Superconductivity Made Easy

Introduction

The resistance in some metals and metallic compounds decreases uniformly with decreasing temperature until a critical temperature is reached where the resistance suddenly falls to zero! At zero resistance, superconducting materials exhibit the unique ability to levitate a magnet. This levitating ability is termed the Meissner effect. In the following demonstration, the fascinating levitating phenomenon and the temperature dependence of superconductors will be displayed.

Concepts

• Superconductivity
• Critical temperature
• Meissner Effect

Materials

Safety Precautions

Use extreme care when handling liquid nitrogen. At atmospheric pressure, the temperature of liquid nitrogen is below its boiling point of –77 K (–196 °C). Severe frostbite can result from contact with bare skin or clothing. Wear a pair of Zetex™ gloves when handling liquid nitrogen. When transporting liquid nitrogen in a car, prevent the filled Dewar flask from tipping and spilling its contents by placing it in a wide based box. Surround the flask with packing material (i.e., crumpled newspapers) to stabilize and protect it. Carefully place the box on a flat and level surface in the car, such as the rear floorboard or the trunk rather than on the seat. Keep the car windows rolled down. This will prevent the displacement of oxygen from the car should the flask accidentally tip. Wash hands thoroughly after handling the superconductivity disk. Wear chemical splash goggles, Zetex gloves, and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Clean and dry the superconductivity disk and magnet and save for another demonstration. Allow the liquid nitrogen to evaporate in the air.

Prelab Preparation

1. Position the demonstration table approximately 10–15 feet from the projection screen. Tip the overhead projector on its side, extension arm up, so that the light projects onto the projection screen. Both the screen and projector should be the same height above the floor. Turn on the overhead projector (see Figure 1).
   {13350_Preparation_Figure_1}
2. Place a ring stand with a ring directly in front of the glass-plated portion of the overhead projector. Turn one of the cups upside down and place it on the base of the ring stand. This cup will be referred to as the bottom cup. Elevate the cup with a textbook if necessary so that the cup is centered in the glass-plated portion of the overhead projector. The bottom half (narrower portion) of the cup should be clearly projected on the screen. Focus the image by moving the cup and/or the projector focus knob. Note: Since the overhead projector is sitting on its side, the image projected on the screen will be upside down.
3. Make a small hole in the center of the bottom of a second cup with a straight pin. Place the second cup securely inside the ring attached to the ring stand. This cup will be referred to as the top cup. Position the ring on the ring stand such that the top cup is about three inches directly above the bottom cup (see Figure 2).
   {13350_Preparation_Figure_2_Gravity feed liquid nitrogen onto superconductive disk}
4. Notice the small “well” that is present on the underside of a polystyrene cup where the sides meet the bottom of the cup. This “well” on the bottom cup will contain the liquid nitrogen as it drips from the top cup.
5. Place the superconductivity disk in the “well” on the bottom cup.

Procedure

Part A. Demonstration of Superconducting Properties

1. Carefully fill the top cup about ¾ full with liquid nitrogen from the Dewar flask. Notice that a small volume of liquid nitrogen will drip through the pin hole in the top cup and collect in the “well” of the bottom cup.
2. Pick up the superconductivity disk with the non-metallic forceps and submerge it in the liquid nitrogen in the top cup. Do not release the disk from the forceps. The introduction of the room temperature disk to the very cold liquid nitrogen will cause the liquid nitrogen to boil profusely until the superconductivity disk is cooled to the temperature of the liquid nitrogen. Once the boiling ceases, remove the superconductivity disk from the liquid nitrogen and return it to its place in the “well” on the bottom cup.
3. Pick up the rare earth magnet with the forceps and place it on top of the superconductivity disk. Observe the magnet as it levitates in midair! The magnet will remain levitated as long as the liquid nitrogen continues to drip from the top cup down into the “well” on the bottom cup because it keeps the superconductivity disk below its critical temperature.
4. Notice that the image is clearly projected on the projector screen. Carefully pass a piece of paper between the magnet and the superconductivity disk to show that there is nothing between the disk and the magnet holding the magnet in midair.
5. Now carefully aim a straw at the corner of the levitating magnet. Blow through the straw. Observe the levitating magnet spin! Be careful not to blow so hard that the magnet is pushed off the disk. The spinning is quite visible if the magnet is marked with white correction fluid before the demonstration.

