Episode 110: Resistance and temperature
its applications.
Demonstration + discussion: Resistance and temperature. (10 minutes)
Discussion: Free-electrons in metals. (10 minutes)
Student experiment: Thermistor behaviour. (20 minutes)
Discussion + demonstration: Conduction in semiconductors. (5 minutes)
Discussion: Superconductivity. (20 minutes)
Student activity: Researching superconductivity. (30 minutes + time for reporting back)
Student questions: Using these ideas. (20 minutes)
Demonstration + discussion:
Resistance and temperature
TAP 110-1: Metal resistance decreases as temperature falls.
Free-electrons in metals
electrical energy by heating and increases the electrical resistance.
resistance depend on temperature?
Student experiment:
Thermistor behaviour
Students can investigate the temperature dependence of
the resistance of a thermistor for themselves.
The results should show a clear decrease of resistance
with increasing temperature. This is the opposite of what
happened with the metal.
NB These thermistors are n.t.c. types (negative
temperature coefficient). Other types exist which have a
non-linear positive temperature coefficient.
TAP 110-2: Calibration of a thermistor
(Diagram: resourcefulphysics.org)
Discussion + demonstration:
Conduction in semiconductors
and the resistance begins to fall.
initial demonstration.
How low can we go?
necessarily zero).
able to manufacture room temperature superconductors.
TAP 110-3: Researching Superconductivity
Student questions:
Using these ideas
TAP 110-4: Filament lamp and thermistor in series
TAP 110-1: Metal resistance decreases as temperature falls.
You could simply use a freeze spray on a coil of constantan wire and show that the resistance
decreases as temperature falls. However, it is more satisfying (and makes a bigger impression) if
you show how current increases when a metal resistor is cooled.
Coil of constantan wire.
Filament lamp.
Power supply.
0-12 V DC/AC
You will need to try this first. Connect all
three components in series. Make sure the
constantan wire is tightly coiled up (but that its coils don’t touch each other) and that the lamp
lights dimly when the wire is at room temperature.
If the freeze spray is applied directly onto the constantan wire the light should noticeably increase
in brightness.
If you have some liquid nitrogen available the effect is even greater but request the CLEAPSS
Special Risk Assessment for safety procedures when using liquid nitrogen. You could use a coil
of fine copper wire instead of the constantan.
(Another neat demonstration involving reduced resistance
at low temperatures is the famous ‘jumping ring
experiment’. This involves the induced emf and current in
an aluminium ring suspended above an electromagnet.
When the electromagnet is switched on there is a sudden
induced emf and current which, by Lenz’s law, results in a
repulsive force. The ring ‘jumps’. If the ring is then cooled in
liquid nitrogen and used before it warms back to room
temperature the jump is much bigger. The drawback of
using this at this stage is that weaker groups will not
appreciate the role of electromagnetic induction.)
(Diagram: resourcefulphysics.org)
TAP 110-2: Calibration of a thermistor
Data and graphs show the difference in electrical behaviour of metals and semiconductors. This
experiment gives direct evidence for the effect of temperature on the resistivity of a
You will need
Thermistor e.g. disc type 4.7 kΩ at 25°C
digital multimeter set on resistance range
two crocodile clips
4 mm connecting leads
250 ml beaker
source of hot water – an electric kettle is simplest
ice cubes
clamp, stand and boss
What to do
Place the thermistor in the beaker. Use crocodile clips and leads to attach the multimeter,
which should be set on the 10 k range. Pour in hot water.
Record temperature and resistance in a table. Cool the thermistor by steps by adding
cold water. You should obtain ten or more pairs of readings as temperature is taken down
near 0 C.
Data analysis
Plot a graph of resistance against temperature in Kelvin. Write in words the pattern you
have found.
Practical advice
Encourage students to plot the graph while the data are collected. To establish a range of values,
they will need to note the thermistor resistance and room temperature before starting.
The resistance will rise as temperature falls, given an n.t.c. (negative temperature coefficient)
More able students may be asked to check whether the curve displays equal ratios of resistance
changes over equal intervals of temperature. Point out that this pattern is called 'exponential'.
Second-year students might return to this activity, to plot ln(R) versus temperature / K and check
whether it produces a straight line.
This is a good practice exercise for the making sense of data task.
Note that the exponential relationship is only approximately correct.
