# The Effect of Temperature on the Resistance of a Metal Wire

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The Effect of Temperature on the Resistance of a Metal Wire

How does different temperatures affect the resistance of a metal
wire/thermistor.

Aim (preliminary):

The preliminary aim of my coursework was to measure what affect
different temperatures have on the change in resistance of a copper
wire.

Apparatus:

* Distilled water

* Heat proof mat

* Tri pod

* Bunsen burner

* Copper wire

* 5v power pack

* Electrical wires

* Crocodile clips

* Thermometer

* 100W resistor

Diagram:

Resistor
========

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HEAT
====

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In theory this was a good experiment however in practice it was not.
Whilst conducting the experiment I realise that from the temperature
changing from 0 degrees to 100 degrees there was only a small change
of 0.01v. I tried with a different resistor but still had no luck.
There was not enough change for me to conduct a worthwhile experiment.
I tried to use a higher current on the power pack but still the
voltage never changed of the copper wire and as a higher current was
used I ad to press the reset button as the power pack continued to
reset. Therefore I concluded that using copper wire to conduct this
experiment was not appropriate as it’s resistively was too high to be
affected by small or even large changes in temperature.

Instead of using a metal I decided to use a thermistor as it can be
subjected to different temperatures and I can expect some results

Aim:
The aim of my investigation is to see whether the resistance of a
thermistor (a type of temperature sensitive resistor) is directly
proportional to the temperature. I'm anticipating that this
experiment will be interesting and relatively challenging because a
thermistor is a type of resistors whose resistance changes

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significantly when its temperature changes, consequently a slight
variation in temperature can lead to a vast change in the resistance,
therefore I have to be extremely accurate when calculating the
temperature as a mistake or inaccurate reading on the thermometer will
affect my results considerably.

Introduction:
I have decided to use a thermistor in this investigation due to its
stability, resolution/sensitivity, suitable range, consistency and low
cost. As a medium in which to test the thermistor's behaviour, I have
chosen distilled water - It is accessible and a high specific heat
capacity. The specific heat capacity of the medium needs to be a lot
larger than thermistor's so that the temperature of the medium remains
constant when the thermistor is immersed in the water.

Thermistors are thermally sensitive resistors and have, according to
type, a negative (NTC), or positive (PTC) temperature coefficient.
They work because at low temperatures, electrons are fixed onto atoms
and so cannot move. As the electrons get hotter they receive enough
energy to break away from their atoms, so the thermistor becomes a
better conductor.

In this experiment I do not know if I will have a NTC or a PTC
thermistor but if the resistance decreases as the temperature
increases then I will know I am using a NTC thermistor, if the
temperature and resistance are opposite to that of a NTC then I will
be using a PTC.

[IMAGE]NTC thermistors offer many features for temperature measurement
and control within their operating temperature range. NTC are also
known as a ceramic semiconductor.

[IMAGE]NTC thermistors show a decrease in electrical resistance with
increasing temperature. Depending on the materials and methods of
manufacture, they are generally used in the temperature range of –50
degrees to 150 degrees, and up to 300 degrees for some glass units.
The resistance value of a thermistor is typically referenced at 25
degrees.

Thermistors are semi conductors so the resistance of a thermistor
varies with temperature. A thermistor can be considered to be a semi
conductor therefore the resistance across it will decrease as the
temperature increases the resistance across it decreases.

Semiconductors are solid materials with conductivities in between the
very high conductivity of metals and the very low conductivities of
insulators. There are a variety of types of semiconductor, including
metal oxides as well as elements like silicon. In insulators, all the
electrons are tightly bound to atoms or ions, and none are free to
move under an external electric field. Therefore these materials do
not conduct electricity at all. In metallic conductors, essentially
all the atoms are ionised, providing free electrons, which move freely
through the ions and can move under an external electric field. These
electrons 'glue' the ions together, and provide non-directional
bonding which holds the material together. They become shared amongst
all the atoms in the material instead of remaining attached to one
atom.

Semiconductors differ from both insulators and metallic conductors.
Only a small proportion of atoms are ionised, so that although there
are conduction electrons they are relatively small in number and the
material conducts, but not well. At higher temperatures, more atoms
are ionised, and the conductivity rises.

Apparatus:

Apparatus

Reason

Thermistor

To find out how the resistance of the thermistor changes as the
temperature increases.

Power pack

The source of power of the circuit, so that the thermistor works and
also to find the resistance of the thermistor.

Ice.

To decrease the temperature of the water that the thermistor will be
in.

Bunsen burner

To heat the water the thermistor will be in.

Thermometer

To know when to take the resistance reading and to measure the
temperature of the water.

Beaker of distilled water

To provide an environment where the temperature change can take place
so the thermistor's resistance can change.

Resistor (100W)

To see what effect it has on the thermistor

Tripod, gauze and heat proof mat.

For the beaker to be heated on.

