Motivate Repulsive Grapes with Neodymium Magnets


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"Well Balanced Grapes" by Mike DeHaan

Well Balanced Grapes: Image by Mike DeHaan

Neodymium magnets are powerful enough to demonstrate the subtle effects of diamagnetism, the weak repulsion that magnets can exert on some substances.

What is Diamagnetic Repulsion?

Magnetism is an attractive force that sticks a fridge magnet onto a refrigerator. However, “like” magnetic poles repel; the “North” ends of two magnets push each other apart. These are the effects of “ferromagnetism” (from “ferrous”, meaning “concerning iron”). Standard magnetism is both an attractive and repulsive force, depending on the circumstances.

"Yomega Raider (R) Yo-Yo" by FHKE

Yomega Raider (R) Yo-Yo: Image by FHKE

Many substances show no obvious attraction to magnetism. We don’t expect a magnet to hold a wood or plastic Yo-Yo.

However, some materials are weakly repelled by magnetism. This effect is called “diamagnetism“, or “diamagnetic repulsion“.

The Power of Neodymium Magnets

Some materials are more powerful than standard iron or steel magnets. Neodymium magnets are among the most powerful permanent magnets. They may be called “rare earth” magnets.

If you don’t have a neodymium magnet, feel free to try this experiment with whatever magnets you might have handy. A second choice is to wind your own electromagnet, powered by a quick-discharge battery. The winding might get quite warm, since there is little resistance in the circuit; it could also damage a rechargeable battery.

A Simple Experiment Demonstrates Diamagnetic Repulsion

The simplest lab demonstration of diamagnetism repels a pair of grapes on a toothpick suspended from a string.

"Suspended Grapes" by Mike DeHaan

Suspended Grapes: Image by Mike DeHaan

List of Materials for this Diamagnetism Experiment

This experiment requires a sewing thread, a toothpick, two grapes, and a neodymium magnet (also known as a rare earth magnet).

The confirmation tests require a non-magnetic object, such as a pencil, and something that can hold an electrostatic charge, such as a comb.

If you also have a weaker, regular magnet, have it ready also. Optional alternate materials include a pair of ice cubes or a pair of raisins.

Steps for the Diamagnetism Experiment

Tie one end of the thread to the center of a toothpick. Impale one grape on each end of the toothpick.

Suspend one end of the thread from a curtain rod, or anywhere that the toothpick can dangle freely without bumping into a wall.

"Repulse the Grapes" by Mike DeHaan

Repulse the Grapes: Image by Mike DeHaan

Adjust the grapes or the point where the thread holds the toothpick, so that it is balanced. The effect is like a suspended dumbbell.

Wait while the thread spins down. It may take several hours to completely relax; I left it overnight for one test.

When the toothpick no longer spins, slowly bring one pole of the neodymium magnet close to one grape. Close in on the same level horizontal plane as the grapes occupy; don’t swoop in from above or below.

Expected Observations from this Diamagnetism Experiment

One grape should move away from the magnet, making the “toothpick dumbbell” rotate.

Confirmation Tests Using Neodymium Magnets

Move the neodymium magnet away. Bring the “toothpick dumbbell” to rest. Approach one grape from the other side, to begin rotation in the opposite direction. Repeat, but in the same direction a few times. Try using either the north or south pole of the neodymium magnet.

Again, bring the “toothpick dumbbell” to rest. Approach one grape with some non-magnetic object; it should not move. Try again, perhaps with a comb that has a static electric charge. Again, it should not move.

Repeat the tests with a weak, standard magnet. It is unlikely to have any visible effect.

If you have patience, try attaching an ice cube to each end of the toothpick. This apparatus should behave like the previous dumbbell. (You might tie an icicle to the string, but the string might not finish unwinding before the icicle melts).

Further Tests of Rare Earth Magnets and Diamagnetism

Feel free to replace the grapes with anything else. Whatever is repelled is also diamagnetic, by definition. Something that is weakly attracted is paramagnetic. A ferromagnetic object will “jump and latch onto” the powerful neodymium magnet.

Try a pair of raisins on the toothpick. These dried grapes should react much more weakly, if at all, compared to the grapes.

Explanations for this Lab Experiment on Diamagnetism

It only seems fair to offer some brief explanations about this experiment. Why use grapes, a toothpick and string to show diamagnetic properties? What is diamagnetism? Could we do “better” science or develop practical applications?

Why are Grapes Diamagnetic?

The water molecules actually are the most diamagetic part of a grape. The water is such a large component of the grape that it shows the effect.

Explaining Diamagnetism

"H2O Water" by fogonthedowns (Justin Zollars)

H2O, or Water: Image by fogonthedowns (Justin Zollars)

Actually, all materials are somewhat diagmagnetic; some simply are more ferromagnetic or paramagnetic than diamagnetic. Let’s use the classic model of “a cloud of electrons orbiting a molecule’s nucleus”.

Under the influence of a magnetic field, the molecule’s electrons change their motion to oppose that magnetism. Diamagnetic molecules have no permanent magnetic “moment”; the electrons freely travel anywhere around the nucleus, but do not easily travel to other molecules.

Water is a diamagnetic substance. Although written as “H2O”, the molecule actually looks like H-O-H as shown in the image. The electrons do not need to stay near either hydrogen, nor near the oxygen. When they experience a magnetic field, they oppose it.

Explaining the Need for a “Toothpick Dumbbell”

The suspended “toothpick dumbbell” offers very little resistance. It takes much more force to roll a grape away, or to make it swing away as a pendulum.

Enhancing the Science of this Experiment

Neodymium sculpture: Image by deramko

This experiment is qualitative, rather than quantitative: it demonstrates an effect without measuring it. For those who care to log numeric results:

  • Record how quickly the “toothpick dumbbell” spins, or how quickly you can move the neodymium magnet forward, for different objects like grapes or raisins.
  • You would also need to record the weight of these test objects, since F=ma (Force = mass times acceleration).
  • Replace the string with a sensitive torque gauge to determine how much force is being exerted.

Exploiting the Science of Rare Earth Magnets

Unfortunately the diamagnetic repulsion effect is too weak for a practical application, such as an invisible water repellant umbrella. A magnet that powerful would also attract unwanted scrap metal, and possibly erase any passing credit cards.

A rare earth magnet is a key component of most mobile telephones. Loudspeakers have long used magnets, but the tiny speaker inside a modern smartphone needs the most powerful available magnetic material.

Conclusion on the Repulsive Force of Diamagnetism

Diamagnetism is a weak repulsive effect of the electromagnetic force, only observed for certain materials when using a powerful magnet. A simple home laboratory experiment can demonstrate diamagnetic repulsion using a neodymium magnet, two grapes, string and a toothpick.

"Neodymium Rare Earth Magnets (mini 3mm cube)" by steven m

Neodymium Rare Earth Magnets (mini 3mm cube): Image by steven m

Warning: Neodymium magnets are not toys. They will pinch flesh “like pliers”. They can splinter and shatter if dropped or collided. They can permanently damage magnetic storage, such as a credit card’s magnetic stripe. If two or more are swallowed, they can be fatal.

Beaty, Bill. “NEODYMIUM SUPERMAGNETS : SOME DEMONSTRATIONS”. (Updated Dec. 2005) AMASCI.COM. Accessed Oct. 5, 2011.
Tatum, Dr. J.B. “Electricity and Magnetism (Chapter 12)” (Updated Aug. 4, 2011). PDF accessed Oct. 5, 2011.

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