Why a magnetic flux in closed surface area is always 0?

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Discussion Overview

The discussion revolves around the concept of magnetic flux through a closed surface, exploring why it is always considered to be zero. Participants examine this topic through various lenses, including theoretical implications, comparisons to electric fields, and analogies with phenomena like Faraday's cage.

Discussion Character

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

Main Points Raised

  • Some participants apply Lenz's law to argue that charges on a spherical hollow surface move to oppose the magnetic field, resulting in cancellation of flux.
  • There is a comparison made between magnetic flux and gravitational effects, though the relevance is not fully explored.
  • One participant mentions Gauss' Law for Magnetism, stating that it implies the absence of magnetic monopoles, which leads to zero net flux through a closed surface.
  • Another participant questions the behavior of magnetic flux in stationary versus non-stationary fields, seeking clarification on whether the net flux remains zero in both cases.
  • Some participants discuss the nature of magnetic flux lines, suggesting that they form closed loops, which contributes to the net flux being zero.
  • There is a distinction made between magnetic flux and electric flux, particularly in the context of Faraday's cage, where electric charges realign to cancel external fields.
  • One participant expresses confusion about how magnetic flux can be zero when a magnetic field exists, leading to further clarification that it is the net flux that is zero, not the magnetic field itself.
  • Another participant raises a question about the behavior of magnetic fields in relation to permanent magnets and their interaction with conducting loops.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement. While some concepts, such as the application of Gauss' Law for Magnetism, are accepted, there are varying interpretations and questions regarding the implications of magnetic flux in different contexts, particularly concerning stationary versus non-stationary fields and the analogy with electric fields.

Contextual Notes

Participants express uncertainty about the implications of magnetic flux in non-stationary fields and the relationship between magnetic flux and electric fields, particularly in the context of Faraday's cage. There are also unresolved questions regarding the behavior of magnetic fields in relation to permanent magnets.

  • #31
Physicsissuef said:
Btw- Here says that B is external magnetic field, thus delta B is change of the external magnetic field. What is http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/fday.gif"
Yes, B is the external magnetic field. That page doesn't talk about delta B, since the field isn't changing. The loop moves in this case, so the flux through the loop changes. But it's the same idea as in all the other pages on this site: What matters is how the flux changes due to the external field. That determines the induced EMF and current in the loop.
 
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  • #32
Physicsissuef said:
So delta B in practical way, doesn't exists, right?
"Delta" just means change. It's just a way of describing how the field in the loop is changing. It's not a separate field.
 
  • #33
Doc Al said:
"Delta" just means change. It's just a way of describing how the field in the loop is changing. It's not a separate field.

And in the http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/fday.gif" , there is delta B * delta A. In this case, delta B is the change of the field of the loop?
 
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  • #34
Physicsissuef said:
And in the http://hyperphysics.phy-astr.gsu.edu/hbase/electric/imgele/fday.gif" , there is delta B * delta A. In this case, delta B is the change of the field of the loop?
There's no delta B mentioned on this link. In http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/farlaw.html" there appears delta (B*A), which is the change in magnetic flux. In your link, only A changes so the change in flux = delta (B*A) = B*delta(A).
 
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  • #35
Doc Al said:
There's no delta B mentioned on this link. In http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/farlaw.html" there appears delta (B*A), which is the change in magnetic flux. In your link, only A changes so the change in flux = delta (B*A) = B*delta(A).
And can I ask you why on the 1-st example there is so much bigger voltage (-16 volts), and in the example below (-0,004 volts)? What is the difference? In the first example there are two coils (it is actually transformator).
 
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  • #36
Physicsissuef said:
And can I ask you why on the 1-st example there is so much bigger voltage (-16 volts), and in the example below (-0,004 volts)? What is the difference? In the first example there are two coils (it is actually transformator).
The first example is not a transformer, it's just two coils. The only difference between them is the number of turns.

The two examples are different! The induced EMF depends on the rate at which the flux changes, which is different in each case. All the values needed to calculate the induced EMF (and the formula to use) are given. Just plug in the numbers.
 
  • #37
Doc Al said:
The first example is not a transformer, it's just two coils. The only difference between them is the number of turns.

The two examples are different! The induced EMF depends on the rate at which the flux changes, which is different in each case. All the values needed to calculate the induced EMF (and the formula to use) are given. Just plug in the numbers.
In the first case, the flux is changing faster, than the example below?
 
  • #38
Physicsissuef said:
In the first case, the flux is changing faster, than the example below?
Yes. The rate at which the total flux is changing is greater in the first examples.
 
  • #39
Doc Al said:
Yes. The rate at which the total flux is changing is greater in the first examples.

Ok, thank you very much.
 

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