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Electromagnetism guidance notes
The following guidance notes cover these practical collections:
Here are five ways of exhibiting magnetic field patterns:
- Large posters of magnetic fields can be plotted out with a compass to make a classroom display.
- Paper can be waxed with paraffin wax and placed on top of a magnet. After the iron filing pattern is produced, melt the wax slightly with a candle placed underneath the waxed paper. When the candle is removed and the paper cooled, the iron filings stick to the paper. This method was used by Faraday.
- An iron filing pattern can be made permanent by spraying gently with hair spray (lacquer).
- Student or teacher can make a quick demonstration using an overhead projector. Instead of covering the magnet with card, before sprinkling iron filings, place a piece of transparent plastic on top of the magnet. Sprinkle iron filings on it.
- Photographs of magnetic fields can be produced by replacing the card with photographic paper. In the dark, or with a red safety light on, sprinkle iron filings onto the paper more densely than normal. Switch on the laboratory lights for 20 seconds. Then switch off the lights and shake the iron filings off the photographic paper. Develop the photographic paper in the normal way, taking the usual safety precautions when handling photographic developer.
Safety note: Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.
Download the following images of magnetic field patterns.
Simple theory of permanent magnetism
Say to students:
Imagine you continue to cut up a magnet into smaller and smaller pieces. If you always found little magnets as a result, you might pretend those little magnets were already inside the big magnet before you started.
Try to sketch a picture of these ‘basic magnets’ in a completely magnetised bar. Start by drawing small rectangular magnets, then change to small compasses as symbols, then omit the round box of the compass and simply show an arrow.
Suggest that the ‘basic magnets’ are already there inside the bar of steel even when it is not magnetised. They would all be pointing in different directions or, rather, they would all be arranged in small closed chains (family groups) pointing head to tail.
The term ‘basic magnet’ is intended to avoid the mistake in early theories of calling the domains ‘molecular magnets’. Tell students, in passing, that there is some structure inside a magnet, though the full story is much more complicated.
The basic magnets are not individual atoms of iron; they are large groups of atoms with all the atoms in a group orientated magnetically the same way. Some of the groups of atoms point magnetically in one direction; other groups point in other directions. The groups are called domains. The domains are far bigger than molecules. They may be big enough to see, or they may be much smaller than that.
The domains are there in unmagnetised magnetic materials too. You can see their boundaries with a microscope when you pour very fine iron filings onto the surface of a sample. The filings collect at the boundaries where the different domains meet, because there are ‘exposed poles’ there. This phenomenon is used in the magnetic method for testing an iron casting for flaws.
In unmagnetised iron, the domains are small and are orientated equally in several directions. When you apply a magnetising field, you can drag the magnetisation of the domains round into a new direction. The favourable domains, those whose natural direction of magnetisation is near to that of the field, are encouraged to grow at the expense of the shrinking of the unfavourable domains magnetised in the wrong way. Thus the bar becomes more and more favourably magnetised along directions near to the general direction of the field. A bit like a country growing and spreading its boundary into neighbouring countries!
In the later stages of magnetising, the domains are large and many of them are in the favourable direction. But their magnetism will still lie along directions that fit ‘crystal axis’ directions. Then, as saturation is approached, a strong magnetising field may wrench the favourable domains’ magnetisation round nearer to the direction of the field itself.
With iron powder on the surface of a steel sample under a microscope, you can see the domain boundaries moving as we increase the magnetising field.
The domains make their conquest of territory with little vibrations of growth and shrinkage. Amplify the output of a search coil, by connecting it to an amplifier or loud speaker. Move the search coil round a specimen which is being magnetised and you will get a hissing sound which is really a succession of small clicks. The clicks just show minor oscillations of the domain boundaries.
How much of this story you tell to students depends on their ability and interest. However, students should be able to answer the following questions.
Teacher: Does the theory tell you what will happen when you cut a magnet in half?
