Capacitor has no magnetic field, Why?

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Discussion Overview

The discussion revolves around the presence or absence of magnetic fields in and around a capacitor, particularly focusing on the behavior of electrons when the capacitor is charged. Participants explore concepts related to magnetic fields generated by moving electrons, the intrinsic magnetic properties of electrons, and the implications of electron spin and orientation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants assert that magnetic fields are generated by moving electrons, suggesting that there should be a magnetic field between the plates of a charging capacitor due to displacement current.
  • One participant questions the existence of a net current on a fully charged capacitor, implying that this may relate to the absence of a magnetic field.
  • Another participant emphasizes that while electrons possess a magnetic moment, they do not generate a magnetic field when stationary, as the magnetic fields from non-moving electrons do not contribute.
  • It is noted that although electrons have an intrinsic magnetic dipole moment due to their spin, the random orientation of these spins in a charge distribution results in a net magnetic field of zero.
  • Participants discuss the concept of transient magnetic fields due to thermal and quantum fluctuations, which are considered too small to have significant practical effects.
  • One participant mentions that fluctuations in electron velocity and spin orientation contribute to noise in electronic circuits, including shot noise and thermal noise.

Areas of Agreement / Disagreement

Participants express differing views on the nature of magnetic fields in capacitors, particularly regarding the role of stationary electrons and their magnetic properties. There is no consensus on the implications of these properties for the presence of magnetic fields in a fully charged capacitor.

Contextual Notes

The discussion highlights the complexities of magnetic field generation in relation to electron motion and orientation, and the assumptions about charge distributions and their effects on magnetic fields remain unresolved.

Ruptor
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We know that magnetic fields are generated by moving electrons so there is a magnetic field between the plates of a capacitor due to the displacement current when it charges up. I don't understand why there is no magnetic field on the plate of a capacitor when it is fully charged because electrons are supposed to have a magnetic moment or field of their own and I would have thought that the increased density of electrons on the capacitor plate would exhibit a magnetic field.
 
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where is there a net current on a charged capacitor?
 
Ruptor said:
We know that magnetic fields are generated by moving electrons so there is a magnetic field between the plates of a capacitor due to the displacement current when it charges up. I don't understand why there is no magnetic field on the plate of a capacitor when it is fully charged because electrons are supposed to have a magnetic moment or field of their own
Where did you get that idea? You just said that magnetic fields are generated by moving electrons. Electrons that are not moving do not have a magnetic field.

and I would have thought that the increased density of electrons on the capacitor plate would exhibit a magnetic field.
 
The electron has a magnetic field as well as angular momentum so they say here
http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html
So even though electrons don't move through space they have a field like a mini magnet so why doesn't a cloud of electrons give a magnetic field? Perhaps because they are inhomogenous?
 
Yes electrons do have an intrinsic magnetic dipole moment, because they have spin 1/2. This is the whole premise behind the Stern-Gerlach experiment, and one of the fundamental building blocks of quantum mechanics.

The reason a charge distribution (whether electrons on a capacitor plate, or along a wire or whatever) doesn't have an associated magnetic field is because the electron spins are randomly orientated, and the contribution of their magnetic fields sum to zero.
 
Ruptor said:
We know that magnetic fields are generated by moving electrons so there is a magnetic field between the plates of a capacitor due to the displacement current when it charges up. I don't understand why there is no magnetic field on the plate of a capacitor when it is fully charged because electrons are supposed to have a magnetic moment or field of their own and I would have thought that the increased density of electrons on the capacitor plate would exhibit a magnetic field.

There is no time averaged magnetic field on a fully charged capacitor. The direction of the magnetic moments of electrons is random. The magnetic fields caused by "individual electrons" add up linearly at every point within the capacitor. The total magnetic field at anyone point adds up to zero vector, or to a fluctuating magnetic field very close to a zero vector.

There are transient magnetic fields in a capacitor on an atomic time scale caused by thermal fluctuations and quantum fluctuations. However, they are generally too small to be significant.

In terms of practical electronics, one can say that part of the "noise" on a circuit is due to random fluctuations in the velocity and spin orientation of each electron. Shot noise is caused by electric currents caused by "quantum fluctuations" in individual electrons. Thermal noise is also caused by the "classical fluctuations" in individual electrons. These sources of noise are have to be modeled using statistics.
 

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