How does a moving magnet induce a current in a coil?

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Discussion Overview

The discussion revolves around the phenomenon of electromagnetic induction, specifically how a moving magnet induces a current in a coil. Participants explore the underlying principles, including the effects described by Faraday-Maxwell's law, and consider different frames of reference to understand the process better.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about the mechanism of induced current when a magnet moves through a coil, seeking clarification on the behavior of electrons.
  • Another participant references Faraday-Maxwell's law, explaining that an electromotive force (EMF) can be induced by an unsteady magnetic field or the movement of a conductor within a magnetic field.
  • A different perspective suggests considering the situation from a frame of reference moving with the magnet, where the magnetic field appears static, and the charges in the coil experience a force due to their motion through this field.
  • Participants acknowledge a linguistic mix-up regarding the terms FEM (Fuerza ElectroMagnética) and EMF (Electromagnetic Force), with one participant clarifying that they understood the intended meaning.
  • The original poster expresses satisfaction with the explanations provided but raises a follow-up question regarding the physical significance of the gradient and area under a voltage versus time graph.

Areas of Agreement / Disagreement

Participants generally agree on the principles of electromagnetic induction discussed, but there are varying interpretations regarding the frames of reference and the implications of the induced EMF. The follow-up question about the voltage graph introduces a new topic that remains unresolved.

Contextual Notes

Some assumptions about the nature of the magnetic field and the specific conditions under which the induction occurs are not fully explored. The discussion does not resolve the complexities of the induced EMF in different reference frames.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics seeking to understand electromagnetic induction and the principles behind it, as well as those interested in the nuances of scientific terminology across languages.

ylem
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Hello!

I've just realized, when trying to do a write up for a Physics experiment (about a coil and a magnet) that I don't know how you get an induced current!

What's actually going on when the magnet moves through the coil? What are the electrons doing to induce a current?

I'm totally confused and my Physics textbook is not helping at all!

Please help!
 
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Take a look at the Faraday-Maxwell's law:

[tex]\epsilon=\oint\overline{E}\cdot\overline{dl}=-\int\int\frac{\partial \overline{B}}{\partial t}\cdot \overline{dS}+\oint(\overline{v}\times\overline{B})\cdot \overline{dl}[/tex]

A FEM can be induced by two effects:

-An unsteady magnetic field inside a steady electric closed circuit.
-The movement of a conductor inside a magnetic field.

In your example, you are causing a [tex]\frac{\partial \overline{B}}{\partial t}[/tex] moving the magnet inside a cylindrical-shaped coil of section S. The FEM is induced internally in the conductor displacing the electrons towards one of the extremes, and positive charges to the other. The vehicle of transmision is a bit heuristic for me, so that maybe a physicist could help you very much.
 
Last edited:
It might help to think about how things look in a frame of reference moving with the magnet. There the magnetic field is static and the charges in the coil are moving through that static field. They will be subjected to the [itex]\vec v \times \vec B[/itex] force and respond to it. In the coil frame, that response is said to be due to the induced EMF.
 
A nice change of reference frame, Tide. Sorry for writting upside down the letters FEM and EMF.

FEM=Fuerza ElectroMagnética (spanish)
EMF=ElectroMagnetic Force (english)

It's an usual error in my spelling to think of my native language... :frown:
 
Clausius,

Gracias!

I knew what you meant by FEM and I was by no means attempting to correct you! Everyone working in the sciences is aware of the linguistic differences used in writing abbreviations.
 
Thanks everyone! It all makes sense now!

I have another question though! The graph of voltage against time: what physical significance is the gradient and the area under the curve?
 

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