Teacher notes ( 1 of 2 )
Hot wires
Curriculum links
England and Wales (Key Stage 3 Science Programme of Study)
key concepts
1.1b, 1.2a
key processes
2.1a-c, 2.2a, 2.3a
range and content
3.1c
curriculum opportunities
4a-c
Northern Ireland (Science Statutory Requirements)
knowledge, understanding and skills
develop: enquiry skills; critical thinking; practical skills
research information
learn about: using electricity
objective 1 – develop as individuals
mutual understanding: team work
objective 3 – as contributors to the economy/
environment
identify skills used in: electrical engineering
Scotland (SQA Science Outcomes)
third level
SCN 312J
fourth level
SCN 417J Phys
fourth level
SCN 439W Phys
Introducing the activity
You may wish to start by discussing what the pupils already know about electricity and circuits. They
should, at least, be able to tell you:
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that we get electricity from mains sockets or batteries
that electricity can be dangerous
how to make a simple, series circuit
how to draw a circuit containing various components using pictures or symbols
how the brightness of light bulbs or speed of motors can be changed
what conductors and insulators are, and give examples.
The pupils may have some knowledge of parallel circuits, but may not have investigated them
thoroughly. Although they may be aware that the ‘strength’ of batteries is defined by their voltage, it
is unlikely they will have done much work involving meters or electrical units. Furthermore, pupils will
have limited understanding of electric current in terms of energy transfer, and how electrical energy is
transformed.
Discuss why it is important to be able to control current flow. Establish that large overloads of
electrical flow can potentially damage equipment and even start fires.
Ask the pupils to think of different situations where electrical current is controlled. Draw attention to
the poster and the wide range of electrical equipment and ways in which electricity is being used.
Explain to the pupils that they are goiwng to investigate one way of limiting current in electrical
circuits to prevent overload. That is, fuses.
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Teacher notes ( 2 of 2 )
The practical activity
• The activity can be carried out individually, in pairs or small groups.
• Try the experiment out beforehand to establish suitable conditions – matching the length of steel wool to the
light bulb rating. The activity was trialled with a 1.5 V 0.3 A bulb and a 7 cm length of fine grade steel wool.
However, the optimum length depends on the dimensions and resistivity of the steel wool, as well as the rating of
the bulb
• Discuss the activity with your pupils so they can carry out the experiment correctly and safely - ensuring they
know how to construct their circuit correctly and can connect and read an ammeter.
• Pupils need to be able to observe the bulb glowing dimly at low current, and progressively brighter as the current
increases, but not burning out before the ‘fuse’ blows. That is, the steel wool must melt before the bulb filament
does.
• Pupils should notice that when the fuse blows, the circuit is broken and the ammeter returns to zero. The bulb
goes out because there is no longer any current flowing through it. To show that the bulb has not burned out, (i.e.
has been protected by the fuse), pupils reduce the voltage setting and reconnect the circuit without the fuse. The
bulb should light up.
• From their results, pupils draw a graph of current against voltage. Depending on the stage at which the activity is
performed, some pupils may expect a straight line graph, showing current to be directly proportional to voltage.
It does not, because as current increases, both the fuse wire and bulb filament become very hot. This increases
their resistance significantly, so the circuit does not follow Ohm’s Law, which applies only to a constant resistance.
Fuse theory
• After the practical, discuss with pupils the significance of their results, explaining that this is the basis of all fuses
‘Fuse’ derives from ‘fusion’, the scientific term for ‘melting’.
• As current increases, the amount of electrical energy transformed into heat and light increases. The very thin fuse
wire eventually melts, breaking the circuit. To protect the other components, the fuse must melt (or ‘blow’) before
the current gets high enough to damage them or start a fire. However, if the fuse wire melts too easily, the fuse
blows before the current is high enough to operate the other component(s). This can be demonstrated by
repeating the experiment with a 6 volt bulb. The steel wool ‘fuse’ will blow before the bulb glows even dimly.
• It is therefore important to match the fuse rating to that of the electrical equipment being used. Hence the use of
different fuses in household plugs – for example, 3 A for a table lamp, 13 A for a kettle. (These cartridge fuses have
a thin fuse wire inside the ceramic tube.)
Equipment (per group)
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DC power supply (variable up to 10 or 12 V)
light bulb in holder (1.5 V 0.3 A or similar)w
ammeter (0 to 1 A, reading to two decimal places)
4 x connecting wires
2 x crocodile clips
Blu-tack
2 x pieces of aluminium foil (1 cm by 2 cm)
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fine grade steel wool (approximately 7 cm single strand) – to act as fuse wire. Proper low-ampage fuse wire may be used, but
it must blow before the bulb. Household fuse wire is not suitable.
heat-resistant mat
eye protection
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Safety note: steel wool gets extremely hot during the experiment. Pupils should wear eye protection as sometimes tiny globules
of hot metal are sprayed around.
Possible extension activities
• Use secondary sources to investigate the development and application of electricity and electric current.
• Investigate the relationship between current (Amps or A), voltage (Volts or V) and power (Watts or W).
• Use an ammeter to investigate any differences in current between series and parallel circuits.
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Visit our web
d careers
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information.