What Causes Rotation in a Non-Rotating Magnet?

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Discussion Overview

The discussion revolves around the mechanisms of electromagnetic induction in relation to the motion of magnets, specifically addressing the conditions under which an electromotive force (emf) is produced. Participants explore concepts related to both rotational and translational motion of magnets and their effects on induced emf, with references to specific devices like the DePalma N Machine.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Mike questions whether an emf is produced when a permanent magnet moves perpendicularly to its magnetic field lines.
  • Another participant suggests that if the magnet is moving in an external field, an emf is induced, but not if it is only in its own field, highlighting a distinction in classical electromagnetism.
  • Mike introduces the concept of a homopolar generator, noting that an emf develops in a rotating electrically conductive disk magnet, which he relates to his question about linear motion.
  • One participant emphasizes the difference between translational and rotational motion, stating that a translating object does not create an emf in its own frame of reference.
  • There is a discussion about the operation of the N machine as a motor, with crisbaird expressing confusion about the initial conditions of motion and the resulting emf.
  • Another participant raises a question about the forces acting on conduction electrons in the disk magnet and how this relates to the absence of rotation at the start, seeking clarification on the differences between linear and rotational cases.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for emf production, particularly regarding the roles of rotational versus translational motion. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about frames of reference and the definitions of motion types. The implications of velocity approaching the speed of light are also mentioned but not explored in depth.

MS La Moreaux
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If a permanent magnet is moved in a direction at right angles to the magnet field lines in the magnet, is an emf produced across the magnet?

Mike
 
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If you mean it is moving in an external field, yes it is induced, just like in a piece of Iron. But if you mean there is no field except for the magnets own field, then in classical electromagnetic no emf is induced because the relative motion of the magnet and its velocity is zero. I don't know what happens whenthe velocity approaches the speed of light.
 
Hassan2,

It seems to be the case that a magnet's field is not attached to the magnet. If an electrically conductive disk magnet (with the faces the poles) rotates like a wheel, an emf is developed from the center to the rim. This is a type of homopolar generator known as a DePalma N Machine. My question relates to the linear version of it.

Mike
 
If you think the problem is interesting, can you please make a simple drawing of it?
 
Rotational motion and translation motion are different things. If you translate an object (move it in a straight line at a constant speed) then the object it still in an inertial frame and therefore acts (to itself) as if it were at rest. A translating object does not create an EMF in its frame of reference because it is not moving in its frame a reference. A rotating frame, on the other hand, is not inertial and so you cannot put yourself in a rotating object's rest frame and apply the standard equations. Homopolar generators work because the motion is rotational.
 
Hassan2,

Sorry, but I do not know how to add a drawing.
 
crisbaird,

I am having trouble understanding this. If the N machine is operated as a motor, it will start from a stationary position. Therefore, at the instant of starting, there is no rotary motion and everything is in the same frame of reference.
 
MS La Moreaux said:
crisbaird,

I am having trouble understanding this. If the N machine is operated as a motor, it will start from a stationary position. Therefore, at the instant of starting, there is no rotary motion and everything is in the same frame of reference.

And that is why at the instant of starting, there is no EMF
 
Yes, but what causes the disk to rotate? It would seem that the conduction electrons in the disk have a force exerted on them by the magnetic field of the disk even though there is no rotation at the start. How does this differ from the linear version of the motor? Why would there not be movement of the magnet in the linear case?
 

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