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Discover the factors affecting resistance in a conductor.

Extracts from this document...

Introduction

Andrew Holgate                 Physics Coursework

Introduction

The aim of this investigation is to discover the factors affecting resistance in a conductor.  I will use what I believe to be the most effective method and experimentation in order to come to a fair conclusion and accurate evaluation.  Secondary sources will be used in order to confirm any theories or motives that I may use, and all conclusions will be explained using scientific theory or the results from the experiments that will be carried out.  Each experiment will be completed safely, and using the best of my scientific knowledge.  Results will be arranged neatly and analysed carefully.  At the end of this investigation I would like to have discovered whether the conductor obeys Ohms Law, in reference to a sound prediction in relation to my conclusion and evaluation.

Basic Plan

As already said, I am going to discover the relationship between the length (and diameter) of carbon putty and the resistance.  This can then be plotted on a graph, using the two factors of voltage and current.  To discover the relationship between the length of carbon and its resistance, the putty will be placed in the circuit at different lengths, and the current and voltage will be recorded at each of these lengths, and then the following equation can be used:

Resistance  = Voltage   OR   R = V

Current                      I

Therefore the resistance can be discovered, however the unit of power used will remain constant this time rather than being altered. Also, to ensure accuracy, different experiments will be set up using a digital multimeter, which converts the voltage and current directly into ohms, making it easier to find the results. We will be using both methods in case one of them gives false indications, so that the experiment is reliable.

Middle

0

0.000

00.00000

0.00

0.000

This table above shows that when the length decreases, the resistance decreases. I hope to see this trend in my main set of results.

Measurements and Observations

After completing the preliminary tests, I was able to confirm some sort of method and a rough idea of what I will be using in each experiment in order to complete the investigation.  After conducting trials in the preliminary tests, I was able to confirm the measurements suitable for each experiment, and also the method that will be used.  One of the first factors that need to be considered is the lengths of the carbon putty. I will star at 18cm, and go down 2cm every time until 2cm is reached( therefore from 18cm – 2cm).

I also needed to decide the different diameters of carbon putty that will be used. I decided to base this on how easily the results would spread over one graph, showing a good range of observations (i.e. a thick diameter as well as a thin diameter). I chose the diameters as 2.3 cm, 2cm, 1.5cm, 1.2cm, and 1cm. Also, for the ammeter/voltmeter experiment, the voltage from the power pack will remain constant throughout.

1.5cm, 2.3cm, and 1 cm were for the ammeter/voltmeter experiment.

2 cm and 1.2 cm were for the multimeter.

As mentioned, two different types of circuits are going to be set up. A multimeter one, and a voltmeter/ammeter one. A brief explanation including a range of observations will be described here.  A more detailed method will be shown later on:

Multimeter experiment

In this experiment, the multimeter is connected directly to the carbon putty. The ohms setting should be used. The lengths of the carbon putty will be altered, and the results recorded in a table as shown below.

 Length (cm) Resistance (ohms)

Also, a circuit will be set up as shown below:

Voltmeter/Ammeter experiment

In this experiment, a power pack will be used, and the setting will need to be kept consistent at 5volts. A voltmeter will be connected in parallel across the carbon putty, and an ammeter will be connection in series in the circuit. Crocodile clips will be connected to two 2p coins, that will be placed at either end of the carbon putty to make a circuit. The voltmeter and ammeter readings will be taken, and then the connections at the d.c. terminal will be swapped around, giving two sets of readings for each measurement. An average will be taken of these readings, and the average derived from them. An example of the results table is shown below:

 Length cm Voltage 1 (V) Voltage 2 (V) Voltage average (V) Amps 1 (A) Amps 2 (A) Amps average (A) Resistance (Ω)

A circuit of this is shown below:

Temperature

The temperature will remain as consistent as possible, that being room temperature (around 20 degrees Celsius). This could change, but this is often beyond our control.

I believe that my readings will be very precise and exactly reproducible. In addition, I may decide to plot my results as I go, and will repeat any reading that looks out of place.

Apparatus

After much consideration and preliminary trials in order to discover a suitable method, I have concluded a range of apparatus that I believe will provide me with the optimum level of safety, accuracy and simplicity.

Carbon Putty: Obviously, this apparatus is essential to the completion of the investigation.  However it is entirely suitable for the process, as it is can be shaped exactly how I want it, therefore allowing a large number of diameters and results to be used. We already know it obeys ohms law. These reasons show why it is superior to a wire in these experiments.

A Power Unit: I concluded that a power unit would be the safest and most simplistic choice to use in this investigation.  If two battery packs were used it would result in a lot more wiring, and much more space taken up as a result, which would mean a crowded working area.  Up to voltage thirteen needed to be used, and the power unit could supply this range effectively, and this can be easily altered and the power can be raised or lowered without any danger to the user.  Only two wires are attached to the equipment, and this can be installed easily and safely.  The power unit increases the simplicity of the experiment to a great degree rather than using another piece of apparatus such as the battery packs, as they are difficult to arrange and cannot supply a continuous power because the batteries could run out during the experiment.  Therefore this apparatus will be able to provide an effective degree of accuracy.