Part B. Demonstration of the Critical Temperature

6. Once the temperature of the superconductivity disk is elevated above the critical temperature, the disk will lose its superconductive properties. To illustrate this phenomenon, pour any remaining liquid nitrogen from the top cup back into the Dewar flask. As the liquid nitrogen remaining in the “well” boils, the temperature of the superconductivity disk will slowly begin rising to room temperature. The moment the temperature of the disk exceeds the critical temperature, the disk will lose its superconductivity properties (i.e., the ability to levitate a magnet), and the magnet will fall to the side of the superconductivity disk.
7. Remove the magnet and the superconductivity disk with the non-metallic forceps. Clean and dry them with a paper towel. Replace the dry, room temperature superconductivity disk back into the “well” of the bottom cup with the forceps. Place the magnet directly on top of the superconductivity disk.
8. Fill the top cup with liquid nitrogen and allow the liquid nitrogen to drip through the hole in the top cup into the “well” of the bottom cup. Do not allow the liquid nitrogen to drip onto the magnet, as that will cause the magnet to become temporarily frozen to the superconductivity disk. The liquid nitrogen that falls into the “well” will boil rapidly at first. As the superconductivity disk is cooled, the rate of boiling will decrease. Eventually, the disk will be cooled below the critical temperature. At that moment, the magnet will “lift-off” and begin to levitate above the superconductivity disk.

Teacher Tips

• The magnet will remain levitated for approximately 5 minutes. As the demonstration continues, a build-up of ice crystals on the cups, the magnet, and the disk may form. These protruding ice particles, known as dendrites, will eventually reduce the effectiveness of the demonstration by interfering with the magnet’s levitation or by plugging the hole in the top cup. If dendrite development occurs, simply remove the magnet and the superconductivity disk with the forceps. Clean and dry them with a paper towel. Carefully wipe off the cups as well. It may be necessary to pierce a second hole in the top cup so that liquid nitrogen can continue to drip down into the “well” on the bottom cup. After cleaning the dendrite-covered pieces, replace the superconductivity disk and the magnet to their respective places and resume the demonstration.
• Two superconductivity disks may be placed one on top of the other while performing the demonstration. The use of multiple superconductivity disks will create a more dramatic levitation effect.
• After completing the demonstration, remove the superconductivity disk with the non-metallic forceps and clean and dry it with a paper towel. Place it under a desk lamp for a few minutes and allow it to dry completely before storage. Constant exposure to moisture may damage the superconductivity disk.
• Once the temperature of the superconductivity disk is elevated above the critical temperature, the disk will lose its superconductive properties.
• Liquid nitrogen is readily available at welding supply stores, industrial supply companies, or from local hospitals, universities, veterinarians, dermatologists, and airports. Pricing varies widely, but, when used for educational purposes, it will often be supplied free of charge. You will need a Dewar flask to transport the liquid nitrogen from the supplier to school.
• Dewar flasks are manufactured specifically for the storage and handling of liquid nitrogen. These extremely well insulated, high quality flasks prevent the rapid boil off of liquid nitrogen which normally boils at –77 K (–196 °C). They are equipped with special vented caps to prevent the buildup of pressure and possible explosion.
• The superconductivity disk has a limited lifetime. After several dozen uses, the disk may begin to crack and lose its superconducting properties.
• The use of a document camera hooked up to a television monitor is an excellent way to demonstrate this to large audiences.

Discussion

Heike Kamerlingh Onnes discovered superconductivity in 1911 while measuring the resistance of metals at extremely low temperatures. He found that the resistance dropped abruptly at 4 K for mercury and at temperatures of a few Kelvins for several other metallic elements. The specific temperature at which a material loses resistance is called its critical temperature. Another surprising characteristic of superconductors was discovered about 20 years later by W. Meissner and R. Ochsenfeld. They observed the Meissner effect—the phenomenon that makes the levitation of magnets possible. It is described in detail below. A superconductor is defined by these remarkable electrical and magnetic properties—the loss of resistance and the Meissner effect.

The Meissner effect can be explained by considering the principles of electromagnetism. According to Faraday’s law, an electric field can be induced by a magnetic field. The Ampere–Maxwell law states that a changing electric field induces a magnetic field. Considering both these laws, the levitation effect is explained as follows.

The magnet possesses an intrinsic magnetic field. As it approaches the cooled superconductivity disk, its magnetic field induces a current in the superconductivity disk (Faraday’s Law). This current, called a supercurrent, contains no resistance. The supercurrent will remain in the superconductivity disk even after the magnet has stopped moving. The induced supercurrent induces its own magnetic field in the superconductivity disk (Ampere–Maxwell Law). This self-induced magnetic field exactly opposes the magnet’s magnetic field so that the inside of the superconductivity disk has exactly zero magnetic field.

Outside the superconductivity disk, however, the induced magnetic field produced by the superconductivity disk still exists. It still opposes the magnetic field produced by the magnet, but no longer exactly cancels it. The result is that the two magnetic fields repel each other in an upward direction with enough force to overcome the force of gravity. How high the magnet is suspended above the superconductivity disk is determined by the relative magnitude of the downward gravitational force versus the upward magnetic repulsive force.

The levitation effect will continue as long as the temperature of the superconductivity disk remains below its critical temperature. Below that temperature, the explanation above occurs as described. Above that temperature, however, the superconductivity disk loses its supercurrent, which in turn means the self-induced magnetic field disappears. As a result, there is no force strong enough between the superconductivity disk and the magnet to overcome the force of gravity.

References

Ellis, A. B. Superconductors; Better levitation through chemistry. J. Chem. Ed. 1987, 64, 836–841.