Why do eddy currents cause oscillations to die away quickly?

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Discussion Overview

The discussion revolves around the phenomenon of eddy currents in a metallic disc oscillating between magnetic poles, specifically addressing why these currents lead to a rapid decrease in oscillation amplitude. Participants explore different explanations for this behavior, including resistive forces and energy dissipation as heat.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant suggests that eddy currents oppose the magnetic field that induces them, creating a resistive force that slows down the oscillation of the disc.
  • Another participant references a textbook explanation stating that eddy currents cause a heating effect, which dissipates energy and contributes to the reduction in oscillation amplitude.
  • Some participants propose that both explanations are valid and can coexist, as they describe different aspects of the same phenomenon.
  • A participant questions whether the heating effect would occur regardless of the direction of the current, leading to a discussion about Lenz's law and the importance of current direction in determining the effects of eddy currents.
  • An analogy involving a bullet in water is introduced to illustrate that both the resistive force and energy dissipation perspectives can be correct simultaneously.

Areas of Agreement / Disagreement

Participants generally agree that both explanations regarding eddy currents are correct, but there is some debate about the implications of current direction and how it relates to the heating effect.

Contextual Notes

There are unresolved aspects regarding the relationship between the direction of eddy currents and their heating effects, as well as the interplay between resistive forces and energy dissipation.

Who May Find This Useful

This discussion may be of interest to students and educators in physics, particularly those exploring electromagnetic induction, energy dissipation, and oscillatory motion in conductive materials.

PFuser1232
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A metallic disc attached to a rod swings between two opposite magnetic poles, and its oscillation dies away very quickly. I thought this happens because the eddy currents formed inside the disc are established in a direction that would cause their magnetic field to oppose the magnetic field which induced them, causing a resistive force on the disc, just like air resistance but much stronger. And since the amplitude is a measure of the total mechanical energy of the disc, and work is done against the "resistive forces", the amplitude decreases. However, my A level book says otherwise. According to my book, eddy currents cause a heating effect which dissipates energy in the disc causing the oscillations to die away. Thoughts on this please?
 
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Both explanations are correct.
 
dauto said:
Both explanations are correct.

Could you please elaborate further?
 
Both things you said are correct. The induced currents produce a magnetic field that opposes the preexisting field forcing the pendulum to slowdown. The energy of the pendulum is dissipated which means it is transformed into heat. That heat is deposited in the metallic disk because the eddy currents have a heating effect. Both things are true. One explanation does not preclude the other explanation. They are both correct.
 
dauto said:
Both things you said are correct. The induced currents produce a magnetic field the oppose the preexisting field forcing the pendulum to slowdown. The energy of the pendulum is dissipated which means it is transformed into heat. That heat is deposited in the metallic disk because the eddy currents have a heating effect. Both things are true. One explanation does not preclude the other explanation. They are both correct.

But a heating effect would be caused by the current anyway, regardless of its direction, right?
 
MohammedRady97 said:
But a heating effect would be caused by the current anyway, regardless of its direction, right?

Yes, but only one direction works. Lenz law gives you the correct direction.
 
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May be an analogy will help. If you shoot a bullet into a water tank the water will slow down the bullet. You can say that the drag produces a force that slows down the bullet but you can also say that turbulent motion converts the energy of the bullet into heat. Both the force and the energy points of view are correct.
 

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