Class practical

An introductory experiment showing that electromagnets can conveniently be switched on and off.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends

• Iron filings in a pepper pot

• Nail, large, made of iron
• Tintacks or paper clips, supply of

• Power supply, low-voltage

• Chemical balance OPTIONAL
• Ammeter, 0-1 A OPTIONAL

Health & Safety and Technical Notes

Read our standard health & safety guidance

The nail should be made of iron which is magnetically soft (cut nails are suitable).

The nail may also have gained some magnetism while it has been lying in a cupboard in the Earth's magnetic field. This is easily remedied by heating the nail to cherry red heat and allowing it to cool in the East-West direction. Alternatively use a demagnetising coil, in which an alternating potential difference is connected to a solenoid, and the nail is then slowly withdrawn from the coil to a distance from it.

Procedure

1. Wind a few dozen turns of insulated wire around an iron nail. (Leave enough wire free at either end to make connections to the power supply.)
2. Connect the ends of the wire to the low-voltage DC power supply, so that a large current flows round the coil.
3. To find out if the nail is a magnet, test it with iron filings. What happens if you turn the current off?
4. Offer your electromagnet some larger bits of iron, such as tintacks or paper clips.
5. What happens each time you turn the current off?

Teaching Notes

• Soft iron is a good temporary magnet. A steel nail will retain a lot of its magnetism once the current in the coil is switched off.
• Iron filings are chips of soft iron which become temporary magnets when in a magnetic field, and so they line up north to south indicating the direction of the magnetic field.
• How Science Works extension: This experiment can produce a valid relationship between the number of coils and the strength of the electromagnet without any measurements, only counting.
• After a demonstration of the procedure above, students could be asked to design a version of the experiment which would allow them to investigate two factors affecting the strength of the electromagnet: the number of coils and the current flowing in the wire. The number of paper clips held by the electromagnet could indicate the strength of the electromagnet.
• This provides an opportunity to discuss the concept of a discrete variable and whether evidence based on discrete variables can lead to a valid conclusion.
• The scope of variables here is limited, so this would be suitable as a first investigation that students might plan and carry out themselves, with little or no guidance. Encourage students to find appropriate ways in which to present their results to make them clear and easy to understand.
• If students use the mass of iron filings picked up as a measure of the strength, making measurements can prove problematic. One solution is to have a mass of iron filings on a balance pan, use the electromagnet to remove whatever it can and then record the drop in the balance reading.

This experiment was safety-checked in January 2007

Class practical

Using a current-carrying coil of wire to make a permanent magnet from a steel rod.

Apparatus and Materials

• Copper wire, PVC-covered, 150 cm with bare ends
• Rod, hard steel

• Iron filings in a pepper pot

• Tintacks or paper clips, supply of
• De-magnetizing coil (300 turns or 2400 turns)
• Transformer or low-voltage AC supply

• Power supply, low-voltage

Health & Safety and Technical Notes

Read our standard health & safety guidance

The steel rod may be a knitting needle or a piece of clock spring. As a poor substitute, short pieces of thick piano wire can be used.

Make sure the hard steel samples are not magnetized. If any are, de-magnetize them by passing them slowly through a coil carrying AC: for the 300-turn coil, use about 6 V AC: for the 2,400-turn coil, use about 20 V AC.

Procedure

1. Use iron filings or a plotting compass to check that the steel rod is not magnetised before proceeding
2. Wind a few dozen turns of insulated wire around the steel rod. (Leave enough wire free at either end to make connections to the power supply.)
3. Connect the ends of the wire to the low-voltage DC power supply, so that a large current flows round the coil.
4. Switch off the current. Test the steel rod again to see if it has become magnetised.
5. Determine where the rod's magnetic poles are.
6. Devise a method for magnetising the rod in the other direction.

Teaching Notes

• How Science Works Extension: Students can make a magnet (by this method, or by the stroking method) and then test its strength. This requires them to devise and evaluate an approach to measuring the strength of a magnet. Here are some suggestions:
• Find how many pins, tacks or paper clips will hang end-to-end from the magnet.
• Lay a pin on the table. Gradually bring the magnet towards it. Measure the distance at which the pin starts to move.
• Place a plotting compass on the table. Bring the magnet towards it from the side (east or west). Measure the distance at which the compass needle points at 45° to its original direction.
• Students should be able to think of other ideas. By trying out several, they can evaluate the sensitivity of each.