A Voltmeter: The voltmeter is a very simple device, which is easily attached to the circuit.  It can measure any relatively high voltage, and as the reading is quite high the apparatus was entirely suitable.  It is attached in parallel to the circuit by two wires, and I concluded that it would be the most suitable apparatus because it is easily accessible, and most of all extremely simple to operate, thereby decreasing any chances of error in the investigation, and increasing accuracy.

Ammeter: Same reasons as the voltmeter, but placed in series in the circuit

Digital Multimeter: This will also be used for different circuits. Useful, because it generates its own power supply and can also convert the amps and volt directly into resistance.

5 wires for ammeter/voltmeter experiment: To connect up the circuit

2 wires for the multimeter experiment: To connect the carbon putty to the multimeter

Two Crocodile Clips: The crocodile clips are attached on the end of two of the wires to make contact with the 2p coins in the ammeter/ voltmeter experiment.

2p coins: Make contact with the carbon putty at either end.

Scalpel: To cut the putty to the desired length.

Transparent mm Ruler: I concluded that an accurate ruler should be used, rather than a ruler that can only measure cm.  This increases my chances of accuracy in the experiment, as the ruler can measure the carbon putty to the nearest mm, and as it is transparent the straightness of the measurement can be observed, to ensure that it is not measured at a slant.

Detailed Strategy

After looking at all the basic requirements for the experiment, we can draw up a detailed method with which we can conclude the preparation of the experiment.  The apparatus, measurements, variables and plan will be taken into account, as well as the highest standard of safety possible.  All wires must be checked for breaks or any bare wire that may show, otherwise this could result in electrocution of the user.  The method will be performed taking into account all of the laboratory safety precautions in order to perform a safe investigation.

My scientific knowledge, as mentioned above, allowed me to conclude the best method to use. I have already concluded the control variables and dependant variables, so need a suitable method to fit them into my experiment.

Scientific knowledge, such the ones shown below, helped me to decide what I should and should not change:

1. Resistance increases as the conductor’s length increases.
2. Resistance increases as the conductor’s cross-sectional area decreases.
3. Resistance varies depending on the type of material being used.

The diagram above showed me how a graph should look with an ohmic conductor, so I will be able to see anomalous results. These will be mentioned later in my project.

As it is known what affects resistance, I need to make sure that these factors need to be kept constant whilst performing the experiments, i.e. the temperature and the diameter of the carbon putty to make sure that my results produced are very reliable. It is important to stress that THE HIGHER THE VOLTAGE FROM THE POWER PACK, THE HIGHER THE TEMPERATURE WILL BECOME. Since I know that the voltage and current flowing through the putty will increase temperature, the voltage needs to remain constant at a fairly low voltage (i.e. 5volts) to ensure that the temperature is the same, and does not increase inconsistently, to produce inaccurate results.

I need to perform two different types of experiment, the ammeter/ voltmeter experiment and also the multimeter experiment. I have therefor divided my method into two parts to shows these different experiments.

1. The Ammeter/Voltmeter experiment:

From my scientific knowledge, I have discovered what I hope to be the best way of producing this experiment. I have learned how to set up circuits, and that the voltmeter needs to be in parallel, and the ammeter in series in the circuits. Also, as mentioned above, I have learnt the different factors affecting resistance, allowing me to produce the best possible procedure:

1. Set up circuit as shown in the diagram in the measurement and observations section of the project. This includes setting the power pack to 5 volts, having the ammeter in series in the circuit and having the voltmeter in series across the carbon putty.
2. Roll the carbon putty to the appropriate length and diameter. Use gloves to ensure safety. Use a scalpel to cut the two ends of the carbon putty to make them equal (N.B. I have found that it would be wiser to roll the putty longer than you need it, so that it can be cut to the appropriate length. Focus on getting a good diameter. Worry about the length afterwards.)
3. Once the putty is at the correct length and diameter, connect the two 2p coins to either end of the putty (making sure they are attached to the crocodile clips.
4. Switch on the circuit, and record the ammeter and voltmeter readings.
5. Reverse the connectors at the D.C. terminal, take the ammeter, voltmeter readings.
6. Repeat until all length at 2cm intervals from 18cm to 2cm have been performed.
7. Work out an average from the two ammeter/ voltmeter readings from each one (when you reversed the d.c. terminals), and then work out the resistance from the diameters.
8. Repeat the experiment with 2 other diameters of carbon putty, making a total of 3 ammeter/voltmeter results. Plot a graph of these 3 sets of results with resistance against length. Draw a line of best fit through each set of results, and label each line with the according diameter.
1. The multimeter experiment

From my scientific knowledge, I have discovered what I hope to be the best way of producing this experiment. I have learned how to set up circuits, and that only the multimeter and the carbon putty is needed to work out the resistance. I have learnt the different factors affecting resistance, allowing me to produce the best possible procedure:

