Comparing the strength of a number of magnets.

Resources

2 or more magnets of different shapes and sizes per group;
10-20 paper clips per group;
lined paper.
Introduction
• Show the pupils a large magnet and a small one.  Ask them to suggest which of these magnets is the stronger and to give reasons for their opinions.  Use questions to explore the ideas behind these suggestions.
E.G. Do pupils think that the larger magnets are always stronger?  Do they believe that horse-shoe magnets are stronger than bar/disc magnets or that darker-coloured magnets are stronger than lighter-coloured ones? Development
As the pupils to work in pairs or small groups to devise a fair way of comparing the respective strengths of two magnets.  The test devised must be conductible in the classroom.  As they work, you could move from group to group, pointing out any factors which would make the tests unfair.
• What follows is a brief description of a number of possible tests which the pupils could do in the classroom.
1. You could attach a chain of paper clips to each magnet and see which magnet can support the longest chain.  So the unit of measurement here is paper clips.
2. Place each magnet on a page of lined paper.  Mark the position of the magnet so that you can return it if it's moved accidentally.
Test each magnet in turns as follows.  Place a paper clip a fixed number of lines away from the magnet.  Again, mark the position.
Move the magnet towards the paper clip, moving up a line at a time.
When the magnet begins to attract the paper clip, mark the magnets position and count how many lines away from the paper clip it was.
The magnet which can attract the paper clip from the furthest distance is the strongest.  Your unit of measurement here is lines, rather than paper clips.
3. Place one sheet of paper over each magnet and check to see whether it still attracts a paper clip.  If so, try another (identical) sheet and another, until there is no longer  an attraction.  The magnet which can attract a clip through the greatest number of sheets is the strongest.  Here the magnetic strength is expressed in sheets/pages.
• Important!
Re-using the same paper clip during these tests mightn't be fair.  each paper clip becomes slightly more magnetised each time it is in contact with the magnet.  This will distort your results.  It will be interesting to see whether the pupils would notice this themselves.  They may observe that some paper clips will attract each other even when the magnet isn't near.
Many of the tests which your pupils come up with will resemble the above.  You could nudge them in the directions although there are other possibilities.
• When pupils have devised their tests let them carry them out.  Encourage pupils to be vigilant for anything which would make the test unfair.
Closure
• Get pupils to record their results and to describe to the class how they reached these results.
• Ask them whether they trust their results.  A discussion of the 'fairness' of each test might be useful.
• Pupils attention could be drawn to the ideas they expressed prior to the activity.  Have they changed their opinions about magnets?