The Meissner Effect: Magnetic Levitation Using Superconductivity
Typically, when the temperature of a conductor is reduced, the resistance is also lowered. We saw in the previous experiment how a magnetic field can cause an object to levitate. For some materials, if we continue to lower their temperature, the resistance continues to drop until it disappears entirely. When this happens, we have what is known as a superconductor. Superconductors have amazing properties and are beginning to find their way into practical applications.
What You Need
- liquid nitrogen
- thin piece of cork (about ¼ inch thick)
- Styrofoam dish (formed by cutting a Styrofoam cup; tThe total height should be about 2 mm)
- cube-shaped neodymium magnet (see Figure 106-1)
- plastic tongs
- superconductor disk consisting of YBa2Cu3O7 ceramic (see Figure 106-2)
- optional: video camera or PC cam connected to a TV monitor (to show this to a larger group)
- optional: thermocouple, voltmeter, DC power supply, superconductor coil, superconductor sample with measurement leads attached
- Place the YBa2Cu3O7 ceramic disk on the table and set the neodymium magnet on top of it to show no repulsive force is occurring at room temperature.
- Place the cork in the center of the Styrofoam dish.
- Place the black YBa2Cu3O7 ceramic disk on the piece of cork.
- Carefully pour liquid nitrogen into the Styrofoam dish to partially cover the ceramic disk.
- The liquid nitrogen will boil for a short while. When the boiling subsides, the disk has sufficiently cooled, as shown in Figure 106-3.
- Using the plastic tongs, pick up the neodymium magnet. Carefully place the magnet over the ceramic disk.
- When the magnet is observed to hover over the ceramic disk, use the tongs to give it a spin, as shown in Figure 106-4.
- Because the parts in this project are small, if the intention is to show this to a larger group, a video camera or PC cam can be used to display this on a monitor.
The magnet is held suspended above the ceramic superconducting material. If the magnet is spun, it continues spinning without noticeable resistance. Eventually, the ceramic will warm up and the superconducting effect will fade.
Why It Works
Normally, at temperatures above what is known as the critical temperature of a material, the material has some electrical resistance. This means a voltage must be applied across the material to push the electrons through the material. The voltage is needed to drive the electrons through what is like an atomic obstacle course, consisting of other atoms vibrating randomly. As (normal nonsuperconducting) resistors cool down, their resistance gets lower. However, superconductors have zero resistance. Not just lower, but zero! This means the electrons no longer need a voltage to push them. This also means the electrons can move about freely throughout the superconductor without energy losses.
Different materials become superconducting at characteristic temperatures that differ for each material, as shown in the following table:
Notice that all the metals listed must be cooled to below 4.2 K. To accomplish this, it is necessary to use liquid helium, which remains liquid up to that temperature, as shown in the following table. In 1987, a breakthrough was achieved by the discovery that YBa2Cu3O7 ceramic became superconducting around 90 K (and below), which can be achieved by immersion in liquid nitrogen. This is far less expensive and easier to work with than liquid helium. Scientists are pursuing materials that can be superconducting at temperatures closer to room temperature, which could open the door to its application in many new areas.
The actual details of how superconductivity works will take us further into quantum mechanics than I think most readers would care to go. The theory known as BCS theory was named after three American physicists: Bardeen, Cooper, and Schrieffer. (Very) basically, the BCS theory describes how electrons are able to more easily navigate through the crystal lattice of matter in a manner that is somewhat analogous to a race car encountering less aerodynamic resistance as it closely follows another car in front of it. As the critical temperature is reached, the electrons are able to go through a material by "tunneling" right through an electrical field in its way. As a result, superconductors have zero resistance. If digital electronics were based on superconductors they would function ten times faster than standard semiconductor electronics.
Magnetic fields can pass through and be present in most materials, including superconductors above their critical temperature. However, as a superconducting material is brought below its critical temperature, the magnetic field is forced out in a process known as the Meissner effect, which serves as the basis for the effect we saw here. To enable the magnetic field to be pushed out of the superconductor, it becomes necessary for a counter current to flow in the superconductor. With no resistance, electrical currents are induced in the superconducting ceramic, which, in turn, creates a magnetic field that repels against that of the permanent magnet.
Superconductors are being looked at to address some of the following challenges in technology:
- Much of the electrical power transmitted throughout the world's electrical power grid is dissipated as resistive heat losses. If superconductors could be used for power transmission and generation, some of the losses could be reduced.
- Magnetic resonance imaging (MRI) equipment uses extremely powerful magnets to help create detailed images of the body. Superconductors allow stronger magnets to be built.
- Maglev trains use superconductors to help produce powerful magnetic fields that raise trains above the track, enormously eliminating friction.
- The extremely powerful magnets used to guide beams of subatomic particles in research facilities (such as CERN) use superconductors.
- Superconductors hold the promise of enabling faster processing of digital information in computers.