Example Student Answer: Yes, the theory tells us that we shall just find new poles. Isn’t that wonderful? However, it is no credit to our theory whatever. Our theory has just given us back the story we put into it. We found out that by experiment ourselves and used it as a basis for building our theory.
Teacher: Suppose you have a bar magnet with poles at the very ends, just on the faces. What is likely to happen to those poles?
Example Student Answer: Like poles at an end push each other away so we should expect to find the pole regions of a bar magnet spreading from the ends round the corners to the last part of the length. This agrees well enough with what one sees. It suggests the usefulness of keepers holding two magnets of opposite polarity close together to provide a circle of magnetism.
You could also ask about heating and hammering and demonstrate some of these effects. However, modern magnetic materials have become so good that they withstand rough handling.
Explaining how a transformer works
Teaching Guidance for 14-16
When an electric current passes through a long, hollow coil of wire there will be a strong magnetic field inside the coil and a weaker field outside it. The lines of the magnetic field pattern run through the coil, spread out from the end, and go round the outside and in at the other end.
These are not real lines like the ones you draw with a pencil. They are lines that we imagine, as in the sketch, to show the pattern of the magnetic field: the direction in which a sample of iron would be magnetised by the field. Where the field is strongest, the lines are most closely crowded.
With a hollow coil the lines form complete rings. If there is an iron core in the coil it becomes magnetised, and seems to make the field become much stronger while the current is on.
The iron core of a transformer is normally a complete ring with two coils wound on it. One is connected to a source of electrical power and is called the
primary coil; the other supplies the power to a load and is called the
secondary coil. The magnetisation due to the current in the primary coil runs all the way round the ring. The primary and secondary coils can be wound anywhere on the ring, because the iron carries the changes in magnetisation from one coil to the other. There is no electrical connection between the two coils. However they are connected by the magnetic field in the iron core.
When there is a steady current in the primary there is no effect in the secondary, but there is an effect in the secondary if the current in the primary is changing. A changing current in the primary induces an e.m.f. in the secondary. If the secondary is connected to a circuit then there is a current flow.
A step-down transformer of 1,200 turns on the primary coil connected to 240 V a.c. will produce 2 V a.c. across a 10-turn secondary (provided the energy losses are minimal) and so light a 2 V lamp.
A step-up transformer with 1,000 turns on the primary fed by 200 V a.c. and a 10,000-turn secondary will give a voltage of 2,000 V a.c.
The iron core is itself a crude secondary (like a coil of one turn) and changes of primary current induce little circular voltages in the core. Iron is a conductor and if the iron core were solid, the induced voltages would drive wasteful secondary currents in it (called
eddy currents). So the core is made of very thin sheets clamped together, with the face of each sheet coated to make it a poor conductor. The edges of the sheets can be seen by looking at the edges of a transformer core.
Oersted, electric current and magnetism
Teaching Guidance for 14-16
In 1820, Hans Christian Oersted performed an important experiment which showed that there was a connection between electricity and magnetism. When a current was switched on through a wire, it made a compass needle turn so that it was at right angles to the wire. The current had produced a magnetic field strong enough to cause the compass needle to turn.
Background to the experiment
It was already known that an electric current in a wire has a heating effect, and may cause the wire to glow. This showed that the three phenomena of electricity, heating and lighting were connected.
Scientists seek to ‘unify’ apparently different phenomena by finding their underlying causes.
Question: How can we show that an electric current has a heating effect, and can also result in light?
Answer: See experiment
It was also known that when lightning, a form of electricity, struck a ship, the ship’s compass might be affected – its polarity might be reversed.
Question: What does this suggest?
Answer: There is a link between electricity and magnetism.
Scientists draw on evidence from everyday life; they try to understand the evidence through experimentation.
How could this link be demonstrated in the lab?
Experimenters imagined that, if an electric current flowed along a straight wire, then the magnetic field it produced would be in the same direction. So they placed their compass needles at right angles to the wire, thinking they would be deflected by the current so that they became parallel to the wire. They saw no effect because, as Oersted was to show, the magnetic field produced is at right angles to the wire. So preconceptions prevented earlier experimenters from seeing the effect. (Note that the available equipment –voltaic piles, typically – would only produce a small current, so the effect would in any case have been very weak.)
