How does a magnetic force affect the movement of charges and magnets?

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Discussion Overview

The discussion centers on the effects of magnetic force on the movement of charges and magnets, exploring concepts related to the Lorentz force, work done by magnetic fields, and the behavior of current loops in magnetic fields. The scope includes theoretical considerations and conceptual clarifications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how a current loop can rotate in a magnetic field if magnetic force does not do work on moving charges.
  • Another participant asserts that the magnetic field does not do work on freely moving charges, as the Lorentz force is always perpendicular to their motion, but acknowledges that in constrained systems like current loops, work can be done.
  • A similar viewpoint is reiterated, emphasizing that the Lorentz force's perpendicular nature leads to the conclusion that magnetic force does not do work on freely moving charges.
  • One participant requests a more detailed explanation rather than references to textbooks, indicating a desire for educational discourse.
  • Another participant agrees with the previous points and references a specific example from a textbook to support their argument.

Areas of Agreement / Disagreement

Participants generally agree on the principle that magnetic fields do not do work on freely moving charges, but there is a lack of consensus on the implications of this principle in the context of current loops and magnets, leading to ongoing debate.

Contextual Notes

Some assumptions about the constraints on charge movement and the conditions under which work is done by magnetic forces remain unresolved. The discussion also references specific educational materials without providing detailed explanations.

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If magnetic force does not exert work on moving charges, how could a current loop immersed in a magnetic field rotate and how could a magnet attract or repel another one?
 
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I think the key aspect here is that the magnetic field doesn't do work on *freely moving* charges. That is, at any given point in time, the force exerted on the charge is perfectly perpendicular to the motion (courtesy of the Lorentz force).
In a current loop, or in a magnet, the charges are constrained in their movement. That means, the Lorentz force and the charge's motion is no longer perfectly perpendicular, with the result of work being done.

Mind you, I'm only 95% sure on this. But it seems in line with this site's explanation:

http://van.physics.illinois.edu/QA/listing.php?id=17176
 
rumborak said:
I think the key aspect here is that the magnetic field doesn't do work on *freely moving* charges. That is, at any given point in time, the force exerted on the charge is perfectly perpendicular to the motion (courtesy of the Lorentz force).
In a current loop, or in a magnet, the charges are constrained in their movement. That means, the Lorentz force and the charge's motion is no longer perfectly perpendicular, with the result of work being done.

Mind you, I'm only 95% sure on this. But it seems in line with this site's explanation:

http://van.physics.illinois.edu/QA/listing.php?id=17176
The Lorentz force is always perpendicular to the motion of the charges. No escape from that. This particular problem where it looks like the magnetic force has done the work, is explained beautifully and in detail in Griffiths, "Introduction to Electrodynamics". As an analogy, Griffiths also points out a similar problem in elementary mechanics.
 
Well ... care to explain it in this thread? Just pointing to some textbook isn't really educational.
 
rumborak said:
Well ... care to explain it in this thread? Just pointing to some textbook isn't really educational.
@Chandra Prayaga is referring to this, which pretty much agrees with what you said in #2.

Screenshot_20170213-230126.png

Screenshot_20170213-230454.png

Screenshot_20170213-230255.png
 
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Thanks. Example 5.3 from Griffiths is indeed what I was referring to.
 

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