Class practical

These investigations provide an introduction to magnets and their interactions.

Apparatus and Materials

For each student group

• Permanent bar magnets
• Cotton
• Plotting compasses
• Magnadur magnets
• Wooden burette stand (optional)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Permanent bar magnets are bar-shaped magnets with opposite poles at either end. Magnadur magnets are ceramic magnets with their poles on their flat faces. (It is interesting to see if the students can discover this for themselves.) See the illustrations. These magnets can come to no harm magnetically, but being ceramic can chip and fracture like china. If two are placed near to each other, oriented to attract, they may move together with sufficient violence to do damage.

Small plotting compasses are very cheap. Badly balanced ones or those with sticky pivots should be discarded. Polarity is easily reversed; students should check which is the north-seeking pole of the compass needle. If their magnetisation is weak, they will oscillate slowly in the Earth's field. To strengthen their magnetisation, place them in a strong field, for example between poles of a pair of strong bar magnets.

Procedure

1. Play with bar magnets, studying the feel of attraction and repulsion. Avoid banging the magnets together or forcing them very close together against their mutual repulsion as this might ultimately weaken them
2. Suspend a magnet freely in a stirrup. This can consist of two loops of cotton, as shown in the picture, arranged to hang the magnet as indicated.
3. A pencil projecting over the edge of a desk or bench and held by weights (for example, books) serves for a support. Alternatively a pencil or wooden rod can be fixed in a non-magnetic retort stand (brass, wood or stainless steel). This should be used to see how the magnet sets by itself. Let them find the effect of bringing another magnet near by.

4. Use the small compass instead of the suspended magnet. This is more convenient. It can be moved around the bigger magnet to show the direction of pull at different places.
5. Arrange two Magnadur magnets as shown; show that the poles are on the large faces.

Teaching Notes

• Students should be able to find out the following...
• Permanent magnets normally have two poles, a north-seeking pole and a south-seeking pole (using seeking pole helps with the problem of what is at the Earth's magnetic poles).
• Similar poles repel and dissimilar poles attract. Students should have the opportunity to feel the forces of attraction and repulsion.
• A suspended magnet will settle in a particular direction in the Earth's field (and at a particular angle to the horizontal).
• A suspended magnet will be deflected by another magnet brought near to it.
• A compass needle is deflected by a magnet.
• How Science Works Extension: You might like to put these findings in a historical context. Compasses were used as practical devices for many centuries without a scientific understanding of how they work. One idea was that the north magnetic pole of a magnet was attracted to the Pole Star. This was a credible idea for people who were used to using the stars for navigation. However, compass-makers noted an interesting phenomenon. When they made an iron compass needle, they carefully balanced it before magnetising it. When it was magnetised, they found that it was no longer balanced; rather, it tipped downwards with its north pole tilting into the Earth. This gave William Gilbert (1544-1603) the idea that magnetism arose from inside the Earth, not in the stars. He made a model Earth with a magnet inside it and showed that small compasses held nearby reproduced sailors’ observations of the Earth’s field.
• Once the Earth’s magnetism was established, scientists went on to use the idea elsewhere. For example, magnetism is an example of action-at-a-distance, so theories were developed that suggested that the Earth was held in its orbit around the Sun by magnetic attraction between them.

This experiment was safety-checked in December 2004

Class practical

Magnets provide an introduction to attraction and repulsion, and to action at a distance.

Apparatus and Materials

For each student group

• Magnets, different types (at least 2 pairs)

• Iron filings in a pepper pot

• Nails
• Other materials for testing magnetic behaviour, including small scraps of paper
• Compasses
• Sheets of paper

Health & Safety and Technical Notes

Iron filings must be kept out of eyes (and sinks). It is worth warning the class to keep fingers away from faces when iron filings are around.

Read our standard health & safety guidance

You could use cylindrical, horseshoe, flat, ceramic, or strong (Eclipse major) magnets. A large permanent magnet should be used with teacher supervision.

One of the pairs of magnets should be strong enough so that, when separated by a few centimetres, students can feel attraction and repulsion.

