Equipment used
- Roll of conducting putty made out of graphite
- Ammeter – This will enable me to measure the amps passing through the circuit at any one time.
- 30cm Ruler – Has to be used to measure the putty accurately to see if there is a correlation between putty length and its resistance.
- Voltmeter - This will enable me to measure the Volts passing through the circuit at any one time.
- Connecting Leads – This will make the circuit full, a very important part of the equipment as the circuit relies on these for the energy to transfer throughout the circuit.
- Cell – This is the power source, the cell is the energy source which once connected to the circuit provides energy to the circuit. The power source should add to 6 volts altogether.
- Two two pence piece coins, the coins are used to attach the connecting leads to
Instructions
- Set up the circuit as shown below
- Set the ammeter at VCD and the voltmeter at 20 volts
- Set up the voltmeter in parallel to the wire
- Measure the potential difference across the wire
- Use ohms law to work out the resistance by using this equation R = V/I
Method
1. The putty is rolled into its first measurement of 30cm, into a cylindrical shape
2. The first crocodile clip is clipped to the coins at either end
4. The power supply is turned on. The voltage and current are then read off the ammeter and voltmeter, and recorded.
5. The power supply is then turned off and putty is re rolled into the next measurement, which is 5cm lower than the previous. The same shaped roll should be used.
Circuit diagram
Ammeter
Voltmeter
Conducting putty
Cell
Results
Resistance measurements to 3 DP
To conclude I have found that in this preliminary experiment the greater the length the higher the resistance. So when the length of putty is increased the resistance also increases. Voltage also increases while the current decreases, this in turn increases the resistance as the length of the wire increases.
I predict that this will happen with my second experiment on nichrome wire. This is mainly due to the fact that this theory applies to most substances, the resistance increases with the length due to previous scientific information.
However there were various flaws with this experiment, there was a number of variables that could have affected the results.
- The thickness of the putty could have changed each time: This will affect the experiment as as electrons pass through a circuit, they give off heat. The surrounding atoms use this heat as energy, which causes them to vibrate. As the electrons give off more energy, the atoms vibrate more, making it harder to allow the electrons to get through the wire. Therefore, a thinner piece of wire will create more resistance (because there is less space for the electrons to get though).
- Air bubbles in the putty would effect the experiment as the electrons wouldn’t be able to pass through them in the same they did through the wire
- The battery may not be fully charged and therefore effect the voltage
- The coins attached to the putty could be oxidized and effect the results
- The cylindrical putty shape may not have an equal distribution of putty throughout
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The temperature of the putty could have differed each reading- Higher temperature means more vibrations. Imagine a hallway full of people. Half of the people (the electrons) are trying to move in the same direction you are and the other half (the protons) are evenly spaced but stationary in the hallway. This would represent a cold wire. Since the wire is cold the protons are not vibrating much so the electrons can run between them fairly rapidly. As the conductor (hallway) heats up, the protons start vibrating and moving slightly out of position. As their motion becomes more erratic they are more likely to get in the way and disrupt the flow of the electrons. As a result, the higher the temperature, the higher the resistance. A prime example of this is when you turn on a light bulb. The first instant, the wire (filament) is cold and has a low resistance but as the wire heats up and gives off light it increases in resistance. As a result we can say that Ohm's law holds true unless temperature changes. http://www.regentsprep.org/Regents/physics/phys03/bresist/default.htm
- The experiment was only carried out once
Since Ohm’s law isn’t accurate if the temperature isn’t constant the results found in the preliminary experiment are very inaccurate based on the limitations.
Evaluation
I would improve my experiment by making the measurement techniques more accurate, for instance I would measure the diameter of the putty exactly using a set of callipers, this would mean that I would know the exact diameter and thickness and keep it the same throughout the putty, I would also repeat the experiment at least three times in order to ensure a fair test. Another improvement would be to keep checking the temperature of the putty in order to ensure that the temperature at which the experiment is carried out is the same each time.
From the preliminary the following things have been learned and can be used to ensure that the actual experiment would be more accurate than it would be otherwise
- The temperature of the wire must be kept the same everytime a reading is taken
- The thickness of the wire should not change throughout the experiment
- The power sources should be fully charged.
- there should be an equal distribution in the wire, e.g. no knots or coils
- The material of wire used should be the same.
Second Experiment (Nichrome Wire)
This is experiment is to deduce whether or not there is a correlation between wire lengths and the resistance in a material. The material that is used to nichrome wire. Due to the previous experiment although it was highly flawed the scientific information that reinforces I predict that if the length of the wire is increased the resistance will also increase.
Equipment
- 100cm length of nichrome wire
- Ammeter – This will enable me to measure the amps passing through the circuit at any one time.
- 100cm Ruler – Has to be used to measure the wire accurately to see if there is a correlation between wire length and its resistance.
