Meissner Effect
Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff
Categories: Magnetism, Liquid Nitrogen
Alternative titles: Magnetic Field Expulsion, Floating Magnet
Summary
When cooled below its critical temperature using liquid nitrogen, a superconductor expels magnetic fields and becomes a perfect diamagnet. This phenomenon, known as the Meissner effect, allows a magnet to levitate above the superconductor until it warms above its transition temperature.
Procedure
- Place a sample of yttrium-barium-copper-oxide (YBCO) superconductor in a container.
- Pour liquid nitrogen over the sample until it is fully cooled below its critical temperature.
- Hold a small permanent magnet above the superconductor.
- Observe the magnet levitate as the superconductor expels the magnetic field.
- Allow the liquid nitrogen to evaporate; when the superconductor warms, the magnet will gradually lose levitation and fall.
Links
Superconductor Meissner effect - pl4nb33:
Meissner Effect - Physlab.org:
📄 Meissner Effect - PhysLab: https://physlab.org/class-demo/meissner-effect/
Variations
- Use different shapes or sizes of magnets to see how the levitation changes.
- Demonstrate flux pinning by slightly displacing the magnet and observing how it locks into position.
- Compare the effect using different superconducting materials.
Safety Precautions
- Wear cryogenic gloves and face protection when handling liquid nitrogen.
- Handle superconductors and magnets with care; avoid direct skin contact with extremely cold surfaces.
- Ensure good ventilation to prevent oxygen displacement from evaporating nitrogen.
- Do not confine liquid nitrogen in a sealed container (risk of explosion).
Questions to Consider
- Why does a superconductor expel magnetic fields when cooled below its critical temperature? (It becomes a perfect diamagnet due to the Meissner effect.)
- What role does temperature play in this demonstration? (Below the critical temperature, the material becomes superconducting; above it, superconductivity is lost.)
- How is flux pinning different from simple levitation? (Flux pinning allows the magnet to remain suspended in fixed positions due to trapped magnetic field lines in the superconductor.)
- What practical applications arise from the Meissner effect? (Maglev trains, frictionless bearings, superconducting power lines.)