Electricity and Magnetism

The big electromagnet

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Demonstrating the strength of electromagnets.

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.

can be analysed using Magnetic Field
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