Electromagnetic induction question, coils and magnets

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Discussion Overview

The discussion revolves around the phenomenon of electromagnetic induction, specifically regarding the behavior of a coil in a closed circuit when a magnet is moved inside it and subsequently remains stationary. Participants explore whether an electromotive force (emf) is induced in the coil under these conditions, considering concepts such as magnetic flux linkage and self-induction.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that when a magnet is moved into a coil and then remains stationary, there is no induced emf because the magnetic field through the coil's windings is not changing.
  • Others argue that while the magnet is moving into the coil, there is a voltage induced, but once it stops, the flux is no longer changing, resulting in zero voltage.
  • A later reply suggests that even after the magnet stops, the inductance of the coil means that current does not drop to zero instantaneously, implying a delay in the cessation of current flow.
  • Some participants express uncertainty about the initial conditions, questioning whether the magnet and coil are moving relative to each other or if the magnet is simply moved into the coil.
  • There is a mention of self-induction in the context of the coil producing voltage after the magnet stops moving, though this is contested by another participant who states that no emf exists once the magnet is stationary.

Areas of Agreement / Disagreement

Participants generally agree that there is no induced emf when the magnet is stationary inside the coil. However, there is disagreement regarding the behavior of current and voltage after the magnet stops moving, with competing views on the effects of self-induction and the time it takes for current to cease.

Contextual Notes

The discussion reflects varying interpretations of electromagnetic induction principles, particularly concerning the conditions under which emf is induced and the role of inductance in current behavior. Some assumptions about the movement of the magnet and coil remain unresolved.

Hannah7h
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Say you have a coil connected in a closed circuit. You then move a magnet inside of the coil and it remains stationary inside of the coil. Is the magnet, even though it is stationary, still inducing an emf in the coil or is it not? I'm guessing it doesn't induce an emf in the coil because there is no change in flux linkage, but I'm not too sure.

Thank you for any help
 
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Hannah7h said:
You then move a magnet inside of the coil and it remains stationary inside of the coil.
I am not sure what you are asking. How can the magnet both move and remain stationary?

Do you mean that both the magnet and the coil are moving at the same velocity so that they have no relative motion? Or do you mean that the magnet remains at rest and the coil moves relative to the magnet?
 
Dale said:
I am not sure what you are asking. How can the magnet both move and remain stationary?

Do you mean that both the magnet and the coil are moving at the same velocity so that they have no relative motion? Or do you mean that the magnet remains at rest and the coil moves relative to the magnet?

Oh sorry! I meant you move the magnet INTO the coil and then it remains stationary inside of the coil
 
Hannah7h said:
Is the magnet, even though it is stationary, still inducing an emf in the coil or is it not? I'm guessing it doesn't induce an emf in the coil because there is no change in flux linkage, but I'm not too sure.

That's correct. Since the magnetic field through the coil's windings isn't changing, there is no induced emf in the coil.
 
Drakkith said:
That's correct. Since the magnetic field through the coil's windings isn't changing, there is no induced emf in the coil.

Ok cool, thank you
 
Hannah7h said:
Oh sorry! I meant you move the magnet INTO the coil and then it remains stationary inside of the coil
Ah, ok. So there will be a voltage while it is moving into the coil, but once it stops the flux is no longer changing in time and so the voltage will be 0.
 
Drakkith said:
That's correct. Since the magnetic field through the coil's windings isn't changing, there is no induced emf in the coil.
Dale said:
Ah, ok. So there will be a voltage while it is moving into the coil, but once it stops the flux is no longer changing in time and so the voltage will be 0.

Yep this is what i thought, thank you very much
 
If the coil is in a closed circuit, then when the magnet stops moving, the current will die off, and the coil will produce voltage via self induction.
 
No, once the magnet stops and the current has died away, there is no longer any EMF.
 
  • #10
True, but it takes some amount of time (after the magnet stops moving) for current to go to zero. The inductance of the coil prevents current from dropping to zero instantaneously.
 

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