1. Set up the circuit as shown in the measurements and observations section
2. Roll the carbon putty to the appropriate length and diameter. Use gloves to ensure safety. Use a scalpel to cut the two ends of the carbon putty to make them equal (N.B. I have found that it would be wiser to roll the putty longer than you need it, so that it can be cut to the appropriate length. Focus on getting a good diameter. Worry about the length afterwards.)
3. Attach the leads to the ________ and the ________ terminals of the multimeter. Set the multimeter to ___________
4. Attach the other ends of these two leads to either end of the putty.
5. Once the circuit is set up, take the first reading from the multimeter. This should be resistance in ohms.
6. Repeat until all length at 2cm intervals from 18cm to 2cm have been performed.
7. Repeat the experiment with 1 other diameter of carbon putty, making a total of 2 multimeter results. Plot a graph of these 2sets of results with resistance against length. Draw a line of best fit through each set of results, and label each line with the according diameter.

Overall, have structured my method by aid of my scientific knowledge to produce precise and reliable measurements. As long as the temperature remains constant, and the shape of the carbon putty is constant throughout, a believe that my results will be accurate.

I considered using many different conductors to allow me to see a range of results. But , from my scientific knowledge, I found that not all conductors at in the same way. The type of conductor I wanted was an ohmic conductor, and I would not be sure if the conductor if I was using followed this trend. With carbon putty, I already knew that it would.

I also considered varying the temperature to see how the resistance changed. This would have been difficult to perform, as making the whole of the carbon putty would have taken time and patience, and also very difficult. I concluded that the results would be inaccurate, so abandoned this idea.

Results

Ammeter/Voltmeter experiment

Diameter 1cm

 Length (cm) Voltage 1 (V) Voltage 2 (V) Voltage average (V) Amps 1 (A) Amps 2 (A) Amps average (A) Resistance (Ω) 18 4.43 4.37 4.40 0.06 0.06 0.06 73.31 16 4.35 4.36 4.36 0.06 0.06 0.06 67.02 14 4.32 4.32 4.32 0.07 0.07 0.07 61.73 12 4.31 4.32 4.32 0.08 0.08 0.08 53.95 10 4.32 4.34 4.33 0.09 0.09 0.09 48.12 8 4.32 4.32 4.32 0.12 0.12 0.12 36.06 6 4.30 4.26 4.28 0.16 0.16 0.16 26.82 4 4.30 4.27 4.29 0.14 0.14 0.14 30.61 2 4.29 4.26 4.28 0.18 0.18 0.18 23.80

Diameter 1.5 cm

 Length (cm) Voltage 1 (V) Voltage 2 (V) Voltage average (V) Amps 1 (A) Amps 2 (A) Amps average (A) Resistance (Ω) 18 4.35 4.36 4.36 0.11 0.11 0.11 39.55 16 4.33 4.32 4.33 0.13 0.12 0.13 33.31 14 4.25 4.24 4.25 0.15 0.14 0.15 28.33 12 4.20 4.20 4.20 0.17 0.18 0.18 24.71 10 4.24 4.24 4.24 0.20 0.21 0.21 21.20 8 4.20 4.19 4.20 0.25 0.26 0.26 16.80 6 4.15 4.16 4.16 0.35 0.34 0.35 11.86 4 4.12 4.12 4.12 0.45 0.46 0.46 9.15 2 3.78 3.78 3.78 1.20 1.21 0.21 3.15

Conclusion

using the same piece of carbon putty, so that the experiment is repeated with exactly the same conductor, and there are no changes whatsoever, in order to produce accurate results

Additional evidence is something being investigated or measured to show the same thing.  To obtain additional evidence that could support my theory on the factors effecting resistance in a conductor, I could do the following:

• Different conductors could be tested to see if they Obey Ohms Law
• Diodes, Filament Lamps and thermistors could be experimented with carbon putty paper to see how they behave in comparison to a linear substance
• Different lengths of materials could be used instead of carbon putty to see if the length increases with the resistance, and see how it differs to that of carbon putty.
• Different voltage outputs from the power unit could be used instead of five volts to see if this applies to different outputs of voltage from the energy source
• A completely different method and apparatus altogether could be used, as there is most likely a way to produce much more accurate results
• Different atmospheric conditions could be used that might increase the chances of finding additional evidence to support my prediction
• A different factor could be observed rather than the voltage and current measured, instead a bulb or motor could be used and see how the electrical or kinetic energy is affected by a decrease or increase in length of the carbon putty.

I believe that my results from the queries within the investigation are accurate enough and sufficient enough to lead me to a firm conclusion.  I am completely happy with the results, although I think the accuracy in the experiment was decreased slightly by the inaccuracy of the voltmeter, however this did not prevent me from obtaining results and a conclusion that fully complied with my prediction.  Therefore I can evaluate this investigation as a success, and after the removal of the anomalous result I am completely happy with what has been produced.

Easter 2001        Page

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