meissner effect ... lenzs law? eddy currents? (1 Viewer)
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morganforrest
Member
The magnet is stationary above the superconductor but remember that Faraday's Law of Induction notes that there has to be RELATIVE movement between a CCC and a magnetic field. In the superconductor, the charged particles are moving. Secondly, Eddy currents only cause an metal to heat up because of resistance. However due to the existence of cooper pairs within the superconducting material, there is no resistance, hence no heat
Forbidden.
Banned
Conflicting information does occur in my subjects too, I take out the common ones then disregard the ones that are not similar ... =\
The Meissner effect is the result of Lenz's law.
If you try to induce a magnetic field into the superconductor (which you can't), eddy currents will be created on the surface to repel the applied magnetic field with its own opposing field, as a result of Lenz's Law.
Even if the eddy currents did heat up the superconductor, it wouldn't be significant as the magnet will still stay afloat above the superconductor.
Please read this for more information:
Click here
The Meissner effect is the result of Lenz's law.
If you try to induce a magnetic field into the superconductor (which you can't), eddy currents will be created on the surface to repel the applied magnetic field with its own opposing field, as a result of Lenz's Law.
Even if the eddy currents did heat up the superconductor, it wouldn't be significant as the magnet will still stay afloat above the superconductor.
Please read this for more information:
Click here
The Meissner effect is analgous to Lenz's Law rather than being exactly the same. It is a property of a superconductor to prevent a magnetic field entering, so the currents are set up in the superconductor.
The reason the currents do not heat up the superconductor is because of how they travel through the lattice as Cooper pairs. This stops the collision with the lattice that produce heat. The currents formed in the Meissber effect are no different to andy other currents passing through a superconductor.
The reason the currents do not heat up the superconductor is because of how they travel through the lattice as Cooper pairs. This stops the collision with the lattice that produce heat. The currents formed in the Meissber effect are no different to andy other currents passing through a superconductor.
the meissner effect is a consequence of lenz's law:
the initial relative motion between the magnet and the superconductor sets up the eddy current (according to Lenz's law). remember that there is no resistance, this means 1. theoretically the current would be infinitely large, creating an infinitely large magnetic field, and hence why it is impenetrable 2. the current would remain in the superconductor indefinitely.
as such, the magnet becomes repelled because it has created an infinitely large opposing force.
now consider that this overcoming force has become so large that the magnet moves the other way.
this creates relative motion, but this time in the opposite direction as to before. this creates an eddy current that would reduce the strength of the previous eddy current.
this process would continue until the magnet can be seen to visibly vibrate in mid-air. an analogy is the self-regulating nature of DC motors, with the back EMF, or the slip speed variation in induction AC motors.
the initial relative motion between the magnet and the superconductor sets up the eddy current (according to Lenz's law). remember that there is no resistance, this means 1. theoretically the current would be infinitely large, creating an infinitely large magnetic field, and hence why it is impenetrable 2. the current would remain in the superconductor indefinitely.
as such, the magnet becomes repelled because it has created an infinitely large opposing force.
now consider that this overcoming force has become so large that the magnet moves the other way.
this creates relative motion, but this time in the opposite direction as to before. this creates an eddy current that would reduce the strength of the previous eddy current.
this process would continue until the magnet can be seen to visibly vibrate in mid-air. an analogy is the self-regulating nature of DC motors, with the back EMF, or the slip speed variation in induction AC motors.
A defining aspect of superconductors is their ability to expel all magnetic fields. Of course, levitation.
I was under the impression that there were two ways this could happen:
1. Dropping a magnet onto a superconductor will induce currents that repel the magnet (Lenz's Law). So it will levitate.
2. However, when you place a magnet on a superconductor ABOVE critical temperature, nothing will happen. Once you pour in the liquid nitrogen, and lower its temp. below Tc, the magnet will suddenly rise into the air. This is NOT a product of Lenz's Law, because there was no changing magnetic field to induce repulsive currents in the superconductor. This relies on superconducting properties that expel all magnetic fields. I am not sure if further currents are induced when the magnet is repelled. Probably.
I was under the impression that there were two ways this could happen:
1. Dropping a magnet onto a superconductor will induce currents that repel the magnet (Lenz's Law). So it will levitate.
2. However, when you place a magnet on a superconductor ABOVE critical temperature, nothing will happen. Once you pour in the liquid nitrogen, and lower its temp. below Tc, the magnet will suddenly rise into the air. This is NOT a product of Lenz's Law, because there was no changing magnetic field to induce repulsive currents in the superconductor. This relies on superconducting properties that expel all magnetic fields. I am not sure if further currents are induced when the magnet is repelled. Probably.
twilight1412
Member
i dont see why you guys call them eddy currents BECAUSE THEY ARE NOT EDDY CURRENTS
they are either persistance currents or superconducting currents
and its not really the relative motion which causes it either
it is the property of superconductors in their superconducting state to expel all external magnetic fields through the persistance currents which run through it
notice how this only works for cylindrical shaped superconductors
they are either persistance currents or superconducting currents
and its not really the relative motion which causes it either
it is the property of superconductors in their superconducting state to expel all external magnetic fields through the persistance currents which run through it
notice how this only works for cylindrical shaped superconductors
Most of what you say is correct, which is why you are better to say analogy to lenz's law.
It doesn't just work for cylindrical superconductors, its just those are the cheapest for schools to buy.
These hexagonal ones are better but cost more.
http://www.coolmagnetman.com/images/sc03.jpg
http://www.coolmagnetman.com/magsuper.htm
It doesn't just work for cylindrical superconductors, its just those are the cheapest for schools to buy.
These hexagonal ones are better but cost more.
http://www.coolmagnetman.com/images/sc03.jpg
http://www.coolmagnetman.com/magsuper.htm
twilight1412
Member
really? i was told that cylindrical worked the best @_@
ouch there goes my confidence
ouch there goes my confidence