K4.1 Electricity and Magnetism
This article covers basic aspects of electricity including conductors, insulators and magnetic materials, electrical circuits and energy. Electrical calculations are included (for secondary). Emphasis is given to barriers to understanding and to alternative conceptions and some useful models for current flow and energy transfer in an electrical circuit are explored. Unusually there are no downloads from this article at present.
This is one of 17 articles whose main aim is to support the processes of teaching/learning between the science education tutor and the trainee science teachers with a focus on “teachers’ knowledge and understanding”. During a primary or secondary BEd, PGCE or GTP we hope that those learning to become science teachers will be able to challenge their own understanding of science and scientific concepts. Unit K0 specifically explores general issues relating to all the knowledge units - to the learning of science.
Standards: This unit specifically addresses Q14 but, appropriately used can contribute to and provide evidence of competence for many others of the standards especially Q4,6,7,8,18, 22 and 25.
Keywords: Electricity, Energy, Circuit, Magnetic materials.
The barriers to understanding magnetism and electricity
1.0 Conductors, insulators and magnetic 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 - and when it occurs naturally in a 'magnetised' form - 'lodestone'.
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)
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.
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 (link needs shockwave installed). 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 bottles per day.
Watts, the power, or rate at which energy is supplied (energy per second), is then easy to work out:
- To know how much milk you get each day, you need to know how many bottles per day (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. In this model the tokens (smarties?) get transferred from cell to bulb (energy) and the cups get passed round and round (coulombs [electrons]). The pupils are the atoms in the wire.
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.
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.
This site from USA has a list of electrical misunderstandings that is worth exploring. It contains a strong critique of the "energy carried by electrons model above" suggesting the cycle chain as a better model.
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
Published: 13 Jun 2006, Last Updated: 13 Sep 2008