Procedure

1. Hold pairs of magnets and feel the forces between them, repulsions as well as attractions.
2. Use the magnets to try to attract nails and other materials. Some of them (such as small scraps of paper) cannot be attracted by a magnet.
3. Place a magnet underneath a piece of paper and scatter iron filings on top to reveal a magnetic field pattern. The purpose of the sheet of paper is to prevent direct contact between magnets and filings, since they can be hard to separate. Tap the paper gently to ensure the filings do not stick together.
4. Place compass needles tip-to-tail near to a magnet. Record their orientations, to plot the maget's field as continuous field lines.
5. Suspend a bar magnet and show it aligns roughly North and South. The pole which points North is the "North-seeking pole" of the magnet.

Teaching Notes

• The observation that a single magnet can experience and exert both attractive and repulsive forces with other magnets is important. It allows introduction of the idea that magnets have two different ends or faces, called poles.
• To show a magnetic field pattern to a whole class, you could place a magnet on an overhead projector, with a piece of transparent material on top of it, and sprinkle the iron filings on to this.
• You can use a compass to develop the idea of magnetic polarity. The compass points towards the Earth's North. The arrowhead end of the needle is a North-seeking pole. All magnets have a North-seeking pole and a South-seeking pole. Poles that are the same always repel each other. Poles that are different always attract each other. Show this with pairs of compass needles.
• Sometimes students get into a tangle about North-seeking and South-seeking poles when they learn that the Earth is a big magnet, and that the pole that is geographically to the North must be a South-seeking pole. So at this stage it is unhelpful to shorten 'North-seeking pole' and 'South-seeking pole' into plain North and South poles.

This experiment was safety-tested in July 2005

Class practical

Iron filings will align with the direction of a magnetic field to reveal the field pattern.

Apparatus and Materials

For each student group

• Permanent bar magnets, 2
• Magnadur magnets, 2
• Steel yoke
• Card

• Iron filings in a pepper pot

• Plotting compasses
• C-cores
• Aluminium rings

Health & Safety and Technical Notes

Warn the class to keep fingers away from eyes. Iron filings inadvertently carried to the eyes can damage the cornea.

Read our standard health & safety guidance

Permanent bar magnets are those bar-shaped magnets with opposite poles at either end. Magnadur magnets are ceramic magnets with their poles on their flat faces.

If iron filings get on the poles of a magnet, they can be removed by rolling Plasticine over the poles.

Procedure

1. Explore the area around a permanent bar magnet with the plotting compass, by moving the plotting compass around the magnet. Concentrate on the bar magnet and the space surrounding it, as well as the compass needle.
2. Sprinkle iron filings on a card and place the card over the magnet. Tap the card gently with a pencil and observe the pattern.
3. Repeat this using two magnets, first attracting, then repelling.
4. Instead of the permanent bar magnets, use two Magnadur magnets. Again investigate the field using:
5. iron filings sprinkled on card
6. the plotting compass
7. The Magnadur magnets can be put on the bench as shown, or attached to the iron yoke.

Teaching Notes

Students will see that the iron filings tend to concentrate close to the magnetic poles of the magnet.

This experiment was safety-tested in December 2004

Class practical

This experiment could be used either to show that magnetic poles always come in north-south pairs, or to introduce a simple theory of permanent magnetism.

Apparatus and Materials

For each student group

• Steel rods, hardened, 2

• Iron filings in a pepper pot

• Sheets of paper, 2
• Plotting compasses, 2

Health & Safety and Technical Notes

Eye protection should be worn.

Read our standard health & safety guidance

The steel rods should be approx 1.5 mm diameter and 10 cm long. They should first be hardened, by heating until they are cherry-red and then plunging them into cold water. When cooled, the rods should be glass hard and unable to be bent. To magnetise them, use a 300-turn coil from a demountable transformer kit supplied with 2 amps from a 4 volt d.c. source. Place the steel rods inside the coil and switch on for a minute.

Procedure

1. You are going to try breaking a long thin magnet in half. Before you break your magnetized steel rod, test it with iron filings to see where its poles are.
2. Also bring each end of the rod near a small compass needle so that you know which pole is north-seeking.
3. Snap the rod in half and again look for poles. Try further snapping and testing.

