Mapping the field of an electromagnet

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
• about 12 transparent compasses

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.