefore have an effect on the resistance
The heat of the wire because as the wire gets hot the atoms vibrate making it more difficult for the electrons to pass through the metal conductor. I will do this by using a low current of 1, 1.5 and 2 Amps so the wire does not get hot.
Reliability and Accuracy
Reliability: I can rely on my results because I have taken 10 different readings and then taken an average. This is so if one of my results goes wrong then I have two other ones to compare it with so I know if a result is abnormal. I am using an Ammeter to get an accurate current reading and not relying on the approximate readings on the power packs variable controls.
Accuracy: I will coil the wire so that there are no short circuits in the wire. I will do this by spacing out the coils and checking it regularly throughout the experiment. This is because if the wire does short-circuit then you will be measuring the resistance of the length of that short-circuited piece of wire and not the whole length of wire. I will also take the reading on the voltmeter as soon as possible so the heat does not affect the resistance.
I think that 10 is a suitable amount of results and 3 different current for each length of wire. Also I think that adding 10 cm on to the length of wire each time is a suitable so the range is from 10 to 100 because this gives us a wide but not too distance range of results.
Results
Conclusion:
Conclusion of Results
After looking the results I think that my prediction was right. The resistance of the wire did change in proportion to the length of the wire. This is because when the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire, which caused more collisions and therefore a higher resistance.
From the results I predict that a piece of wire that is 110cm long and is the same width as the wire that I used should be around 3.1Ω.
Evaluation
Anomalous Result
The one point that was not that close to the line was an anomaly. The reason for this could have been due to a number of different variables/factors.
1. The metal wasn’t that pure and had other less conductive metals in it.
2. The heat of the wire.
3. The heat of the surrounding area.
4. The equipment used wasn’t accurate enough. e.g. the voltmeter could have been more accurate and digital or different ways to connect the resistance wire to the circuit because the crocodile clips slipped and came off easily.
All the reasons stated above are acceptable but 4 is the most likely one because 1,2,3 would be the same all the way through so the trend would still be the same but lower on the results than before.
Accuracy
I feel that overall our results were quite accurate. Although there are a few different results but I think this is because of the apparatus or the heat. The only way I can think of to stop the heat from occurring is to use an even lower voltage. To stop the apparatus from not working properly see details below at the end of the reliability section.
Reliability
Most errors in our experiment were encountered in the measuring of the wire. This is because it was hard to hold a piece of wire straight and measure it with a 1m ruler and then trying to fix crocodile clips to the right part on the wire. Also I do not feel that the crocodile clips were always fixed securely to the wire with a good connection. This also meant that they were easy to move about on the wire changing the length of it.
More Results
I do not think that doing any more results in our experiment would have made it any more accurate. I feel that the only way to make it more accurate would be to use a different method – perhaps were we had a thin metal bar that was strong and did not bend like the piece of wire. We could have used a different instrument like an ohmmeter to measure the resistance instead of measuring the voltage and current and then dividing the voltage by the current.
