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The barriers
to understanding magnetism and
electricity
1 Conductors, insulators
and magnet materials
Most children will happily describe metals as conductors
and all other substances as insulators. This is good enough
for primary school but there are some non-metallic conductors:
- Ionic solutions and melts
- Graphite and other 'conjugated' organic molecules (conducting
plastics were discovered in 1977)
- Gases also become conductors at high voltages (think of
lightning strikes, and fluorescent lighting)
Some materials are classed as semi-conductors
and are used to make electronics
More problematic are magnetic materials. Many children
think all metals are magnetic, and nothing else is. But only
a few metals and alloys show magnetic properties (principally
iron), yet several oxides and other compounds are also magnetic,
notably the magnetic oxide of iron Fe3O4 called magnetite.
2 Electrical energy
Electricity is a way of transferring energy - rather like
a bicycle chain. We talk of using electricity by which we
mean making 'use of' the energy. Energy comes in as fully
useable, but once transferred (or converted) it has begun
its degradation into waste heat.
We need to avoid using the term electricity, and instead
say electrical energy (or electric current - see below)
3 The electrical circuit
The big confusion here is the idea that although electrical
energy is transferred from the cell to the bulb, electrical
current travels round 'transferring' the energy. When
students say electric current is 'used up, what they are saying
is that electrical energy is being degraded - energy stored
in the cell, or 'generated' at a power station is transferred
to light and heat at the bulb. If you want to light the bulb
again you must 'use up' more of the energy stored in the cell.
Students and pupils should make a light bulb glow using a
cell a wire and a naked bulb (not in a holder) so they realise
that a complete circuit is needed which, includes the wire
in the bulb.
4 Why is a circuit needed?
We can then ask the question - why is a circuit needed? If
energy is transferred from the cell (which eventually becomes
'flat) why do we need a return wire?
This is where the milk
bottle model is so powerful. Energy (the milk) is transferred
from the cell (dairy) to the bulb (the house) but the bottles
must be returned to enable the process to continue. (For information
on how to obtain the CD ROM please click here Science
Issues.
Milk in pints represent energy in joules,
Bottles (which carry the energy) represent the coulombs (packets
of electrons)
Voltage (joules per coulomb) tell you how full each bottle
is (they have to be very big for the model to work, allowing
high voltages - eg 240 volts would be 240 pints in one bottle!)
Current in coulombs per second is represented by bottle per
day.
Watts, the power, or rate at which energy is supplied, is
then easy to work out:
- To know how much milk you get each day, ask how many bottles
(current) and how much is in each bottle (voltage); thus
power is (coulombs per second)x(energy per coulomb) or (amps)
x (volts), giving you how much energy is 'delivered' per
second - the power, in watts.
Other models are available - eg the smartie model where children
stand in a circle all holding one cup each (coulombs) and
one (the cell) has a pile of smarties (joules). They pass
the cups from hand to hand round the circle. When a full cup
reaches the person representing the bulb they empty the cup
which return empty. Two cells in series fills the cup twice
as much - double the voltage.
The supermarket
model from the home of research into children's ideas
(Leeds University) makes the same point - it is energy that
is 'delivered' and containers or carriers that go round and
round.
5 Electrical calculations
(for secondary teachers in training)
Until students and teachers have grasped the basic mechanism
of energy transfer by carriers that go round and round, any
attempt to do calculations on V=IR and P=VI etc are pointless
exercises in memorising meaningless formulae.
Early work on calculations should use expressions such as
"If I double the number of bulbs …", and refer
students back to the analogies/models you have introduced.
Other misconceptions
This site in South Africa has a list of electrical
misunderstandings that is worth exploring. The whole site
is good.
Giving Practical experiences
Secondary teachers in training will need experience handling
some of the more unusual apparatus: Van der Graaf generator,
electroscopes, electromagnetic devices, electrolysis, power
cable demonstration, etc used in teacher demonstrations, though
normally this experience will come from their time in school.
Primary teachers in training will need to do all the experiments
they are likely to ask their pupils to do, though it will
not take them long! Much better if the circuits they make
are 'useful'. Thus quiz boards, burglar alarms and various
other projects all help to make the ideas real, for both trainee
teacher and pupil.
Section Developed by:
Keith Ross, University of Gloucestershire
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