Kirchhoff's Law Derived from Faraday's Induction - EMROZ

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    Faraday Kirchhoff
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Discussion Overview

The discussion centers on the derivation of Kirchhoff's Voltage Law from Faraday's Induction Law, exploring the conditions under which Kirchhoff's law holds true and the implications of changing magnetic fields on this law. The scope includes theoretical reasoning and conceptual clarification related to electromagnetism.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • EMROZ presents a derivation of Kirchhoff's Voltage Law from Faraday's Induction Law and seeks feedback on its validity.
  • One participant acknowledges the reasoning as valid, noting that it expresses the electrostatic condition for the curl of the electric field.
  • Another participant questions the applicability of Kirchhoff's Voltage Law under non-electrostatic conditions, specifically when a changing magnetic flux is present.
  • A subsequent reply asserts that a changing magnetic flux would induce an electromotive force (emf), indicating that the closed loop integral would not be zero.
  • Another participant references textbooks that apply Kirchhoff's loop law in the context of changing magnetic fields, suggesting that the induced emf is accounted for in the equations used.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which Kirchhoff's Voltage Law is valid, particularly regarding the influence of changing magnetic fields. There is no consensus on whether the law holds in such scenarios.

Contextual Notes

The discussion highlights the dependence of Kirchhoff's Voltage Law on specific conditions, such as electrostatic versus dynamic scenarios, and raises questions about the interpretation of Maxwell's equations in relation to circuit analysis.

emroz92
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Please see the attachment. There I have derived Kirchhoff's Voltage law from Faraday's Induction law. Reply me if I have done something wrong and also notify me if this proof has been made somewhere else.

Thanking you,
EMROZ
 

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Yeah, the reasoning is fine. What you're doing is stating the electrostatic condition for the curl of the E field.

Essentially, you're taking:

[tex]\vec{\nabla}\times\vec{E}=0[/tex]

and expressing it in integral form.

This is a well known fact. So, yes it has been done before.

Also, the validity of Kirchoff's Voltage law is not something that is questioned or "unproven." It is a direct consequence Maxwell's equations under electrostatic conditions (as you showed) or, (in my opinion ) more fundamentally, of conservation of energy.

Still, its cool that you derived this result yourself, without any knowledge that the relationship was there! These are the moments that make studying physics really great!
 
G01 said:
under electrostatic conditions

Why electrostatic? Under what conditions, if any, is Kirchhoff's voltage law no longer correct? For example, isn't it still true if a changing magnetic flux is passing through the loop?
 
mikelepore said:
Why electrostatic? Under what conditions, if any, is Kirchhoff's voltage law no longer correct? For example, isn't it still true if a changing magnetic flux is passing through the loop?

No, the closed loop integral wouldn't be zero. The changing flux would induce an emf and that means del cross e isn't zero.
 
Yeah right,
Many books (Halliday, for example) use Kirchhoff's loop law under changing magnetic fields and they use $\E_{L}$ in the equation. Actually, this very emf is the right hand side of the Maxwell's equation and they toggle it to the left hand side and denote it beside the solenoid in the circuit diagram.
 

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