Oersted's discovery of electromagnetism

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
• overhead projector
• 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.