This demonstration extends the discussion of magnets in Building an Electromagnet with two simple but interesting exhibits of magnetic forces.
(1) Levitated MagnetsSeveral ring magnets, wooden dowel or pencil
Place several of the ring magnets on the dowel. Hold the rod in an upright position. If any of the magnets stick together, remove them, turn them over and place back on the rod. Have students observe the levitation of the magnets.
(1) Why do the magnets float?
(2) Where are the poles of the magnets?
(3) What do you observe about the spacing of the magnets? Can you explain the spacing?
The poles are on the flat sides of the magnets. The combination of gravity and magnetic forces creates the spacing. Each magnet supports the weight of all of the magnets above it-thus, the lower magnets require a stronger repulsive magnetic force than the upper magnets. This stronger force is supplied by the smaller separation distance for the lower magnets, since magnetic force increases with decreasing separation distance. This demonstration can also be performed as an activity if desired.
(2) An Attraction for DollarsAmerican dollar bill, neodymium magnet
Hold a dollar bill in a horizontal orientation. Bring a neodymium magnet close to one corner of the dollar bill and watch the result.
(1) Is there an attraction between the magnet and the dollar bill?
(2) Would you characterize the attraction as strong or weak?
(3) How close to the bill must you bring the magnet in order to observe an effect?
(4) Is there any difference in the result for a $5 bill or a $10 bill?
(5) Why do you think the bill is attracted to the magnet?
The corner of the bill will be attracted to the magnet. The ink in the dollar bill contains small amounts of an iron compound, which is attracted to the magnet. The attraction is weak because the amount of iron is very small. The neodymium magnet has a strong enough magnetic field to make the attraction visible.
