Drawing magnetic field patterns - an activity

What the Activity is for

The aim of this activity is to develop the idea of a magnetic field by engaging pupils in drawing some magnetic field patterns.

What to Prepare

• bar magnets
• plotting compasses
• iron filings

What Happens During this Activity

Remember that some care needs to be taken in leading pupils to draw magnetic field patterns. Rather than getting the pupils to start with iron filings and make their own drawings, we would suggest starting with a compass needle demonstration, which helps pupils to understand and recognise what it is that they are drawing, and then progressing to drawing the iron filings.

Compass needle demonstration

Take a bar magnet and place it on the glass of an overhead projector. Switch on the projector and bring the magnet into focus on the screen. The idea is to use about 12 small plotting compasses to act as magnetic field detectors to find out the magnetic field pattern.

Teacher: Now then, we know that there is a magnetic field around the bar magnet. But how does the magnet actually pull and push magnetic objects in that space or field? In what direction? To find out we can use these small plotting compasses, specially made to find out magnetic field patterns.

Teacher: If you remember, each of these compasses is simply a suspended magnet.

Teacher: If I place this first plotting compass close to the north pole of the bar magnet, notice what happens to the plotting compass needle: it lines up so that the south pole of the compass needle is attracted to the north pole of the magnet.

Teacher: If I place this second compass at the other end of the magnet, the north pole of the compass needle is attracted in towards the south pole of the magnet.

Teacher: How about if I place this compass down the side of the magnet. It points along the side and towards the south pole of the bar magnet.

By placing the plotting compasses in key positions around the bar magnet it is possible to start sketching out the shape of the magnetic field pattern, with magnetic lines of force following continuous loops from the north pole of the magnet round to the south. To make this as plain as possible, choose the new position of the compass by moving it in the direction indicated by the needle to form one continuous curved line. It is a good idea to place the bar magnet on an overhead transparency so that you can start sketching out (with a felt-tip pen) the magnetic field lines, which the pupils can then see on the screen. This is the first time that the pupils will have seen this distinctive magnetic field shape.

Teacher: So we can see the plotting compasses lining up and the lines of force looping round from the north pole of the magnet to the south. We call these lines that I'm drawing lines of force and these show the direction in which the force due to the bar magnet acts on the compass needle.

Iron filings activity

Finally, and with the pupils in a position where they are aware of what they are actually looking for, the class can have a go at using iron filings to pick out the magnetic field patterns for various magnets and combinations of magnets.

This activity can be carried out in pairs or fours, with one set of apparatus per group. In briefing the pupils to make their drawings be clear about what it is that you want:

Teacher: Place the magnet on the desk. Put the sheet of white paper over it and scatter iron filings lightly onto the paper. Not too many! Tap the paper now and again so that the filings move into position and you can see the field pattern beginning to show.

Teacher: In making your drawings, I want you to draw lines of force. Do not try to draw individual filings. Do not put in lots of shading. Just look for the pattern of lines of force.

Given the build up to this activity, the pupils should have no problem in producing a drawing for the single bar magnet. It is then worth testing their ability to spot more complicated magnetic field patterns by setting them the task of investigating the following fields:

• Two bar magnets attracting.
• Two bar magnets repelling.

Focus the pupils' attention on what happens with the lines of force between the magnets:

• For magnets attracting, the lines of force link from the pole of one magnet to the opposite pole of the other magnet.
• For magnets repelling, the lines of force push away from each other between the similar poles of each magnet.