Investigating the Relationship Between Temperature and Resistance in a Themistor
Length: 1748 words (5 double-spaced pages)
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To investigate the relationship between temperature and resistance in
A current is the flow of charge round a circuit, this can be in the
form of ions in a liquid or electrons in a metal.
Resistance is anything that slows the flow of electrons round the
Ohm's law states that the voltage is equal to the current multiplied
by the resistance - V=IR
This can be re-arranged to say R=V/I.
Ohms law states that in a metal component the ratio of voltage to
current remain constant, meaning that the resistance stays the same as
long as the temperature remains the same. In this experiment I will be
changing the temperature therefore this rule will not apply, however
the equation R=V/I is always true as it is the way resistance is
In a wire when the temperature is increased the resistance increases.
This is because at a higher temperature the lattice atoms are
vibrating faster and are colliding with the electrical current and
slowing down the flow of charge.
This is also true in a thermistor but there is another competing
effect because it is a semiconductor. Being a semiconductor means that
the outer electrons are not free at room temperature but when heated
the get more energy and are freed. This means that there are more
electrons available to conduct. This does not apply in a normal wire
because the outer electrons are free and the inner electrons are
tightly held in the atom. The two competing effects in the thermistor
mean that when the temperature is increased the resistance decreases.
If the resistance decreases then the current increases.
In this experiment I measured the current round the circuit when the
thermistor was at different temperatures, I heated the water in the
beaker and read its temperature with the thermometer. I then measured
the current at each 50C interval from room temperature (200C) to 1000C.
I did this to see at what temperature I should start my experiment.
When the temperature is low the current does not change a lot and
because the numbers are such small fractions any errors would be a
really high percentage and it would not give me accurate results with
the equipment I am using.
Preliminary Experiment Diagram
Text Box: Thermistor
These results tell me that when the temperature was increased the
current increased and therefore the resistance must decrease (R=V/I)
To calculate the resistance in the next experiment I will also measure
the Voltage using a voltmeter. I need to do this because although I am
keeping the voltage the same on the power pack this may not be
accurate due to the change in resistance round the circuit.
I will set up the equipment as shown in the diagram.
I will fill the beaker with water of room temperature.
I will light the Bunsen burner and when the temperature reaches 250C
will read the current through the thermistor and the voltage across it
and record them.
I will do this at each 50C interval up to 1000C.
I will repeat the whole experiment three times to achieve the most
accurate results I can.
Each time I repeat the experiment I will make sure I use the same
For safety I will stand up whilst doing the experiment ,wear safety
glasses, I will tie my hair back and I will follow the normal lab
I will measure the temperature, voltage and current.
The start temperature will be 250C because from my preliminary
experiment I found this to be the point where the current was changing
enough to draw reasonably accurate results. The step size between
temperatures will be 50C because this is what I used in my preliminary
experiment and I found that it gave me a good number of points to
The stop temperature will be 1000C because I want to measure as high
as I can so as I can see the bigger rises in current. I have to stop
at 1000C as water can not be heated past this in a beaker.
To plot the graph resistance, I will find the resistance readings by
dividing the voltage by the current at each point. Then I will find
the average for the three experiments at each point and plot it on the
graph. I will plot the resistance against the temperature.
I predict that when the temperature of the thermistor is increased the
resistance will decrease.
I think this because when a thermistor is heated there two competing
effects which are; a) that, as in a wire, the lattice atoms vibrate
faster and collide with the flow of charge ( which is the flow of free
electrons) slowing it down and reducing the current. And b) that
because it is a semi conductor the outer electrons get more energy as
it is heated and are freed, they and then free to conduct current.
I think that the second effect will be stronger because it increases
the number of charge carriers. The second effect it stronger and this
means that less of the current is stopped when the temperature
increases and therefore the resistance goes down.
If the first effect was isolated and there was no competing effect, as
in a wire, the resistance would increase as the temperature increased.
When the lattice atoms collide with the flowing current it reduces the
current meaning that it increases the resistance.
I predict that the graph will be a curve. I think this because at a
low temperature there are lots of electrons which can be freed as the
atoms start to vibrate more. At a higher temperature the atoms have
less electrons which can be freed as some have already been freed.
Therefore I think that the reduction in decrease will be largest at
low temperatures and then get smaller as the temperature rises.
Average Resistance (Ohms)
From my graph I can clearly see that as the temperature increases the
resistance decreases. This is because as the thermistor increases in
temperature the lattice atoms move faster and are colliding with the
electrical current and slowing down the flow of charge. In a normal
wire this would make the resistance decrease but because a thermistor
is a semiconductor there is a second stronger effect which out weighs
this. This effect means that the outer electrons are not free at room
temperature but when heated the get more energy and are freed. This
means that there are more electrons available to conduct. This does
not apply in a normal wire because the outer electrons are free and
the inner electrons are tightly held in the atom. The two competing
effects in the thermistor mean that when the temperature is increased
the resistance decreases. If the resistance decreases then the current
The graph shows the resistance dropping from about 300 ohms to about
This is a drop of 250 ohms. The temperature went from 250C to 1000C.
By 500C the resistance has already dropped to about 50 ohms which is
half the decrease in ohms under a third of the rise in temperature
which proves that the resistance decreases most at lower temperatures
and less at higher temperatures. This is as my prediction. It is
because at a low temperature there are lots of electrons which can be
freed as the atoms start to vibrate more. At a higher temperature the
atoms have less electrons which can be freed as some have already been
freed. Therefore the reduction in decrease will be largest at low
temperatures and then get smaller as the temperature rises.
These results support my prediction because the line of best fit is a
curve showing the resistance decreasing as the temperature rises.
Therefore the gradient is greatest at about 300C when there are lots
of electrons to be freed and the gradient is smaller at about 1000C
when the electrons have been freed.
I thought the theory behind the experiment quite complex as it
involved two competing effects but I found the experiment itself quite
straight forward and easy. Our results are reliable enough to support
my prediction and to draw a detailed analysis. They are reliable to a
reasonable degree of accuracy because we took three results and then
found the average. We also know they are quite accurate as the line of
best fit goes close to all the points. In the three experiments we did
the resistances were very similar with very little disparity between
them which shows the experiment was precise. There is only one clear
anomalous result which is at 300C I think this is because the
thermistor had not got the a high enough temperature for many of the
atoms to free electrons. Sources of error could have been that between
experiments the thermistor or thermometer did not cool down properly
to the same temperature. If this was the case it did not have a big
effect on the results as the reading in the three experiments are very
similar. Other sources of error could have been the our reading of the
thermometer. It was difficult to read the ammeter and voltmeter at the
sama time as looking at the thermometer and I could not have read the
voltmeter and ammeter at exactly the same time. I did the experiments
over two lessons so I made sure I kept the same thermistor but I could
not keep the same thermometer, ammeter or voltmeter which may be the
source of some error. The other results are all close to the line of
best fit but I could make it better by doing more experiments to take
the average resistance from, use a more accurate voltmeter and ammeter
or use a data logger to record the results. To build on this
experiment I could investigate what would happen after 1000C and see
if the resistance would start to increase again after the thermistors
atoms have lost all their outer electrons and the lattice atoms
continue to move faster slowing down the flow of current. I could not
use water to do this as water can not be heated beyond 1000C, I would
have to use a liquid with a higher boiling point such as oil. For this
experiment I would set up the equipment in the same way but use oil
instead of water and continue beyond 1000C. I could also investigate
whether the length of a wire had an effect on the resistance of the
circuit there would be more lattice atoms and may be a higher
I would also like to try lower temperatures to see if there was a
minimum energy needed to free the electrons if so the graph would be-