Alternative approaches
If you cannot afford the teaching time for students to do their own experiment, you could either do
it as a demonstration or simply provide raw data, as tabulated below:
Temperature / K
Social and human context
Because thermistors are cheap and they have a low thermal capacity, they can be used in
temperature sensors, e.g. to make a disposable thermometer.
External references
This activity is taken from Advancing Physics Chapter 5, 320E
TAP 110- 3: Researching superconductivity
Student instructions
You are to use books, articles or websites to research superconductivity. At the end of this you
should summarise your findings on a single sheet of A4 that must be handed to your teacher by
…………… (some time in advance of the feedback session). This will be checked, photocopied
and distributed to the class. In the lesson you will be asked to talk us through your findings (be
prepared to speak for up to 5 minutes - by all means prepare some PowerPoint slides to illustrate
your points). Your research should focus on (one of the questions below):
How did Kammerlingh Onnes discover superconductivity?
How can the low temperatures for superconductivity be reached?
What are typical transition temperatures for metals and high temperature
superconductors? Include a table and explain why high temperature superconductors
are exciting.
Why are superconductors used in strong magnets? (Find at least two examples and
explain how superconductivity is maintained).
Superconductors can be used to store energy. How?
How would room temperature superconductors change the world?
How are superconducting materials used in electronics and what advantages do they
have over conventional conductors?
Useful websites:
Kammerlingh Onnes: http://nobelprize.org/physics/laureates/1913/onnes-bio.html
Superconducting magnets:
High temperature superconductors:
Make your own! http://www.webcom.com/cfsc/scpart1.html
Applications: http://www.physnet.uni-hamburg.de/home/vms/reimer/htc/pt4.html
Energy storage: http://www.doc.ic.ac.uk/~mpj01/ise2grp/energystorage_report/node8.html
TAP110- 4: Filament lamp and thermistor in series
Before starting
Make sure you are familiar with the characteristic graphs of the filament lamp and the thermistor.
Surge current protection
Sketch graphs of current vs. potential difference for a filament lamp rated 230 V; 25 W.
Explain the shape of the graph in terms of the effect of temperature on the conductance /
resistance of the lamp.
Sketch graphs of current vs. potential difference for a thermistor whose resistance
decreases with temperature.
resistance of the thermistor.
This circuit contains a filament lamp and thermistor in series with a 230 V supply. When
the switch is closed, the lamp glows dimly at first, but then gets brighter and brighter until
the lamp is lighting normally. Explain these observations.
230 V
filament lamp
4. How does connecting a filament lamp in series with a thermistor protect the components from
a surge current?
Practical advice
This question can follow activities on electrical characteristics. The student is expected to
remember the shape of the characteristics. It is important for students to relate the characteristics
to the change in conductance / resistance with temperature. The observations in question 3 can
be demonstrated using a torch bulb of high resistance connected to a low voltage supply. It is not
wise, for safety reasons, to use the mains voltage for laboratory demonstrations of the effect. A
pair of circuits, one with and one without a thermistor in series and switched on together, neatly
shows the delay.
Social and human context
Many devices, from modern lighting systems to computers, need protection against surges in
current, particularly when turning them on.
Answers and worked solutions
With current on the y-axis and pd on the x-axis, the graph should slope steeply for low
currents (low resistance, high conductance), and then curve to become flatter at high
currents (higher resistance, lower conductance).
With current on the y-axis and pd on the x-axis, the curve should have a small slope to
begin with (high resistance, low conductance at small currents), and then start to rise
more steeply (lower resistance, higher conductance, as the thermistor becomes hotter).
As the filament wire gets hot the conductance decreases and the resistance increases.
As the thermistor gets hotter its conductance increases and the resistance decreases.
When the switch is closed both components are cold. The thermistor has a high
resistance which limits the current. Most of the potential difference is across the
thermistor and so most of the power is dissipated in the thermistor. This heats up the
thermistor so its conductance increases and more of the potential difference is applied
across the filament lamp. More power is dissipated by the lamp which now glows
brighter. The bulb glows more and more strongly as the thermistor's conductance
increases. As the filament gets hotter its resistance increases. Eventually the situation
stabilises when the resistance of the filament is far greater than that of the thermistor, so
that nearly all the power is dissipated by the lamp.
Surge currents often occur when circuits are turned on and this large initial current,
although only lasting for a fraction of a second, can still overload a component. The high
resistance of the lamp (when cold) limits the size of the surge current.
External references
This activity is taken from Advancing Physics Chapter 2, 280X

Resistance and temperature - Teaching Advanced Physics