Lenz's law and instantaneous change in inductor's current

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Discussion Overview

The discussion revolves around Lenz's law and the behavior of induced current in a coil when a magnet approaches and recedes from it. Participants explore the implications of instantaneous changes in current within inductors and the relationship between the motion of the magnet and the induced current.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions the interpretation of Lenz's law, suggesting that the text implies an instantaneous change in current direction, which conflicts with the understanding that current in an inductor cannot change instantaneously.
  • Another participant clarifies that changes do not occur instantaneously, explaining that as the magnet approaches and then recedes, its velocity must change continuously, leading to a gradual change in the induced current.
  • A third participant introduces the idea of a 180 degrees phase difference between the movement of the magnet and the induced current, assuming a specific pattern of motion for the magnet.
  • A later reply confirms the understanding of the current's reaction based on the additional assumption regarding the magnet's movement.

Areas of Agreement / Disagreement

Participants generally agree that the changes in current are not instantaneous, but there is a discussion about the specific assumptions regarding the magnet's motion and its effects on the induced current.

Contextual Notes

The discussion includes assumptions about the nature of the magnet's movement, such as the suggestion of simple harmonic motion, which may not be universally accepted or applicable in all scenarios.

PainterGuy
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Hi,

I will use this text for my question: https://imagizer.imageshack.com/img921/1874/nUBPY6.jpg
Source: https://www.electrical4u.com/lenz-law-of-electromagnetic-induction/

The text says that when the north pole of a magnet approaches the coil, the induced current in coil is such that it creates a north pole to repel the approaching north pole of magnet. In other words, the coil resists the approach of magnet.

When the magnet's north pole starts receding from the coil, the induced current in coil is such that it creates a south pole to attract the receding north pole of magnet toward it. In other words, the coil resists the recession of magnet's north pole.

The following is my question statement. In an inductor or coil the current cannot change instantaneously. Therefore, as the magnet was approaching coil, it induced a current in the coil and this induced current generated its own magnetic field. So, when the magnetic starts receding away from coil, the current shouldn't switch its direction instantaneously. But the text suggests, or Lenz's law, as if the switch in direction of current is instantaneous. Where am I confusing it?

Thank you for your help!
 
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The linked text does not say things change instantaneously, and they don't.
Consider the magnet approaching the coil. For it to subsequently recede from the coil, its velocity towards the coil must continuously decrease from positive, through zero and become negative. As that happens, the current through the coil will likewise reduce to zero then grow in the opposite direction.

Hope that helps.
 
Last edited:
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Thank you, @andrewkirk .

I think that I understand it now. There is a 180 degrees phase difference between the magnet movement and induced current. I have assumed that the magnet follows simple harmonic motion and the system has been operating for some time. Please let me know if I have it correct. Thank you.
 

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Yes, given the additional assumption you state regarding the magnet's pattern of movement, that diagram agrees with my understanding of how the current reacts.
 
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