How can a torque be derived for a moving dipole without external fields?

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    Dipole Torque
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Homework Help Overview

The discussion revolves around deriving the torque for a moving dipole without external electric or magnetic fields. The context involves point charges arranged in a specific configuration and their behavior when the system is in motion.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationship between the motion of the dipole and the resulting electric field, questioning how the absence of external fields affects the torque. Some suggest that relativistic effects may play a role in the distortion of the electric field.

Discussion Status

The conversation is ongoing, with participants examining the implications of the problem setup. There is a proposal to consider relativistic corrections, and some express skepticism about the possibility of deriving torque without these corrections. Multiple interpretations of the problem are being explored.

Contextual Notes

Participants note that the charges are fixed in a dielectric and cannot be neutralized, which may influence the analysis. The discussion also touches on the potential application of the problem to measure absolute velocity in space.

tavsaito
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I am at a loss for how to start this problem.
t = r x F and F = q(E + v x B) i know but there is no external E or B field its just a dipole that moves with a speed v in the x direction the dipole is oriented with +q at (d,d,0) and -q @ (-d,-d,0)

how do i derive a torque?
 
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tavsaito said:
I am at a loss for how to start this problem.
t = r x F and F = q(E + v x B) i know but there is no external E or B field its just a dipole that moves with a speed v in the x direction the dipole is oriented with +q at (d,d,0) and -q @ (-d,-d,0)

how do i derive a torque?

If there is no external E or B field, there should not be any torque. Are you sure you've stated the problem correctly?
 
yes there are no external E or B fields i think it has to do with relativity. the one particle creates a E field in its own frame which becomes distorted when it moves allowing there to be a non parallel component.
 
the problem is.

Consider two point charges +/- q embedded on a square dielectric in the x-y plane the dielectric is a perfect insulator and the charges cannot move or be neutralized. Assume that i) the origin of the coordinates is in the center of the square and the positive and negative charges are respectively at (d,d,0) and (-d,-d,0) and ii) the square dielectric is moving at velocity (V,0,0).
a) derive an expression for the torque on the system. Calculate E and B from maxwell's equations but without relativistic corrections
b) It was proposed to use a similar set up to measure the absolute velocity of a body in space. For example this could be used to detect the velocity of Earth in its rotation around the sun. Do you think such an experiment would work? If necessary in your explanations, use the relativistically correct expressions for the E and B fields cause by each charge at the others position.
 
Solution: Basically were asking to prove that it wasnt possible unless taken in a relativistic regime. See the Right-Angle (Lewis-Tolman) Paradox where you can observe a torque in the moving frame but it does not result in any actual rotation
 

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