Experiment: Set a compass pointing north-south; place a wire above it, lying east-west. Connect to a 1.5 V cell. There should be no effect.
Preconceptions (existing ideas) may make it difficult to make progress.
Oersted imagined an electric current ‘struggling’ through a wire. As it flowed, this ‘conflict’ gave rise to 'heat' (or infra-red radiation, as we now understand it) and light, which radiated away from the wire. Might it not also result in a magnetic field, radiating away?
One idea leads to another, which can then be tested.
Question: How would you draw magnetic field lines to represent Oersted’s idea?
Answer: Radiating out from the wire.
Question: How should a compass needle behave, if this idea is correct?
Answer: It might be expected to point radially to the wire. Try it, and the magnet does not do this. So magnetic field lines do not radiate out from a current-carrying wire.
Oersted was giving a public demonstration of the connection between electricity, 'heat' and light. He connected a voltaic pile to a platinum wire and showed that it became hot and glowed.
As he lectured, his idea about a connection between electricity and magnetism resurfaced in his mind. He had a compass to hand (for other experiments he intended to demonstrate), so he decided to test his idea there and then.
Experiment: Set up the previous experiment, but with the wire lying north-south above the compass, parallel to the compass needle. Switch on – the needle rotates to lie east-west.
Oersted reversed the current; the needle moved the other way.
The effect observed by Oersted and his audience was small; no-one was very impressed. Oersted was aware that other scientists had been distracted by similar elusive phenomena, and it was three months before he spent any more time on his discovery.
Scientists are often working at the limits of sensitivity of their instruments. A technique may have to be refined greatly before a phenomenon can be observed reliably.
What Oersted did next
Oersted showed that the magnetic field around a current-carrying wire was circular; i.e. the lines of force are circles, centred on the wire.
He went on to show that a thicker wire produced a greater effect. He also showed that materials placed between wire and compass had no effect.
Answer: Thicker wire has less resistance, so greater current. Non-magnetic materials have no effect on a magnetic field.
In July 1820, he sent a 4-page paper outlining his results, in Latin, to several scientific journals.
Scientists are expected to share their findings as soon as possible, by publication in journals. In 1820, Latin was a common (shared) language which allowed scientists of different nationalities to understand each other’s work.
Ampere read Oersted’s report, and in the space of a week had repeated the observations and developed a mathematical theory describing how the magnetic field depends on the strength of the current and the distance from the wire.
In principle, scientific results are checked by other scientists who repeat the experiments, to see if they get the same results.
Oersted had observed and described an experimental phenomenon. Ampere took it further by writing a mathematical equation to account for it.
Question: Oersted had shown that an electric current has a magnetic field around it. What practical applications were developed from this idea?
Answer: Electromagnets, motors, dynamos, transformers, etc.
More about Oersted
Oersted founded the Danish Society for the Dissemination of Natural Science, a society aimed at presenting scientific ideas to the general public. Oersted also played a key role in the founding of the Technical University of Denmark, with the intention of improving the scientific basis of engineering.
Many scientists seek to share their work with a wider audience. They may also seek to share the technological benefits of their discoveries. He described 1820, the year of his great discovery, as the happiest year of his life.
Scientists can get great pleasure from their work.
In 1802, an Italian lawyer called Gian Domenico Romagnosi published an account of an observation similar to Oersted’s. However, his article appeared in a newspaper and it wasn’t taken up by the scientific community. Romagnosi doesn’t appear to have followed up his preliminary findings.
There are many examples of ‘prior claims’ in the history of science. In practice, the acknowledgement usually goes to the person who publishes a careful, detailed and repeatable account of their observations, at a time when other scientists are ready for the idea, and in a place it will be read and taken seriously.
We are grateful to David Sang, author of this Case Study.