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Recorded: 2017/03/05 Published: 20017/10/21
Randy shows Jim five different ways in which a body can be levitated: by magnetism, by superconductors, by Lenz' Law, by acoustics, and most recently by thermophoresis.
1. The particular magnetic levitation we talk about is diamagnetic. A very, very large magnetic field induces a large magnetic moment in an object as a reaction. If the diamagnetic component of the object's magnetic susceptibility is larger than its paramagnetic component, the resulting magnetic moment will oppose the field.
Superconductors also expel magnetic fields from their body by the Meissner effect, which makes a "perfect dielectric." This allows the superconductor to levitate.
2. Using Lenz' Law, eddy currents that are induced by time-varying electric fields produce magnetic fields that react with their source, levitating the object.
3. Acoustic levitation occurs when standing sound waves are set up in a region, providing areas where particles can levitate.
4. Optical traps work in a similar way to acoustic levitation with electromagnetic fields instead of pressure fields.
5. Thermophoretic levitation occurs when temperature gradients produce a force that allows small object to float.
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Notes:
1. The papers we read for this program:
- Brandt, E.H., "Levitation in Physics." Science 243 349 (1989).
- Zuza, Fuisasola, Michelini, and Sani, "Rethinking Faraday's Law for Teaching Motional Electromotive Force." European Journal of Physics 33 397 (2012).
- Brandt, E.H., "Suspended by Sound." Nature 413 474 (2001).
- Ashkin, A., "Optical Trapping and Manipulation of Neutral Particles Using Lasers." Proceedings of the National Academy of Sciences 94 4853 (1997).
- Fung, Usatyuk, DeSalvo, and Chin, "Stable Thermophoretic Trapping of Generic Particles at Low Pressures." Applied Physics Letters 110 034102 (2017). [arXiv]
2. Videos of levitating objects:
- Levitating Frog
- Superconducting Levitation
- Lenz' Law
- Acoustic Levitation
- Optical Tweezers
- Thermophoretic Leviatation
3. Our subreddit.
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