This experiment was safety-checked in July 2007

Class practical

Exploring the magnetic field pattern for a C-core.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends
• C-core, laminated iron
• Cardboard

• Iron filings in a pepper pot

• Plotting compass

• Power supply, low-voltage

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

Procedure

1. Take an iron C-core, place a card on top, and sprinkle with iron filings. Is there any magnetic field pattern? Test with a plotting compass.
2. Wind twenty turns of PVC-covered copper wire round one arm. Connect it to the DC terminals of the low-voltage power supply.
3. Switch on, and investigate the magnetic field produced. Identify N and S poles.

Teaching Notes

• There is no field pattern around the C-cores until a current passes through the coil wrapped round the C-core.
• A compass needle will indicate that the two ends of the C-core have opposite magnetic poles.

This experiment was safety-tested in July 2007

Class practical

An illustration of magnetic induction. Students could go on to investigate how an electromagnet's strength varies with the current.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends
• C-cores, laminated iron, 2

• Power supply, low-voltage

Health & Safety and Technical Notes

Read our standard health & safety guidance

For this experiment to be effective it is imperative that there be no grit, such as iron powder, between the touching faces of the two C-cores. If necessary, slide a clean piece of paper between the faces (with the current off) and withdraw while gripping it gently with the faces, which are thus wiped clean. Alternatively, remove grit, dust, or iron filings by wiping the faces clean with the thumb.

Care should be taken to keep the C-cores in their original pairs and their faces undamaged, especially for accurate results in induction experiments.

Procedure

1. Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect to the d.c. terminals of the low-voltage power supply.
2. Switch on. Bring up the second C-core, and investigate the attraction between the two C-cores.
3. Switch off the current and investigate the attraction again.

Teaching Notes

• Students should observe that the iron of the C-cores retains no magnetism, unlike the permanent bar magnets. However, a current through a few turns of wire produces a magnetic field which magnetizes the iron strongly.
• The magnetic field of the first C-core induces opposite poles in the second C-core, so that they attract strongly. It will take a considerable force to separate the C-cores while there is a current through the coil. They will just fall apart when the current is switched off.
• Students could go on and investigate how an electromagnet's strength varies with the current. Attach small nails or paper clips, head to tail, from the electromagnet (the first C-core). Estimate the electromagnet's strength by counting the number of paper clips the C-core can support. Repeat this procedure for different values of current, then analyze the data.

This experiment was safety-tested in April 2006

Class practical

Electromagnets of all shapes and sizes can be found doing useful jobs in machinery such as buzzers, bells and relays.

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends
• C-core, laminated iron
• Hacksaw blade

• Power supply, low-voltage

Health & Safety and Technical Notes

Although hacksaw blades are traditionally used for this activity, some schools may consider it necessary to use strips of hard steel without teeth.

Read our standard health & safety guidance

The blades must be demagnetised before each lesson because they could display an assortment of magnetic poles along their lengths.

Procedure

1. Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect to the 1 V AC terminals of the low-voltage power supply.
2. Clamp a strip of steel such as a hacksaw blade under a spare terminal of the low-voltage supply, taking care that it does not accidentally short-circuit other terminals.
3. Put the C-core under the projecting blade, but not quite touching it. When the supply is switched on, you will feel distinct vibrations in the blade.
4. Adjust the length of the blade to give the largest amplitude of vibration.

Photograph courtesy of Mike Vetterlein

Teaching Notes

• The length of the blade can be tuned to resonance and will give a noticeable amplitude. The length is critical and will depend on the blade.
• The vibrating blade can be allowed to hit against a bell or tin can to provide a ringing tone, if the teacher can stand all the noise - which is twice the frequency of the supply.
• Rather than supporting the blade under a spare terminal of the power supply, you could use a wooden block to support it, as shown below.
• The principle of polarizing may be shown by using Magnadur (ceramic) magnets as shown. In this case, the blade is attracted continuously to the electromagnet. This gets stronger and weaker with the frequency of the mains supply. How does the pitch of the note change?

