Teacher’s notes: Magnetic Fields
Magnetic field pattern in bar magnets (students’ enquiry)
Aim
In this investigation students will observe the patterns of the magnetic field around different combinations of bar magnets. They will use their observations to make generalisations about magnetic fields and develop the ability to think critically and analytically.
Equipment
-Iron filings, or ferrofluid for an interesting alternative (
-Acetate sheets (A4)
-Bar magnets
-Compasses
-Small aluminium trays
The investigation
Students should work in pairs to observe and draw the pattern of the magnetic field lines in the three situations below.
Time allowing, students would really benefit from the opportunity to complete their observations using both iron filings and ferrofluids. The latter will show clearly to them the 3D nature of magnetic fields. Also, using a compass around the magnet will ensure they will be able to appreciate that magnetic field lines have a direction (N to S).
The method of this enquiry is detailed in the students’ worksheet.
Safety
-Iron filings can be dangerous if they enter the eye, so extreme care needs to be taken when handling them. It is advisable that students wear goggles during their investigation. They should also wash their hands at the end.
-Ferrofluids don’t represent a serious hazard, but contact with skin, eyes and clothes should be avoided, as they can stain fabric very badly. Ideally small sealed containers made of Perspex could be prepared and retained for future use, but in the absence of these clear instructions about how to safely handle ferrofluids should be given to the students, i.e. use box, or small basin, with relatively high walls, just pour a small amount of ferrofluid in the container (this will also produce better observations), always lower the box slowly on top of the magnet(s) and never allow the magnet and ferrofluid to come in contact with each other. Good containers for this part of the investigation are small aluminium trays (like the ones you get from Chinese take away restaurants).
Answers to questions
This generalization exercise is useful for the students in order to develop analytical thinking and independent study skills. A class discussion at the end of the activity could be a good way to formally assess individual pupils and for AfL.
Some rules written by the pupils could include:
-Like poles repel and unlike poles attract
-Magnetic field lines are all around the magnet (3D) and not just on the plane of the acetate sheet
-Magnetic field lines converge at the poles
-The strength of the magnetic field is greater near the poles
-The closer together the magnetic field lines, the stronger the field, i.e. the strength of the field depends of the number of magnetic field lines. In regions of higher density the field will be stronger
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Created by Alessio Bernardelli