Magnetic Fruit
When we think about objects that respond to magnets, fruit usually doesn't come to mind. Watch a rare-earth magnet repel a grape and discover different kinds of magnetism.
- Two large grapes
- Drinking straw
- 2-foot (60-centimeter) length of string
- Tape
- Neodymium (rare-earth) magnets—a 1-inch (2-cm) cube works well; the larger, the better
- Stand to hang the grapes from—a ring stand works well, or you can make a stand using PVC
A note on magnets: Only extremely strong magnetism will be powerful enough to push the grape, so neodymium magnets are the best—the bigger, the better. But be careful! Large neodymium magnets are so strong that they can give you a painful pinch if you get your flesh caught between two of them or between the magnet and a piece of iron.
- Tie or tape one end of the string to the middle of the straw.
- Tie or tape the other end of the string to your stand, ensuring the straw can rotate freely in space. You’ve just created a type of torsional pendulum.
- Slide the grapes onto each end of the straw. (It helps to make small cuts in the stem ends of the grapes and insert the straw ends into the cuts.) Adjust the position of the string and grapes so the ends are balanced, but the straw doesn't need to be completely level. To level the straw, try pushing the grapes further onto the ends of the straw.
Hold one side of the magnet near a grape, but do not touch the grape with the magnet. The grape will be repelled by the magnet and begin to move slowly away.
Remove the magnet and let the grape stop moving.
Turn the magnet over and hold the other side near the grape. Once again, the grape will be repelled and will begin to move slowly away. You’ll notice that both poles of the magnet repel the grape—evidence of diamagnetism!
Diamagnetic materials are repelled by both poles of a magnet—you saw this in the movement of the grape. In diamagnetic materials, all the electrons pair with electrons of opposite spin. Examples of materials in which all the electrons are paired include helium, bismuth, graphite, and water. Water is a main component of grapes.
When you bring the magnet toward the grape—the diamagnetic material—you induce an electric current in the atoms of the grape that make them magnetic in a way that will repel the approaching magnet. This is the same result as predicted by Lenz's law: When you move a magnet toward an electrical conductor, it will create electric currents in the conductor, turning it into an electromagnet that repels the magnet.
Diamagnetic repulsion is very weak—a hundred thousand to a million times weaker than ferromagnetism. Only since the invention of rare-earth magnets, which are several times stronger than traditional magnets, have we been able to easily demonstrate the weak diamagnetic force.
Try mounting other objects such as aluminum, wood, prunes—yes, even prunes!—on your torsional pendulum. What are their respective magnetic properties?
There are three types of magnetism: diamagnetism, ferromagnetism, and paramagnetism. Iron is ferromagnetic, which means it’s attracted to both poles of a magnet. In atoms of iron, cobalt, and nickel, the spins of electrons in one atom will align with electrons in neighboring atoms, making regions called domains that have very strong magnetization.
The third type of magnetism is paramagnetism. Atoms and molecules that have single, unpaired electrons—such as aluminum, hydrogen, lithium, and liquid oxygen—are paramagnetic. Their electrons orient in a magnetic field, causing them to be weakly attracted to both magnetic poles.
Every electron is a magnet because electrons carry charge and spin. In addition, an electron in orbit can be an electric current, turning the electron into an electromagnet.
To accurately model the behavior of ferromagnetic, diamagnetic, and paramagnetic materials requires quantum mechanics. We encourage you to research this!
For further reading, we suggest Edward M. Purcell’s Electricity and Magnetism: Berkeley Physics Course, Volume 2, published in 1984 by McGraw-Hill.
