# 02Electromagnets at work

Em02TA of the Electromagnetism topic
• ## 01 Oersted's experimentEm02TAnugget01 Activity

What the activity is for

Oersted's experiment was of fundamental importance to the development of physics in making the link between electrical and magnetic effects. The aim of this activity is to demonstrate what was involved in Oersted's discovery of electromagnetism.

What to prepare

• transparent compass
• long wire
• a power supply capable of providing about 5 ampere

What happens during this activity

Even though Oersted's experiment is of ground-breaking significance in the history of physics, it can seem unimportant to pupils. The secret here is to try to generate some of the surprise and interest that Oersted himself experienced when he first witnessed this phenomenon.

Try carrying out the demonstration on the glass of an overhead projector as follows:

1. Place a plotting compass on the projector and agree with the class that it is pointing along a north-south line.
2. Draw the attention of the class to the circuit you have set up which consists of a single, long loop of wire.
3. Start moving the wire (circuit switched off) over the top of the compass and along the line of the compass needle. Ask the class what they think will happen. Nothing!
4. Now keep the wire along the line of the compass needle and switch on the electric current. The compass needle swings around so that it now lies across the line of the wire (at right angles to its original position).
5. The key question here is: What does the movement of the compass needle tell us? The answer is simply that the electric current in the wire must be producing a magnetic field.
6. Switch off the electric current and the compass needle swings back again. Line up the wire with the needle once again but this time with the wire under the compass. Prepare to switch the circuit on again.
7. Ask the class to predict what will happen to the compass needle.
8. When the electric current is switched on the compass needle is once again deflected across the line of the wire, but this time in the opposite direction. If the class has already used plotting compasses to map the fields of permanent magnets, they may be able to work out that there is a magnetic field around the current-carrying wire and that it is circular in shape.

This demonstration might take no more than 10 minutes. Make the link to Oersted's experiment and then quickly move on to explore what happens with lots of coils of wire in the big electromagnet.

• ## 02 The big electromagnetEm02TAnugget02 Activity

### Demonstrating the strength of electromagnets

What the activity is for

The main purposes of this activity are to capture pupils' interest and imagination and to develop the idea that the strength of an electromagnet depends on the number of coils of wire.

What to prepare

• a demonstration electromagnet (Made from an iron C-core of the type used in Westminster Electromagnetism kits and a long length of insulated wire (about 5 metre) which is coiled around the core.)
• a low voltage power supply (about 12 volt direct current)
• a matching C-core
• 100 gram and 1 kilogram slotted masses to hang from the electromagnet assembly

The electromagnet is assembled and then hung from the ceiling with some very strong string. The matching C-core is introduced to the electromagnet and masses are added to it, suspended from a loop of strong string.

What happens during this activity

This is a teacher demonstration which serves to illustrate just how powerful magnetic forces can be. Rather than have the pupils working with iron nails and small coils of wire (to build relatively weak electromagnets) we would recommend this approach.

Assemble the electromagnet and switch on the electric current. Make the point that you are using a low voltage supply and speculate as to how strong the electromagnet is likely to be.

Teacher: Will it hold the second C-core?

Teacher: Yes, it will!

Start adding masses to the string round the second C-core:

Teacher: Will it take an extra 100 g?

Teacher: Yes, it will!

Go through the whole process of adding masses in a slow and deliberate way, building up the tension. Ask the pupils to predict how much the electromagnet will hold. A science teacher colleague actually performs this demonstration in a darkened room, with a strong light focusing on the electromagnet (high drama!). Eventually you will be adding 1 kilogram at a time as it becomes evident that this electromagnet is much stronger than any pupil might have anticipated:

Teacher: Isn't that amazing? By far the best thing you'll see today – or all week for that matter!

The electromagnet we use will hold about 4 kilogram. Given the size of the load you need a soft landing site (we use a waste-paper bin with a piece of old blanket inside) to catch everything when the electromagnet can no longer hold up the load.

• ## 03 Mapping the field of an electromagnetEm02TAnugget03 Activity

### Exploring the field of a solenoid

What the activity is for

The field of a solenoid is just like that of a bar magnet, except that you can get inside the coil. By using a high current power supply you can explore this field with compasses and with iron filings.

What to prepare

For the demonstration

• solenoid with only a few well spaced turns mounted on a transparent base for the OHP
• OHP
• power supply capable of supplying about 8 ampere
• iron filings

Per pair

• 1 metre of bare-ended wire
• 10 centimetre of broomstick dowel around which to wind the wire to make the solenoid
• a power supply capable of providing about 5 ampere
• compass

What happens during this activity

Pupils wind their own solenoid, taking care to leave enough wire at the ends to connect it to the power supply. Then remove the dowel. Run the current through the solenoid only for short lengths of time, applying about 2 volt across the ends. Warn the pupils that the wire will get hot enough to burn them. Explore the magnetic field around and through the solenoid. Ensure that similarities with the bar magnet are noticed, and that pupils switch the solenoid on and off with the compass near it. Use the reaction of the compass to the switching to emphasise that the solenoid forms a temporary magnet. Use the demonstration solenoid on the OHP to summarise their findings.

• ## 04 Electromagnetic devicesEm02TAnugget04 Activity

### Investigating electromagnetic devices

What the activity is for

Electromagnetic machines are very common. The aim of this activity is to make clear how a small selection of these work, drawing on the behaviour of electromagnets that pupils have seen.

What to prepare

• a mounted relay with power supplies, switch and lamp (You should aim to have a mounted demonstration relay with the two circuits quite separate, and run from two separate power supplies, with one small (for the low current circuit), and one large (for the high current circuit which might conveniently drive a large lamp.)
• a mounted bell (The mounted bell should display as much of the mechanism as possible.)
• a video camera to provide an enlarged display of the demonstration
• two interactive objects