Equal and opposite electric fields?

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Discussion Overview

The discussion revolves around the behavior of electric fields generated by moving charges, specifically examining the forces between two electrons, one stationary and one moving towards it. Participants explore the implications of electromagnetic interactions, the role of magnetic fields, and the conservation of momentum in this context.

Discussion Character

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

Main Points Raised

  • One participant notes that the electric field at a moving electron due to another stationary electron is proportional to 1/d², suggesting that the forces on the two particles may not be equal and opposite.
  • Another participant argues that Newton's third law (NIII) does not always hold for electromagnetic interactions due to the changing momentum of the electromagnetic field.
  • A subsequent reply introduces the concept that the momentum of the electromagnetic field can be expressed in terms of the electric field (E) and magnetic field (B), raising questions about the conditions under which these fields interact.
  • One participant proposes using fluid dynamics as an analogy to understand the behavior of moving electric fields, suggesting that the flow lines of displaced water could resemble electric field lines.
  • Another participant reflects on the idea that an accelerating particle's field takes time to adjust, leading to unbalanced forces and the notion that momentum is conserved only if the field itself contains the missing momentum.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of Newton's third law in electromagnetic contexts, and there is no consensus on how to reconcile the forces experienced by the moving and stationary electrons. The discussion remains unresolved regarding the implications of these interactions.

Contextual Notes

Participants mention the dependence on specific frames of reference and the potential influence of external magnetic fields, indicating that assumptions about the system may affect the conclusions drawn.

Who May Find This Useful

This discussion may be of interest to those studying electromagnetism, particularly in understanding the complexities of electric and magnetic field interactions and the implications for momentum conservation in dynamic systems.

granpa
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electron one is moving directly toward stationary electron two. the field at electron one due to electron one is proportional to 1/d^2. but the field at electron two due to electron one is less because electron ones field is compressed. it would seem from this that the forces on the 2 particles arent equal and opposite.

now I can see that there will be some frame in which both particles are moving at the same speed and in which the forces will be equal and opposite but seems like it should hold for all frames. it probably does and I'm just overlooking something simple.

the magnetic field can't play a role since they are moving straight toward each other.
 
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NIII does not always hold for EM interactions, because the EM field can have changing momentum. The magnetic field of electron one does enter, because the EM momentum depends on the integral of EXB,
 
thanks for the reply. I would never have figured that out myself. (not that I understand what it means but at least I know where to look now).

so the momentum of the field at any given point equals EXEXV?

this formula makes more sense since the particle might be moving through an external magnetic field in which case I assume EXB wouldn't hold.
 
speaking of moving fields, I've been thinking about what sort of equations to use to understand what a moving field would look like. it occurred to me that fluid dynamics might be useful. if a sphere appeared magically in a body of water the displaced water would move outward. the flow lines of this displaced water would look identical to electric field lines. one might be able to use this to figure out what the field would look like in the case where the sphere/charge is moving.
 
oh. now I am beginning to get it. its the same reason they give for charges producing light. an accelerating particles field takes time to adjust so in the meantime the forces arent balanced. momentum isn't conserved unless you assume that the field itself contains the missing momentum.

where did I just read that? it will come to me later.
 
here is where I read it:

http://en.wikipedia.org/wiki/Field_(physics )
 
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