Permanent magnet thought experiment, help with hypothesis.

[CasualKilla]

If we placed some conducting coils inside a magnetic field created by 2 permanent magnets, Ampere's law tells us that the current in those coil will be proportional to the flux flowing through the coils.

 

This is fine if the coil is a super conductor, but as soon as you add a resistor to the coils, the current will create a potential across the resistor and thus the coils. Faraday's law tells us this will produce a changing flux, in this case the flux in the core will reduce.

 

This seems to lead to an odd conclusion, either the permanent magnet strength must decrease to reduce the flux or the core must become extremely resistant to flux at steady state. My money is on case 2, but it still seems very weird since reducing the magnetic field to 0 would create a magnetic Faraday cage, or what I shall name CasualKilla Cage .

 

 

 

[studiot]

Have you watched Prof Lewin's lecture on Faraday and this stuff?

 

I have linked to google becase there are several presentations of this famous lecture.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&q=Lewin+on+Faraday&gbv=2&oq=Lewin+on+Faraday&gs_l=heirloom-hp.3...1687.7297.0.7906.16.8.0.8.8.0.219.1422.0j5j3.8.0.msedr...0...1ac.1.34.heirloom-hp..7.9.1468.WSnY9qLg1T4

[CasualKilla]

Have you watched Prof Lewin's lecture on Faraday and this stuff?

 

I have linked to google becase there are several presentations of this famous lecture.

 

https://www.google.co.uk/search?hl=en-GB&source=hp&q=Lewin+on+Faraday&gbv=2&oq=Lewin+on+Faraday&gs_l=heirloom-hp.3...1687.7297.0.7906.16.8.0.8.8.0.219.1422.0j5j3.8.0.msedr...0...1ac.1.34.heirloom-hp..7.9.1468.WSnY9qLg1T4

Gonna start with "M.I.T.-Walter Lewin- Complete Breakdown of Intuition - Part1" since that how I am have been feeling after deciding to study electromagnetism. lol

[swansont]

If we placed some conducting coils inside a magnetic field created by 2 permanent magnets, Ampere's law tells us that the current in those coil will be proportional to the flux flowing through the coils.

 

 

No, Ampere's law tells you the field created by a current. To create a current from a field, the field must be changing, and is given by Faraday's law.

[Enthalpy]

Nearly paraphrasing Swansont...

 

The current drops due to the wires' resistance, yes. The flux that you deduce from the current is only the contribution by this current. The flux created by the permanent magnets stays. And since the coil's behaviour is to oppose the flux variations:

• It will first prevent the flux by the permanent magnets enter the coil (if the magnets approach so quickly that the coil achieve it, which is difficult)
• As the induced current vanishes, only the flux by the permanent magnets stays.

A superconducting coil would prevent indefinitely the flux entering... if the induction is small enough for the material. This is used by some gravity gradiometers of astounding sensitivity.