Prediction:
From the information above I predict that as I increase the temperature of the thermistor by heating it in a beaker of oil in a water bath, the resistance, measure in Ohms (Ω), will decrease. This is because a thermistor is a semi-conductor and so as the particles in the thermistor are heated the electrons gain more energy to jump the gap between the valence band and the conductive band and so more electrons are in the conductive band meaning there is more conduction. In turn I also predict the lower the temperature of the thermistor the higher the resistance because there is less energy to transfer the electrons to the conduction band and so it is hard for the electricity to travel because there are less electrons to move.
Variables:
There are a variety of factors that could affect the results of my experiment into the resistance of a thermistor and because I want to get results that are as accurate and reliable as possible I need to assess all these factors and decide what things I will keep constant in my experiment, what I will change and what I data I will record.
One factor that must be kept constant during my experiment is the voltage and current in the circuit. This need to stay the same because I am changing the temperature in order to investigate the resistance of the thermistor and because both voltage and current affect the resistance in a thermistor this means that two variables are being changed and so the results wont be accurate. The current and potential difference (voltage) in affect the resistance in a thermistor because the greater the voltage the more energy the electrons have and so there are more electrons moving in the conduction band and therefore more heat.
All equipment must be kept the same in my experiment. The thermistor must be the same one throughout because different sizes of thermistors react differently to heat and they can be made in different ways to react more or less with heat than others. The amount of water and the amount of oil must be kept the same because oil does not conduct electricity, some of the energy in the circuit will be lost in the oil just as some of the energy from the heat will be lost and so the more oil there is the more energy that is lost. The type of flame I use to heat my water/oil bath must also stay constant because different types of flames give off more heat for example a roaring blue flame gives off a lot more heat than a large yellow flame (the safety flame) and so the hotter the flame the more energy will go into heating the oil that heats the thermistor. The thermometer must also stay the same during the experiment because if it changes it may take some time to adjust to the actually temperate and if the scale isn’t exactly the same throughout the experiment then the accuracy of the data may suffer.
The intervals in which I measure the resistance of my thermistor must also stay constant. For example the resistance should be measure every time the temperature of it is raised by 50C so that the data can show any relationship between heat and resistance as clearly as possible.
Preliminary Test:
In my preliminary tests I needed to find out;
- The best possible way of heating the thermistor
- The range and size of data I record
- What the resistance need to be measured in ie. In kilo ohms or in ohms
I went through a series of options to decide on the best way to heat the thermistor and decide that it was to put the thermistor in a test tube of oil and heat that oil in a beaker of water over a Bunsen burner. I decided on this method because if I was to just heat the thermistor over the Bunsen burner then inevitably the plastic coating around the component would melt and damage the equipment. Neither could I heat the thermistor in a water bath because if then I was just be heating the water around the thermistor and not in the beaker that has the thermistor in it so too much energy would be lost in the air. Also, the results wouldn’t have been as accurate because it is a lot harder to measure the temperature of the thermistor in this method because the temperature in the component is not that close to the temperature of the water of the air surrounding the thermistor. Another method I thought of was to heat the water in a beaker of water but this method would definitely not of been that effective because water conducts electricity and so the electricity being past through the circuit to get to the thermistor would be mostly lost into the water and so the voltage and current in the circuit would change. Oil does not conduct electricity and so I decide this was the best substance to heat my thermistor in but controlling the temperature of oil is quite difficult when heating on its own so I decide to put it in a water bath so that it would heat too quickly and was easier to control so that it doesn’t get too hot.
I decide that the range of my data should be from 100C-400C in intervals of 50C because this gives me a range of seven pieces of data which is enough needed to prove my theories for this investigation and anymore would not be a constructive use of time but any less may not be enough to show the relationship properly. I think that 100C is a good temperature to start with because it will not take to longer time to reach that temperature but it is a still a temperature below the average room temperature and so I can show that the resistance also changes when you lower the temperature as well as increase it. I also think that 400C is a suitable temperature to stop heating the thermistor at because I will already have a good range of data that fits well into the time that I have to do this experiment and also once u begin to heat oil above about 500C it becomes a safety hazard.
The results of my preliminary experiment were as follows;
These results seem to follow what I have already predicted which is that the resistance decreases in a thermistor as the heat increases. I decide to record my data in ohm rather than any other (for example kilo ohms) because this gave the best accuracy of numbers and I thought it better to work in hole numbers rather than decimals. I found that the best way to decrease the temperature of the thermistor is to put ice in the water bath and wait for the temperature to drop so this is what I am going to do and then from 100C I am going to heat the water bath up again with the Bunsen burner. I discover that the best and most accurate way of getting the resistance measurement is using a ohmmeter instead of a full circuit and measuring the current and voltage and then putting it into the equation.
