A question about Faraday's law

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    Faraday's law Law
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Discussion Overview

The discussion revolves around the application of Faraday's law in a specific system involving a ring and a coil. Participants explore the nature of the forces acting on the ring due to changing magnetic fields and induced currents, focusing on the direction and origin of these forces.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions how an upward force on the ring arises when the induced current flows clockwise due to an increasing magnetic field, suggesting that the force should be sideways according to the Lorentz force equation.
  • Another participant proposes that the magnetic field has a component parallel to the ring, which could explain the force direction.
  • A subsequent reply corrects this by stating that the magnetic field has components both radial and axial in the region of the ring.
  • Further, a participant discusses the implications of an infinitely long core, suggesting that the Lorentz force on charges in the ring is directed inward, questioning whether this results in repulsion of the ring.
  • This participant also notes that the radial component of the magnetic field at the end of a solenoid diverges outward, which may contribute to the ring being repelled.
  • They conclude by emphasizing that this phenomenon occurs only with an alternating current in the coil, linking the changing flux to the induced electric field and current in the loop.

Areas of Agreement / Disagreement

Participants express differing views on the direction and nature of the forces acting on the ring, indicating that the discussion remains unresolved with multiple competing interpretations of the magnetic field's effects.

Contextual Notes

There are unresolved aspects regarding the assumptions about the magnetic field configuration and its components, as well as the specific conditions under which the forces are analyzed.

LiorE
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My question is about the system in the picture. There is an upwards force which acts on the ring. My problem is that I don't see how it comes about. I mean, suppose that at time t, the current through the coil flows counterclockwise and is increasing. So there is an increasing, upward pointing magnetic field through the ring. That should induce a current on it in a clockwise direction. But then, according to the Lorentz force equation, the force on the ring should be sideways, isn't it? I mean it doesn't make sense that the force will be in the same direction as the magnetic field anyway, because \vec F = q(\vec{v} \times \vec{B}), so it has to be perpendicular to both v and B.

So how is that force created?

Thanks in advance,

Lior
 

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This should give you an idea:
http://img253.imageshack.us/img253/3589/76749370cc6.png

(sorry for the ugly sketch, any reference to a sketch making program would be appreciated =x )
 
Last edited by a moderator:
Oh, I get it - because B has a component parallel to the ring. Thanks!
 
k3d
"google sketchup"
 
LiorE said:
Oh, I get it - because B has a component parallel to the ring. Thanks!

No... B has a components perpendicular to the ring. In the region of the ring, B has both radial and axial components.
 
This is actually a very interesting question, guys.

With an infinitely long core, the B field will be purely axial, following along the core. the Lorenz force on the charges moving in the ring are anti-radial--directed inward. So is the ring repelled in this case?? I don't think so. There's no preferred axial direction. But at the end of the core, or the end of a solenoid, the field diverges outward, radially. This would seem to be the cause for the ring being repelled.

Remember, this only works for an AC current in the coil. From what I can deduce, the changing flux passing through the ring induces a circumpherential electric field inducing a current in the loop. But the radial component of the magnetic field in the region of the ring acts under the Lorentz force,

F_z = q v_phi B_r

to push the ring away.

Any Yays, or Nays, your just another lunatic on the internet?
 
Last edited:

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