- Voltmeter - This will enable me to measure the Volts passing through the circuit at any one time.
- Connecting Leads – This will make the circuit full, a very important part of the equipment as the circuit relies on these for the energy to transfer throughout the circuit.
- Cell – This is the power source, the cell is the energy source which once connected to the circuit provides energy to the circuit. The power source should equal 6volts
Instructions
- Set up the circuit as shown below
- Set the ammeter at VCD and the voltmeter at 20 volts
- Set up the voltmeter in parallel to the wire
- Measure the potential difference across the wire
- Use ohms law to work out the resistance by using this equation R = V/I
Method
- The wire is stretched out on the ruler and measured to its first measurement of 10 cm
- The crocodile clips are attached to either end of the 10cm wire
- The power supply is turned on. The voltage and current are then read off the ammeter and voltmeter, and recorded.
The power supply is then turned off and putty is re rolled into the next measurement, which is 10cm higher than the previous.
- It was also decided to allow the wire to cool between experiments as considerable heat was noticed at lower lengths and, as mentioned above, an increase in temperature results in an increase in resistance. By allowing the wire to cool between experiments a fair test could be assured.
Nichrome wire
Voltmeter
Ammeter
Cell
Results
Averages for each wire were then calculated to give these results, which were then graphed:
Conclusions
Having performed the investigation, the following conclusions were drawn:
- As predicted, an increase in length resulted in an increased resistance.
- The wire shows a strong trend of a straight line, i.e. the length of the wire is shown to be directly proportional to the resistance – double the length and the resistance doubles.
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The overall resistance of the three experiments seems to differ considerably. Due to the strong correlation of the results, the explanation of this is likely to be the method used to obtain the results such as faults in the measurements or the stretching out in the wire, which was defiantly a problem creating knots and coils. Obviously this is a rather important oversight and this will be discussed more in the Evaluation section. The reason why this is the likely explanation is because resistance is known to be inversely proportional to the cross-sectional area, i.e. if you increase the cross-sectional area (by increasing the diameter) then you decrease the resistance. This is because a wider wire means less likelihood of the free electrons having collisions and losing energy.
In turn this also means that if increased length increases resistance then it would be theoretical that the thicker a piece of conducting material is the higher increase in resistance you would get. This would possibly be another experiment to try in order to solidify a conclusion on the relation between resistance and material qualities.
Evaluation
- As mentioned previously, the biggest downfall of the investigation was the apparent mistakes when choosing the wire, in that they would appear to be of differing diameters. This did not, in this case, cause a big problem as the same wire was used for each set of results so it is known that the results for each wire are correct.
- Generally speaking, the wire would appear to contain the most accurate results due to the fact that all of its points bar one sit on the line of best fit for that wire. The only one that does not is the point at 90cm, which was exactly at the point that the black mark (mentioned previously) was found to be.
Some outliers could be because:
The length of wire for that particular measurement was not correct. At some measurements it is possible that the length was shorter, causing a lower resistance, and at 90cm it is possible that it was longer, causing a higher resistance. The solution to this is to measure the lengths more carefully and ensure that the wire is pulled tight against the meter rule.
For a particular result, one or more of the connections could have been faulty, causing extra resistance at the connections. A solution to this would be to, before each experiment, connect the connections together without the wire in place and measure the resistance then. If it is higher than it should be then the connections could be cleaned.
Whilst extremely unlikely, it is conceivable that the power supply was providing a different voltage for some of the results.
The other three readings have almost the same inaccuracy, an average of 10%, which again, is fairly accurate. The inaccuracy could have been because of the wire coming from a different manufacturer to the predicted results, as there is some discrepancy between the amount of copper and nickel in different brand’s wire. The ammeters and voltmeters could have been damaged and reading falsely on both the meters used.
Measuring the lengths of the wire is also an inaccuracy as the rulers used are not exact, and it is difficult to get an accurate reading of length by eye, as the wire might not be completely straight, it may be of different thicknesses throughout the length. These would have contributed as well to the error. These results would be difficult to improve on as they are reasonably accurate, and there were no anomalous results. But if I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings and more readings so that a more accurate average can be taken.
I would also investigate other factors, such as temperature, voltage and current, and see how these affect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance. As these results had a range of only 5 readings, from 0-100cm, and were only repeated three times, and that the results are not 100%, accurate due to the errors discussed earlier, then I would say that these results are not strong enough to base a firm conclusion on because there are so many sources of error, which are explained earlier.
If one were to assume that Ohm’s Law applies, then another possible explanation could be that at some points (more likely in the lower lengths), the wire was not allowed to cool completely so that the temperature was higher for that measurement. This would cause a higher resistance as explained previously. The wire was allowed to cool but not to a definite temperature, it was just based on how hot the wire felt, usually the cooling period was between 2-3 minutes.