Apparatus:
- Thermistor
- Bunsen burner
- Heatproof mat and gauze
- Beaker
- Test tube
- Oil (regular cooking oil)
- Ohmmeter
- Thermometer
Diagram:
Method:
As in the diagram I will firstly set up all my equipment so that I have a tripod, mat and gauze set up. I will have the beaker half filled with water because this is enough to heat the test tube effectively with out the hazard of the water boiling over or spilling out. Then I will attach the thermistor to the ohmmeter will metal clips and insert the thermistor into the test tube. When doing this I need to be carefully that once the thermistor is inside the test tube the positive and negative wire are not touching because this would cause it to short circuit and then the ohmmeter will not work effectively. After this is down the test tube will be filled up with oil until it covers the thermistor but being careful not to put too much in it because this is not economical and is at risk of spilling or boiling over.
After all equipment is set up and the test tube with the thermistor has been put inside the beaker of water I will put ice in the beaker until the temperature is at 100C. I will measure this temperature using a thermometer will a scale of 10C that goes up to 1000C; anything that has a smaller scale is not necessary because I am only measuring whole temperatures (not decimals) but anything that has a larger scale may not be as easy to read and so could not give results that are as precise. When it has reach this temperature I will record the first resistance using the ohmmeter and then I will continue to heat the thermistor will the Bunsen burner, record the resistance every 50C . I will repeat this experiment twice more to ensure that my results are reliable and so I can plot my graph using an average resistance for each temperature.
Results:
*RETEST – original resistance = 672Ω
Graph:
(on following page)
Data Analysis:
My data shows a clear resemblance to my prediction and fits in with the theory that as the temperature of a thermistor increases the resistance across it decreases. In my graph the data is a curve that decreases quite rapidly from 100C to 150C but decrease at a more constant rate from 150C to 350C. This means that from a temperature of absolute zero, where the resistance should be infinite, to a temperature of about 150C the rate at which the resistance in the thermistor decreases very rapid whereas after this point the rate at which the resistance decreases is a lot slower and more constant. In my data I did have one anomaly which was the resistance at 400C and I can only think of one factor that could have affected this piece of data. The resistance seems to be smaller than it should be in which case one thing that could have affected it was in one of the results record I could have taken down the resistance at a higher temperature than it should have been which would therefore affect the average. In which case, the anomaly is due to carelessness and inaccuracy.
If I ignore my anomaly and assume that the graph continues as it is I would image that at one point it would even out and the resistance would stop decreasing. This would either be at the point of no resistance because after this the resistance would be going into negative number and as this is impossible I would assume that from that point the resistance would be 0Ω if the temperature went any higher. Another possibility is that I would not reach a point of no resistance but instead level out at a resistance slightly above that because it could be the point at which the electrons left in the Valence band need more energy that can be given through heat in order for them to jump the gap to the conduction band.
Conclusion:
From my data that I have obtained from my investigation into the relationship between the temperature of a thermistor and the resistance across it I can conclude that my prediction was correct. A thermistor is a semi-conductor and so this means that as the temperature across it and the amount of energy being transferred into the electrons increases the resistance decreases. This is because as the energy from the heat increases, more electrons jump the gap between the valence band and the conduction band so they increase the current. If you look at the equation I mentioned in my background information (Resistance (Ω) = Voltage (V)/ Current (I)) then you will see that when the voltage in a circuit involving a thermistor is constant, as it was in my experiment, and the current increases (as a result of heating) the resistance will become small because the more the voltage is being divided by the smaller the resistance will be.
I also conclude that the behaviour of a resistance is that from a low temperature the resistance of a thermistor drops quickly and then begins to decrease at a slower rate until it reaches a point at which the resistance stays at a constant unit which I think would be 0Ω but I do not have enough evidence to support this statement.
These properties in a thermistor make it very useful in industry for things such as machinery that can be damaged if it over heats. Putting a thermistor into a circuit such as that could mean that if the machine reaches a certain temperature (probably just before it becomes so hot that it is a risk) it can automatically shut down or trigger off a fan that can be power to cool it down again to a safe temperature.
Evaluation:
Even though during my experiment I didn’t have any problems in obtaining the data I found that the data I did obtain wasn’t as accurate as it could have been and so as a result of this I had a few anomalies. This inaccuracy of my data was mostly due to incorrect readings of the thermometer; this problem could be resolved if the accuracy of the readings were improved by instead of using the type of thermometer that I used I could use a electronic thermometer with probes which is a lot more accurate and sensitive to temperature so the readings would be more reliable.
I would also have like to have obtained a wider range of data because I did not think about the consequences of getting anomalies I think the number of results I needed to obtain should have been more. To improve this I would begin by measuring the resistance of the thermistor at 00C to emphasis the rapid decrease in resistance that I have in the start of my results and to see if 00C is the point of infinite resistance for a thermistor. I would always go on to obtain the resistance from temperatures a lot higher than 400C but in doing this I would have to take a lot more safety precautions because, past a certain temperature, oil becomes quite dangerous to heat. If I did this I would be a definite improvement on my result because it would mean that I could investigate further on the behaviour of a thermistor and try to find out at what temperature the thermistor I was using reaches no resistance or the point at which the resistance across it stops decreasing.
Apart from these points I think my method in my experiment was effective and allowed me to obtain my results safely without too many faults. The units in which I recorded my results were correct and showed the results of the data I collected clearly so if I was to do it again then I would keep the method and units in which I measure the temperature and resistance the same; only changing some of the equipment so that the data could be more accurate.
