Magnetic Shielding

Magnetic lines stop here.

Magnetic-field lines pass through cardboard, air, and certain other materials, depending on whether they're permeable or nonpermeable. Test different materials to see which gather magnetic lines of force and act as magnetic shields, and which allow magnetic lines of force to pass through them.

Subjects:

Physics
- Electricity & Magnetism

Keywords:

magnetic field

magnet

Tools and Materials

Three ceramic donut magnets (rectangular magnets will also work)
Two pieces of cardboard of equal size, about 2 x 2 3/4 inches (5 x 7 centimeters)
Two pencils, 2 3/4 in (7 cm) or longer
Five or six paper clips
Wood craft stick, plastic straw, or any other nonmetallic material
Steel butter knife
Hot-glue gun and glue (alternatively, you can use rubber bands)

Assembly

First, make a cardboard-pencil sandwich: Attach the pencils to one piece of cardboard, placing the pencils parallel to each other and close to the opposite edges of the cardboard. Glue the second piece of cardboard to the pencils so that you end up with a cardboard-pencil-cardboard sandwich.
Secure one of the magnets to the top piece of cardboard with hot glue. Center the magnet near one of the edges (see photo below).
Add two more magnets on top of the first one so that they are held in place by magnetic attraction. Don't glue them!

To Do and Notice

Hold your shielding sandwich with the magnets on top. Raise the paper clips one at a time to the bottom side and notice what happens. The paper clips should be attracted to the magnet and so will hang from the bottom of the cardboard sandwich.

As you add more paper clips, notice what happens. If you add them carefully, they will arrange themselves so that they are evenly spaced.

Insert the craft stick or plastic straw into the shielding sandwich, move it around, and notice what happens. The paper clips should be unaffected.

Now insert the flat blade of a steel butter knife into the shielding sandwich, move it from side to side, and notice what happens. Do the paper clips fall off? Try experimenting with different materials, such as various metallic coins. Make a list of the materials you test and note what happens.

What's Going On?

The magnetic-field lines from the magnet pass through the cardboard, the air, and other materials like the craft stick and straw. Materials that allow magnetic lines of force to pass through them are called nonpermeable because magnetic fields do not form within them.

In contrast, the metal knife acts as a magnetic shield, meaning the force lines coming from the pole of the magnet do not pass through it. Instead, they are gathered in, travel down the metal strap, and re-enter the magnet at the other pole. Materials that gather magnetic lines of force are said to be permeable, because they support the formation of magnetic fields within those materials. Only magnetic materials are permeable.

The photos below show the effect of a nonpermeable material (at left) and permeable material (at right) on magnetic lines of force in our cardboard sandwich.

Going Further

Try using only one or two magnets. As the magnetic field strength gets weaker, it gets harder to keep the paper clips from falling. How could you modify the sandwich to make a weaker magnet work better?

Insert an open pair of steel scissors between the pieces of cardboard, then close the scissors as if cutting the field lines. The paper clips will drop off when the field lines interact with the closed scissors.

Scientists model magnetic forces using field lines. A compass lines up along a field line, so you can trace field lines with a compass.

Iron filings also line up along field lines. By sprinkling iron filings on a piece of paper placed on top of a magnet (or a couple of magnets), you can see the field lines. Our Snack, Magnetic Lines of Force, demonstrates this phenomenon.

Field lines come from the north pole of a magnet and return to the south pole. Magnets have field lines that exert a force on magnets or iron. If there are no field lines, there is no force, and the result is magnetic shielding.