Oscillation related to electromagnetic induction

[songoku]

Homework Statement

A copper ring is suspended by a long, light rod pivoted at X so that it may swing as pendulum, as shown in the diagram below. An electromagnet is mounted so that the ring passes over it as it swings. The ring is set into oscillation with switch K open. What happens to the motion after switch K has been closed?

a. the periodic time will decrease
b. the oscillation will be heavily damped
c. the amplitude will increase because the ring is accelerated towards the magnet
d. the oscillation will continue at constant amplitude while the battery can supply energy
e. the ring will attracted to the magnet and stop moving

Homework Equations

not sure

The Attempt at a Solution

I think the answer will be (b). The motion will be damped because there will be induced current in the ring so some of the kinetic energy of the motion will be converted to thermal energy causing the movement of the ring becomes slower.

I am not sure why (a) is wrong. The periodic time will be constant? I don't know why

And by "heavily damped" does it mean the ring won't oscillate and takes a long time to go back to vertical position (equilibrium position)? If yes, so option (b) is also wrong?

Thanks

 

[mfb]

The amount of damping will depend on details of the setup.

Technically the damping can also influence the period but that is a small effect.

 

[Delta2]

Just to say c and d and e are wrong by Lenz's law the induced current will be such as to oppose its cause which is the movement of the ring. The amplitude is definitely going to decrease, not increase or stay constant. The only way to increase or remain constant is to offer directly energy to the ring but this cannot be done by the battery of the circuit.

 

[songoku]

The amount of damping will depend on details of the setup.

Technically the damping can also influence the period but that is a small effect.

Just to say c and d and e are wrong by Lenz's law the induced current will be such as to oppose its cause which is the movement of the ring. The amplitude is definitely going to decrease, not increase or stay constant. The only way to increase or remain constant is to offer directly energy to the ring but this cannot be done by the battery of the circuit.

So basically we assume that the effect of the damping is negligible and the period stays the same? It means that for this question we only choose the best answer possible, not 100% correct answer?

Thanks

 

[mfb]

Damping should be the dominant effect for every realistic setup.

 

[songoku]

Damping should be the dominant effect for every realistic setup.

Ah sorry I don't mean to say the effect of damping is negligible. What I mean is the effect of damping is significant but it is not enough to alter the period.

But I learn there are 3 types of damping: light damping, critical damping, heavy damping.
For light damping, the change to the period is small and for critical and heavy damping the change is significant.

So if option (b) is true then it means the period will also change but instead of decreasing, it should be increasing because the movement of the copper ring will be slower and it takes more time to complete one oscillation?

Thanks

 

[mfb]

With heavy damping you don't even have a proper oscillation. And the period would increase with all types of damping, so (a) is wrong anyway.

 

[songoku]

Thank you very much for your help mfb and delta