Properties of symmetric magnetic field around ##Z## axis (cylinder)

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Discussion Overview

The discussion revolves around the properties of symmetric magnetic fields around the Z-axis, particularly focusing on the implications of cylindrical symmetry for the magnetic field components, especially in the ##\hat \phi## direction. Participants explore both physical interpretations and mathematical derivations related to this symmetry.

Discussion Character

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

Main Points Raised

  • One participant expresses confusion about why a symmetric magnetic field around the Z-axis results in a zero component in the ##\hat \phi## direction, seeking both physical understanding and mathematical derivation.
  • Another participant clarifies that cylindrical symmetry means the field depends only on ##\rho## (and possibly ##z##) but not on ##\phi##, suggesting that it does not imply the field is necessarily in the ##\hat \phi## direction or that it is zero.
  • A participant requests examples of when the magnetic field in the ##\hat \phi## direction is not zero, indicating a desire for further clarification on the nature of cylindrical symmetry.
  • A later reply provides an example of a magnetic field from an infinitely long thin wire along the Z-axis carrying current, which has a non-zero component in the ##\hat \phi## direction, demonstrating that it depends only on ##r##.
  • Another example is presented regarding the electric field of an infinitely long thin charge density along the Z-axis, which is in the ##\hat \rho## direction, further illustrating the concept of cylindrical symmetry.

Areas of Agreement / Disagreement

Participants generally agree on the definition of cylindrical symmetry and its implications for the magnetic field components. However, there is ongoing discussion about the specific conditions under which the ##\hat \phi## component may or may not be zero, indicating that multiple views remain on this topic.

Contextual Notes

The discussion reflects varying interpretations of cylindrical symmetry and its effects on magnetic field components, with some assumptions about the nature of the fields and their dependencies remaining unresolved.

sagigever
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I am trying to understand but without a succes why symmetric magnetic field around ##Z## axis make that ##\hat \phi## magnetic field is zero
I can't understand why it physically happens and also how can I derive it mathematically?
What does the word symmetric means when talking about magnetic field?
 
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Cylindrical symmetry around z-axis essentially means that the field depends only on ##\rho## (and possibly ##z## but not ##\phi##). It doesn't necessarily means that it is in the ##\hat\phi## direction or that it is zero.
 
Last edited:
Delta2 said:
Cylindrical symmetry around z-axis essentially means that the field depends only on ##\rho## (and possibly ##z## but not ##\phi##). It doesn't necessarily means that it is in the ##\hat\phi## direction or that it is zero.

can you give me example when it not zero in ##\hat \phi##?

ohh so you mean that it can have component in the ##\phi## direction, but the derivative with respect to ##\phi## will always be zero?
 
sagigever said:
can you give me example when it not zero in ##\hat \phi##?

ohh so you mean that it can have component in the ##\phi## direction, but the derivative with respect to ##\phi## will always be zero?
yes. An example where we have cylindrical symmetry and the field is not zero (however its in the ##\hat\phi## direction) is the magnetic field of an infinitely long thin wire that lies at the z-axis and carries current I. Then the field is $$\mathbf{B}=\frac{\mu_0}{2\pi r}I\hat\phi$$. As you can see it depends only on ##r## (not on ##\phi## or ##z##) and it is not zero.
 
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An example of a cylindrical symmetric field that is in the ##\hat\rho## direction is that of the electric field of an infinitely long thin charge density ##\lambda## that lies again in the z-axis. The Electric field is given by
$$\mathbf{E}=\frac{\lambda}{2\pi\epsilon_0\rho}\hat\rho$$.
 

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