• A number of similar models could be constructed to illustrate a range of practical applications of electromagnets: for example, the buzzer, the bell, telephone earpieces, control solenoids. These might be given to the students as optional project work for them to see what they can achieve using equipment of this sort.

This experiment was safety-tested in July 2007

Class practical

A clever application of feedback: a switch that opens and closes due to temporary magnetism in a current-carrying coil of wire.

Apparatus and Materials

For each student group

• C-core, laminated iron
• Hacksaw blade
• Plasticine
• Copper wire, PVC-covered, 150 cm with bare ends
• Adhesive tape

• Power supply, low-voltage

Health & Safety and Technical Notes

Although hacksaw blades are traditionally used for this activity, some schools may consider it necessary to use strips of hard steel without teeth.

Read our standard health & safety guidance

The blades must be demagnetised before each lesson because they could display an assortment of magnetic poles along their lengths.

Procedure

1. Take one iron C-core. Wind twenty turns of PVC-covered copper wire round one arm and connect one end to one of the 1 V d.c. terminals of the low-voltage power supply.
2. Clamp a strip of steel such as a hacksaw blade under a spare terminal of the low-voltage supply, taking care that it does not accidentally short-circuit to other terminals.
3. Attach a mass (e.g. some Plasticine) to the free end of the blade, to slow its vibrations to about four vibrations per second.
4. Put the C-core under the projecting blade, but not quite touching it.
5. Connect one end of a short length of wire to the other DC terminal of the supply. Tape the other end so that the bare wire protrudes along the length of the blade.
6. Position the unconnected end of the coiled wire so that it makes gentle contact with the shorter wire.
7. When the supply is switched on, the blade will be attracted downwards. This will break the circuit, so that the blade springs upwards again, re-completing the circuit.

Teaching Notes

This model makes it easier for students to understand the automatic switching which is at the heart of operation of the traditional electric bell.

This experiment was safety-tested in June 2007

Class practical

Switching a small current through one circuit causes a larger current through another circuit to be switched on (or off).

Apparatus and Materials

For each student group

• Copper wire, PVC-covered, 150 cm with bare ends
• C-core, laminated iron
• Hacksaw blade
• Adhesive tape
• Support blocks or clamps, 2
• Cell, 1.5 V
• Switch
• Battery, 12 V or low-voltage DC power supply
• Lamp (12 V 24 W) in lamp holder

Health & Safety and Technical Notes

Although hacksaw blades are traditionally used for this activity, some schools may consider it necessary to use strips of hard steel without teeth.

Read our standard health & safety guidance

The blades must be demagnetised before each lesson because they could display an assortment of magnetic poles along their lengths.

Procedure

1. Construct the first circuit as follows:
   • Take one iron C-core.
   • Wind twenty turns of PVC-covered copper wire round one arm.
   • Connect one end to one terminal of the 1.5 V cell.
   • Connect the other end to the switch.
   • Complete the circuit by connecting the other end of the switch to the free terminal of the cell.
2. Clamp one end of the hacksaw blade.
3. Tape a length of insulated wire to the blade. Its free, uninsulated, end must project a few centimetres beyond the end of the blade.
4. Position the C-core under the projecting blade, but not quite touching it.
5. Now construct the second circuit as follows:
   • Connect the end of the insulated wire to one end of the 12 V DC battery or supply.
   • Connect the other terminal of the supply to the lamp.
   • Connect a length of wire to the other terminal of the lamp.
   • Tape the bare end of this wire to the top of the second wooden block.
   • Switch on the 12 V supply. (The lamp will not light, since the circuit is incomplete.)
6. Position the two bare wires as shown in the illustration. They should not quite touch at this stage. Adjust the separation of the two bare wires by moving the support block nearer to the electromagnet or further from it.
7. When the switch is closed, the first circuit is complete and the blade will be attracted downwards. The two bare wires will touch. This will complete the second circuit, so that the lamp will light.
8. Open the switch. The first circuit is broken, and so the electromagnet is no longer energized. The blade moves upwards and the second circuit is broken. The lamp goes out.

