# Magnetic Levitation Help

#### SuperAcid

In an experiment I am carrying out I am levitating metal rings using a solenoid with a clamp stand going through the solenoid acting as a support for the ring and a core for the solenoid (seen in the attachment). I have one simple question? Theorectically, should an Aluminium or Copper ring levitate higher (at a fixed current through solenoid) ? Would any other factors like ring diameter etc. affect the height of levitation (I am keeping the voltage/current constant)?

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#### clem

The force on the ring is inversely proportional to its resistance. The ring will rise to a height where F=mg, so its height will depend on 1/mR. Calculate this ratio for each ring.

#### SuperAcid

The force on the ring is inversely proportional to its resistance. The ring will rise to a height where F=mg, so its height will depend on 1/mR. Calculate this ratio for each ring.

How do you know this ? Is it derived from an equation?

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#### clem

There is a complete equation, but the dependence on R is simple. The current in the ring is given by I=EMF/R. The EMF depends on the area of the ring, which I assume is the same for each ring.

#### Bob S

Pure aluminum is the best of all. I think it is called 2S aluminum. The reason is that it has the highest ratio of electrical conductivity to density for any metal. Its conductivity is about as good as copper, and its density is about 1/3 that of copper. For a given dB/dt, higher frequencies generally are better.

There is another form of levitation, using superconducting materials. Superconducting materials exclude dc magnetic fields from the metal entirely, while in eddy current levitation, the eddy currents near the surface are cancelling the ac magnetic fields inside. The depth of the eddy current penetration varies inversely with frequency (I don't think is like skin depth which is sqrt(frequency)). Look at transformer laminations.

#### soulsoldier

how does it come that the copper ring is levitating? I have seen a demonstration which is very alike with your drawing but the "clamp" was a ferromagnetic material.
I understand that an induced eddy current opposes the magnetic field, but i don't understand which force is working on it and why it comes to an equilibrum whitout oscillating.. Which electromagnetic laws are involved? And what has the ferromagnetic core to do with it?

#### Antiphon

There is a complete equation, but the dependence on R is simple. The current in the ring is given by I=EMF/R. The EMF depends on the area of the ring, which I assume is the same for each ring.
This equation isn't correct. The ring will not have an infinite current if the resistance is zero. The current attains the value needed to cancel the inducing magnetic field.

#### soulsoldier

The current attains the value needed to cancel the inducing magnetic field.
but if there is an iron core within, there are still magnetic field lines going through... ???

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