Teaching Notes

• You might start this experiment by asking, "Can you make a magnet with a north pole at one end, and no south pole?"
• Students should dip their thin bar magnet into iron filings to check where the poles are, and then break the magnet into two pieces. When they have broken their magnet into two pieces they will find that they now have two smaller magnets. They can continue breaking their magnet into smaller and smaller pieces. Each time there is never a single pole in the broken magnet. That is different from the way in which electric charges behave. You can separate negative and positive charges by rubbing insulating materials together.

This experiment was safety-tested in April 2006

Demonstration

A demonstration to link theory and experiment.

Apparatus and Materials

• Plotting compasses, as many as possible
• Bar magnet

Health & Safety and Technical Notes

Some magnetising and de-manetising coils are powered by mains a.c. Only use them if they have been inspected and tested for electrical safety.

Read our standard health & safety guidance

Procedure

1. Arrange as many compasses as possible in a regular crowd to fill a rectangle on the bench top, as a model of a bar magnet.
2. To magnetize the model, draw a large magnet once over the compass needles from end to end of the crowd.
3. To demagnetize, wave the large magnet arbitrarily over the model, taking the magnet slowly farther and farther away while waving it.

Teaching Notes

• This model illustrates an early, simple theory of a magnet (but does not do justice in detail to the modern domain theory). It is so important that students should see it clearly at close range, not at a distance on a remote high lecture bench. Students should be encouraged to take turns in small groups to try it. Before students try the model, warn them not to bring the large magnetizing magnet very near the compass needles.
• If the compass needles are mounted in transparent cells, a crowd of them can be shown on an overhead projector. A projection model can be made cheaply by pushing points of sewing needles through a thin sheet of Perspex, as pivots for compass needles from cheap plotting compasses. Install a transparent lid over the array to avoid losses in storage.
• This has the advantage of providing a large picture for a demonstration, and the disadvantage of being more special than a crowd of ordinary compasses.
• If compass needles acquire reverse magnetization, demagnetise them using an alternating current. Then remagnetize them by placing them in a coil in which a direct current is turned on momentarily. Not all will be magnetised the right way at the first try. Remove the ones that are, and repeat with the rest.

This experiment was safety-tested in December 2004

Class practicle

This experiment raises an interesting question. Can something be magnetised and not show poles?

Apparatus and Materials

• Steel rings, magnetized, 2

• Iron filings in a pepper pot

• Sheets of paper, 2

Health & Safety and Technical Notes

Eye protection must be worn.

Read our standard health & safety guidance

The rings are small, flat rings, about 0.25 mm thick, 1 cm internal diameter and 2 cm external diameter. They are stamped out of thin steel and hardened by heating them to cherry red and then plunging them into cold water. The rings should be glass hard and easily broken in the fingers.

The rings need to be magnetised with circular magnetism so that no poles are seen. For that, the circular magnetic field produced by a current in a straight wire is used. There needs to be a wire carrying a current of 100 amps going through the centre of the rings. Multiple wires (e.g. a wire carrying 10 amps threaded 10 times through the rings) may need to be used. To ensure even magnetisation with no poles, pass the wire through the centres of the rings; the rings could be placed on a wooden dowel which has a hole down the centre for the current-carrying wire.

Procedure

1. Test the ring with iron filings. Does it have magnetic poles?
2. Snap the ring in two with your fingers and test the pieces with filings.

Teaching Notes

You might introduce this experiment by saying:

"Can a ring of steel be magnetised even though it shows no sign of poles? Here is a ring of steel. I believe that I have magnetised it yet I can find no poles. You see no clumps of iron filings hanging on the ring when I dip it into iron filings. I find no magnetic field near it. When I put this compass needle nearby it is not affected. So I find no poles, no field and yet I thought I had magnetised it."

"How does the theory help to explain the appearance of the poles at the break? (The "basic magnets" are nose to tail around the ring and are not exposed until the ring is broken.)

This experiment was safety-tested in December 2004