# Solenoid's effect on a nearby iron ring

1. Sep 6, 2011

### serverxeon

1. The problem statement, all variables and given/known data
You have an iron ring, hung near a solenoid which has an iron core inserted within.
[PLAIN]http://img843.imageshack.us/img843/1623/imgsole.jpg [Broken]

Below are the questions and answers my book provided. I highly suspect the reasoning and i seek PF members to evaluate how true it is.
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a) What happens when the switch is closed?
Ans: There will be a change in the magnetic flux going through the ring, and according to lens law, a current will be induced such that the resulting magnetic field opposes the change.
Hence, the ring will swing away momentarily.

b) What happens when the potential divider is brought to X quickly?
Ans: There will be a change in the magnetic flux going through the ring, hence, the ring will swing towards the solenoid momentarily

c) What happens when the potential divider remains at X?
Ans: The ring will remain at rest (ie. at equilibrium position)

d) What happens if the potential divider is brought between X and Y at 2Hz?
Ans: The ring will oscillate back and forth at 2Hz

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My intuition tells me that the ring should be attracted to the solenoid, just like a normal metal to a bar magnet, regardless of change in flux, or polarity.

Last edited by a moderator: May 5, 2017
2. Sep 7, 2011

### Spinnor

There are two effects going on here and I think you are right that the iron ring will be attracted in parts a, b, c, and d. Should have been a copper ring to avoid confusion?

3. Sep 8, 2011

### serverxeon

i have come up with the following. Can anyone evaluate how correct i am? Correct me if i am wrong please.

When the switch is on, the split second duration for which the magnetic flux is changing within the ring, will cause an induced current, and a Bfield in the ring to oppose the change. HENCE, the ring will not move throughout this 'split second' as the Bfield cancels out.
After the magnetic flux becomes steady, the iron ring, like all ferramagnetic material, gets attracted to the stronger side of the Bfield, which is towards the solenoid.
The iron ring will remain attracted to the solenoid despite other changes in resistance.