by Paul Doherty
Exploratorium Staff Scientist
Magnets can exert forces on one another over large distances.
- pencil with a steel eraser holder (make sure the magnets attract the holder)
- two donut magnets that fit onto the pencil (available from Radio Shack)
To do and notice:
- Hold the pencil vertically with the point up. Make sure the bottom of the pencil
is not touching any surfaces.
-Slide one magnet onto the pencil until it sticks to the steel eraser holder.
- Continue holding the pencil, and drop the second magnet onto the pencil so that
it repels the first magnet. (Position your hands as shown in the picture.)
- Notice that the falling magnet stops at the eraser holder and the bottom magnet
pops off the bottom of the pencil.
What's going on?
Magnets either attract or repel one another by exerting forces, known as magnetic
forces, upon each other. Magnetic forces can affect magnets over large distances,
which explains how the falling magnet can make the bottom magnet pop off without
In contrast, certain forces that subatomic particles exert on one another work
only over very short distances. For instance, the force that holds quarks together
in the atomic nucleus, called the strong force, has almost no effect on quarks
that are farther than 0.000000000000001 meters apart.