Place a bar magnet in the center
of a piece of paper. Draw its outline so that you will know where
it belongs in case it gets bumped (you could tape it lightly in
place, too). Keeping the magnet in the same place, move a compass
around it, stopping at several dozen locations. At each stop,
slightly lift the compass off of the paper and draw a small arrow
right under the compass, recording which way the needle is
pointing. Make enough arrows that you can clearly see the pattern
they make. This map or pattern of arrows is called the magnetic
field.
The mapping technique described above tells us about the magnetic
field in the horizontal plane.
However, it actually fills all of three-dimensional space
above and below the paper, and not just
where the paper is, and the arrows might point upwards or downwards, too.
What does the field of a single bar magnet look like at other places,
like above or below the magnet?
(This will require a little imagination, because
compasses are designed so that they can't point up or down. For their intended
use, to guide a scout back to camp, having the compass point up or down isn't
very helpful!).
Suppose a small bug with a compass starts walking
on the paper (anywhere!), and keeps walking, always heading in the
direction that the compass needle points. Where will the bug end
up (always!)?
Suppose you went outside (anywhere!) with your compass (but not the magnet)
and starting walking always heading in the direction that the
compass needle points. Where will you end up (always!)?
If the bug leaves a trail as it walks, it will form a smooth curve that
connects together the arrows. This is called a magnetic field line.
It tells you the same thing as the set of arrows, and is a little easier to
draw.
on the paper (anywhere!), and keeps walking, always heading in the
direction that the compass needle points. Where will the bug end
up (always!)?