Teaching Notes

• A relay is an automatic electric switch. This model is intended to make it easier for students to understand how a relay operates: switching one circuit causes one or more other circuits to be switched. A small current sent through the relay's coil makes the relay switch on (or switch off) a big current. Or a small current may make a different relay connect up several other circuits.
• A relay hands a switching signal on from one circuit to another. That is why it is called a relay, after a relay race in which one runner hands the torch on to the next. Relays were once found by the thousand in telephone exchanges. There are huge relays in a power station, and controlling relays in many factories with automated manufacturing systems. But these functions are increasingly performed by solid-state electronic devices.

This experiment was safety-tested in July 2007

Demonstration

If students have made a model relay, this demonstration will reinforce their learning.

Apparatus and Materials

• Cell, 1.5 V
• Commercial relay, small
• Power supply, low voltage, DC
• Switch
• Iron yoke

• Electric motor, fractional horsepower

• Magnadur (ceramic) slab magnets, 2
• Copper wire, stiff, bare, SWG 32 and SWG 26
• Clamp, or wooden support blocks
• Crocodile clips, 2
• Leads, 4 mm, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Choose a relay which will control a 12 V circuit. It is desirable to choose one whose internal workings can be seen, or which can be revealed by later dissection.

Procedure

1. Connect up the control circuit: cell, switch and relay.
2. Connect up the circuit to be controlled: battery and motor.
3. Show the small-current control circuit switching the large current motor.

Teaching Notes

• If students have made a model relay, this demonstration will reinforce the class experiment rather than spoil it. A commercial relay operates quickly and surely and can control a large current.
• You could insert ammeters in both circuits. Both instruments should have the same range, so that the different sizes of the two currents will be graphically evident.

This experiment was safety-tested in April 2006

Demonstration

To introduce and revise electromagnets.

Apparatus and Materials

For each student group

• Unilab coils (or equivalent), 60 + 60 turns, 2
• Clamp stand & boss, 2
• Power supply, low voltage, variable
• Aluminium rod (e.g. from clamp stand)
• Steel rod (e.g. from clamp stand)
• Ruler (30 cm, 50 cm or metre rule)

Health & Safety and Technical Notes

Read our standard health & safety guidance

This involves switching on the lab pack with the voltage turned up. Though warned against doing this in the literature accompanying our Unilab power supplies, we havent had any problem over the years weve been doing this.

This involves running the coils at a greater current than they are rated for, though only for a brief time. If students were to try, they could melt the coils.

The coils initially "kick" apart, then "creep" apart if the supply is left on. Students shouldn't leave the supply on: this will either make the lab pack trip out, or melt the coils. The experiment investigates the "kick", not the "creep".

Procedure

1. Clamp the steel bar horizontally between two clamp stands, after putting on it two 60 + 60 Unilab coils. Adjust the coils so that they are a few mm apart.
2. Connect the coils to the power supply so that, when the supply is switched on, the coils attract. Switch off.
3. Reset the distance between the coils to a few mm and change the connections so that, when the supply is switched on, the coils repel. Switch off and note the ‘kick distance’, i.e. the new distance between the coils.
4. Repeat step 3, for a range of outputs from the power supply, starting with 12 V and working down. Record the kick distance for each output.
5. Replace the steel rod with one made of aluminium and repeat step 4.

Teaching Notes

• Steps 1 to 3 can be used to introduce this as a class experiment. It provides a good quick activity if the results are not recorded, but also produces good graphs if results are recorded.
• The point of doing step 2 before 3 is that students may have to change their connections to make the coils repel. It is important to start with the maximum potential difference (pd) across the coils, so that students see something significant happen. If you start with a small pd across the coils, nothing happens and students are likely to lose interest.
• Likewise, it is best to start with the steel rod before trying the aluminium rod.
• This experiment is particularly good for helping students appreciate the effect of core material on the strength of an electromagnet.

This experiment was submitted by John Myers from Ilkley Grammar School.