Why Does the Aluminum Ring Hover Unsteadily in a Magnetic Levitation Experiment?

[jsmith613]

Homework Statement

see attachment

Homework Equations

The Attempt at a Solution

ok, so clearly an emf will be induced in the aluminium ring as the alternating current produces a magnetic field. The induced emf will be equal to the rate of change of emf (which will vary with time as the current in the coil varies with time and hence the magnetic field it produces varies). This means that the induced current in the aluminium ring varies too. The two fields interact with each other and the aluminium ring feels a magnetic force upwards.

Using this logic, the magnetic force (F = BIl) should ALSO vary with time (as the magnetic field and induced current are continuosly varying with time) and the ring should NEVER float at a stable position because the NET force on the aluminium ring is ALWAYS changing?

I am clearly missing something but I don't know what

 

[NascentOxygen]

Good question, jsmith613! I expect inertia plays a part here. A good demonstration would provide an AC supply where you could vary the frequency. If force is time-varying, as you drop the period down to approaching ~ 0.2 seconds the ring should be seen to waver up and down.

If you have this as a lab demo, lightly touch the ring and you may feel it vibrating.

 

[jsmith613]

Good question, jsmith613! I expect inertia plays a part here. A good demonstration would provide an AC supply where you could vary the frequency. If force is time-varying, as you drop the period down to approaching ~ 0.2 seconds the ring should be seen to waver up and down.

If you have this as a lab demo, lightly touch the ring and you may feel it vibrating.

so are you suggesting the question is 'wrong'