Magnetism in Coils: Why PD Changes Direction?

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Discussion Overview

The discussion revolves around the phenomenon of induced electromotive force (emf) in a coil when a magnet passes through it, specifically addressing why the potential difference (p.d) appears to change direction as the magnet enters and leaves the coil. The scope includes theoretical aspects of electromagnetic induction and the application of Lenz's law.

Discussion Character

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

Main Points Raised

  • Some participants note that an emf is induced in the coil due to changes in magnetic flux when the magnet enters and leaves the coil.
  • One participant explains that the direction of the induced current is such that it creates a magnetic field opposing the change in flux, referencing Lenz's law.
  • Another participant questions why the poles of the coil appear to switch when the magnet leaves, suggesting that a North magnetic field is generated in the coil to attract the magnet's South pole.
  • There is a repeated inquiry about how the poles on the coil should be oriented to oppose the magnet's motion when it leaves the coil.
  • One participant expresses understanding after discussing the attraction of the coil's North pole to the magnet's South pole, indicating a shift in comprehension regarding the interaction.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and curiosity about the mechanisms at play, but there is no explicit consensus on the reasons behind the switching of poles or the implications of the induced emf direction.

Contextual Notes

Some assumptions regarding the behavior of magnetic fields and the specifics of electromagnetic induction are not fully explored, leaving room for further clarification and discussion.

Hannah7h
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upload_2016-6-1_20-41-58.png

This is a graph for a magnet falling through a coil, it shows the p.d induced in the coil (the induced emf) when the magnet enters and then shows the induced emf in the coil when the magnet leaves.
My question is why does the p.d appear to change direction? (i.e why the positive and negative values?)
 
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When the magnet is entering the coil, what is happening to the magnetic flux through the coil? What about when it is leaving the coil? What do you know about induction and its relation to the change in the magnetic flux?
 
Drakkith said:
When the magnet is entering the coil, what is happening to the magnetic flux through the coil? What about when it is leaving the coil? What do you know about induction and its relation to the change in the magnetic flux?

I know that an emf will be induced in the coil when there is a change in magentic flux, and that if the coil is connected in a closed circuit, a current will also be induced in the coil. I also know that the direction of this current is in such a direction to create a magnetic field which opposes the change that caused the induced current in the first place (Lenz's law). So I guess when the magnet enters the coil, there is a change in magnetic flux which induces an emf in the coil in one direction, and then when the magnet leaves the coil there is another change in flux and it induces an emf in the coil in the opposite direction, hence the positive and negative values? Is this the reason for the +ve and -ve values?

2) Also if the magnet entered the coil with its North pole, there would be a change in flux in the coil, an emf would be induced and then a current would be induced in the coil, which would generate a North magnetic field in the coil, to oppose the motion of the magnet. But I was wondering why, when the magnet leaves the coil, is another North magnetic field generated in the bottom of the coil, i.e. why do the poles on the coil switch? I have attached a picture below to try and show what I'm talking about
upload_2016-6-2_16-7-52.png
 
In the second case, the magnet leaves the coil. How would you have the poles on the coil in order to oppose the magnet leaving the coil? Would you have the coil attract or repel the magnet, in the third drawing?
 
nasu said:
In the second case, the magnet leaves the coil. How would you have the poles on the coil in order to oppose the magnet leaving the coil? Would you have the coil attract or repel the magnet, in the third drawing?
nasu said:
In the second case, the magnet leaves the coil. How would you have the poles on the coil in order to oppose the magnet leaving the coil? Would you have the coil attract or repel the magnet, in the third drawing?

You would have the coil attracting the magnet, therefore the the pole on the coil would be a north pole to attract the south pole of the magnet to oppose its motion. Ah yes I get it now, thank you
 

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