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

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The discussion centers on why the magnetic flux through a closed surface is always zero, referencing Maxwell's equations which state there are no magnetic monopoles, only dipoles. Participants explain that magnetic field lines form closed loops, meaning any flux entering a surface must also exit, resulting in zero net flux. Comparisons are made to Gauss's law for electric fields, which can yield non-zero flux when enclosing a net charge. The conversation also touches on the Faraday cage, clarifying that it relies on electric charges realigning to cancel external fields, unlike the magnetic flux scenario. Overall, the net magnetic flux through any closed surface remains zero due to the nature of magnetic field lines.
  • #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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