Mounted on the frames sitting on the projector and its stand in the photograph above, are acrylic sheets with a bar magnet, a pair of bar magnets with like poles facing, a pair of bar magnets with unlike poles facing, and a U magnet, glued to their undersides. Sprinkling iron filings (in the cubby at right) onto the sheets and gently tapping or shaking the frame, reveals the pattern of field lines around the magnet(s), which you can project for the class to see.

The behavior exhibited by the magnets in demonstration 68.03 -- Magnets on swivels, in which one magnet brought close to the other causes it to turn on its stand (the direction depending on whether the pole being brought toward the second magnet is approaching a like pole or an unlike pole), indicates that there is some kind of field that surrounds the magnet. We can represent this magnetic field by means of a set of lines, drawn so that at any point the direction of the magnetic field is tangent to the line at that point, and the number of lines passing through unit area at that point is proportional to the magnitude of the field. The direction of the magnetic field at any point is the direction in which a compass needle points when it is placed there. Thus, for a bar magnet, the field lines leave the north pole of the magnet, loop around the sides of the magnet, and enter through the south pole.

A good way to make the pattern of field lines around a magnet visible is to place a stiff card on the magnet and sprinkle iron filings over the card. The iron filings become polarized in the field of the magnet, and align themselves with each other to lie along the magnetic field lines. In this demonstration, the magnets are mounted to the bottom surface of clear acrylic sheets. When you place one of these on the overhead projector and sprinkle iron filings on it, you can project the resulting pattern for the class to see. As noted above, when you sprinkle the iron filings onto the sheet, you will probably need to give the frame a gentle tap or shake to make them align to form a pattern. The photographs below show the field pattern for each magnet or combination of magnets listed above.

The field around a bar magnet

The field around two bar magnets with like poles facing

The field around two bar magnets with unlike poles facing

The field around a U magnet

References:

1) Giancoli, Douglas C. Physics: Principles with Applications (Upper Saddle River, New Jersey: Prentice-Hall, 1998) pp. 588-590.