Can magnetic field exist without electric field?

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Discussion Overview

The discussion revolves around the question of whether a magnetic field can exist independently of an electric field. Participants explore various scenarios and theoretical frameworks related to electromagnetism, including static and dynamic conditions, the behavior of charged particles, and the implications of Maxwell's equations.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants assert that magnetic fields are typically generated by moving charges or time-varying electric fields, suggesting that a magnetic field cannot exist without some form of electric field.
  • Others argue that under static conditions, a magnetic field can exist without an electric field, citing examples such as a superconducting loop with a steady current or a charged capacitor in open circuit.
  • A participant mentions that while electrons create electric fields, the electric field may not extend outside of a permanent magnet, leading to the idea that a permanent magnet may only exhibit a magnetic field.
  • Some contributions highlight that neutrons possess a magnetic moment despite having no electric charge, raising questions about the relationship between electric and magnetic fields.
  • Another viewpoint suggests that there are reference frames where the electric field can be zero while the magnetic field is non-zero, indicating a more complex interaction between the two fields.
  • Participants discuss the implications of idealized conditions in physics, noting that while perfect scenarios may not exist, they can provide useful approximations for understanding electromagnetic phenomena.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether magnetic fields can exist without electric fields. Multiple competing views are presented, with some emphasizing static conditions and others focusing on dynamic interactions.

Contextual Notes

Limitations include the dependence on specific conditions (static vs. dynamic) and the assumptions about idealized systems versus real-world scenarios. The discussion reflects a variety of interpretations of electromagnetic theory without resolving the underlying complexities.

  • #31
Classically you can certainly have an electric field without a magnetic field or a magnetic field without an electric field. On the quantum level, since all fundamental charged particles also have spin you cannot have either without the other.

In either case the answer to the question is the same for the magnetic field as it is for the electric field, but the answer differs between classical and quantum EM.
 
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  • #32
DaleSpam said:
Classically you can certainly have an electric field without a magnetic field or a magnetic field without an electric field. On the quantum level, since all fundamental charged particles also have spin you cannot have either without the other.

It is possible also for neutral particles to have a non-zero magnetic moment. Of course, if you are moving relative to such a particle an electric field still appears ...

Does it really make sense to talk about electric and magnetic fields on the quantum level other than as a semi-classical approximation?
 
  • #33
Orodruin said:
It is possible also for neutral particles to have a non-zero magnetic moment. Of course, if you are moving relative to such a particle an electric field still appears ...

I don't think we would call them neutrons if they could acquire electric fields every time something is moving somewhere.
 
  • #34
Just as moving charges give you magnetic fields, anything giving rise to a magnetic field that moves is going to give you an electric one. This is due to how the electromagnetic field transforms under Lorentz transformations. The divergence of the electric field from the moving magnetic source would still be zero and so also the charge density.
 
  • #35
cabraham said:
Under static conditions, either can exist w/o the other. A superconducting loop w/ a steady dc current has magnetic field, but no electric field. The counterpart of this setup is a charged capacitor open circuited, as there is an electric field w/o a magnetic field.

Again these are static conditions, i.e. not changing with respect to time. Under dynamic conditions, i.e. changing wrt time, neither can exist w/o the other. If one field is time-changing, the other must be non-zero.

Claude

What about transformer cores? the magnetic field oscillates at 50 hz but there's no electric field in a metal. There's eddy currents but no electric field, or am I missing something?
 
  • #36
Thierry said:
What about transformer cores? the magnetic field oscillates at 50 hz but there's no electric field in a metal. There's eddy currents but no electric field, or am I missing something?
Yes there certainly are eddy currents in the core metal. But why would you conclude that there is 0 electric field? Core metal is conductive with value ##\sigma##, eddy current density has value ##J##, and remembering Ohm's law in 3 dimensions: ##J=\sigma E##, so that ##E=J/\sigma##.

Does that help? Best regards.

Claude
 
  • #37
Thierry said:
there's no electric field in a metal

...under electrostatic conditions.
 
  • #38
cabraham said:
Yes there certainly are eddy currents in the core metal. But why would you conclude that there is 0 electric field? Core metal is conductive with value ##\sigma##, eddy current density has value ##J##, and remembering Ohm's law in 3 dimensions: ##J=\sigma E##, so that ##E=J/\sigma##.

Does that help? Best regards.

Claude
ah yes, it does. Thanks!
 
  • #39
jtbell said:
...under electrostatic conditions.
I see. of course, thanks!
 

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