New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
doing the stroking. Bring up the north pole of the magnet, and see that it repels the like pole of
the clip and attracts the opposite end. The stroking magnetizes the clip because the strong
magnetic field causes the magnetic domains of which the clip is composed to align with the field.
We can plot the lines of force around our magnet using the paper clip hung on the thread.
The clip orients itself along such lines, the poles at opposite ends being forced in opposite directions
to provide the torque. Hold the thread supporting the paper clip as far up as possible to
minimize the thread’s restoring torque. A small piece of tape on the clip airdamps vibrations.
Tape the magnet to the centre of a sheet of paper, support the clip somewhere near the magnet,
and lower it gently to the paper. Draw a pencil mark along the line where the clip touches the
paper as shown in figure 24c. This is a field line. Now, move the clip a little farther along, and try
to make a continuous field line. Do this all around the magnet, (figure 24d) holding the thread
closer to the clip as you approach the magnet, to prevent it being drawn in. For a very small
magnet, the lines of force follow curves of the form drawn in figure 25. Compare them with the
lines you obtained. A larger magnet tends to make the loop flatter
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Figure 25.
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