Tongs

[IMAGE]

Voltmeter

To safely remove hot beaker

To measure the p.d across the thermistor

Diagram:

Method:

1. Collect and set up all apparatus as shown

2. Record the resistance of the thermistor at room temperature.

3. Place ice in beaker to get to lower temperatures.

4. Wait for temperature to get as low as 10 degrees Celsius

5. Place thermistor in the beaker of water

6. Record resistance from voltmeter.

7. Remove ice from the beaker and switch on Bunsen burner.

8. Allow the temperature to increase

9. Record results in 10 degree intervals up until 100 degrees Celsius

10. Repeat steps 3 – 9 twice for repeats.

11. Work out average and plot calibration graph whit resistance
against temperature.

Risk Assessment/safety:

There is only element that provides a major risk in this experiment
this being the extremely hot water. There is no real protection
against this but if precautions and care is taken with regards to the
surroundings and others working in the vicinity. If scalding occurs
then the skin will have to be submersed in ice water to minimise the
pain. Although water and electricity are not usually a good
combination the fact that only 5V are passing round the circuit there
is no real danger from the charge.

Fair test:

In order to make this experiment a fair test I will have to perform
the experiment under the same conditions as each other and be as
precise with my results as possible. To do so I will write my results
correct to the 3rd decimal place, which is rather precise. Also I will
try and conduct them under the same room temperature as the slightest
of alteration could affect my results considerably. I will conduct
repeats in order to make my results more reliable. I will leave the
thermistor in the certain degrees of water for the same amount of time
each time when recording my results as sometimes it may take longer
for an accurate reading to be taken down and it takes time for the
thermistor to get used to the new temperature.

A couple of other important things were ensured in order to keep the
results fair and accurate. One was to keep the thermistor as close to
the thermometer's bulb as possible. This was so that their readings
were being affected by the same part of the water because (due to
convection currents and uneven heating) the temperature of the water
was not equal throughout the beaker.

Results:

Temperature (°C)

P.d (V)

P.d (V)

P.d (V)

Average P.d (V)

10

4.29

4.19

4.22

4.23

20

3.95

3.92

3.96

3.94

30

3.79

3.83

3.74

3.79

40

3.51

3.47

3.53

3.50

50

3.09

3.13

3.14

3.12

60

2.82

2.90

2.85

2.86

70

2.32

2.36

2.41

2.53

80

2.08

2.02

2.01

2.04

90

1.85

1.70

1.72

1.76

100

1.45

1.42

1.51

1.46

Conclusion:

Firstly I found out that I was using a NTC thermistor as I found out
that as the temperature of the water increased the p.d across the
thermistor decreased. This is expected with a NTC thermistor. This is
due to the semiconductor material of the thermistor. The material
becomes more conductive as the temperature increases. The reason for
this is because some of the electrons of the semiconductor are able to
cross from the valance band to the conductance band. The sensor
performed relatively well as only one anomalous result was found, this
was at 10°C, the reason for this was maybe that the thermistor could
not perform as accurately under such cold temperatures as for all the
other readings the thermistor seemed to be working in perfect order.
The resistance at different temperatures were very similar all three
times I took a set of readings, when the temperature was increasing
and when the temperature was decreasing. The sensor performed as
expected and produced very accurate and precise results correct to the
third decimal place.

I plotted my results on a graph where the shape of a straight line
appeared which indicated that there was no real fault or indication
that my thermistor was not working properly. All results came up as
they should have been coming up. I calculated the gradient of the
straight line at 0.03, which indicates that it was quite sensitive and
could detect small changes in temperature.

Therefore I can conclude that my thermistor performed fairly well,
this is because the results followed a pattern, my thermistor
performed successfully because the response time was good, there was
an unsystematic error and therefore I can conclude that my readings
were fairly accurate

Evaluation:

There was an anomalous result at 10°C. This is due to a number of
reasons described below. For these reasons, this system will not be
useful where extreme accuracy is required, but it was adequate for the
proposed application.

Below I have described the factors, which will have affected the
accuracy of the thermistor's performance.

I chose water as convection occurs quickly and the heat is fairly
evenly distributed. I also made sure that the thermometer and
thermistor were very close to each other. However there still may have
been some slight differences in temperature between the point, which
was being measured by the thermometer, and the thermistor. Therefore,
next time it would be a good idea to stir the water constantly.

Further more the voltmeter may have needed to settle, but because the
water was always being heated or cooled, it would have not had time to
do so and so was not reading the resistance/voltage at the actual
temperature, but rather it was behind.

In addition to this the thermistor itself has a relatively slow
reaction time to the change in temperature. By using a resistance
thermometer, this response time can be decreased dramatically. The
thermistor may have been releasing some energy also which it will have
then sensed, and will have affected the thermistor's resistance.

The thermometer has a 0.03% error margin and the thermistor has a 10%
error margin. This gives a total maximum percentage error of 10.03%,
which is very high. The accuracy and stability of the voltmeter is an
error margin is likely to have been the most influential source of
inaccuracies in the results.

Further more I could do other investigations having done this in the
future I could use a PTC thermistor instead of the NTC thermistor
used. Also I could use other semi conductors or even I could change
the resistance to see its effects on temperature, I could also go back
to my original idea, which was to see what effect the temperature of
water has on a coiled metal wire.