Will this dipole rotate or change position?

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Discussion Overview

The discussion centers around the behavior of a dipole in an external electric field, specifically whether the dipole will rotate or change position. Participants explore concepts related to forces, torques, and stability in the context of electric fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests calculating the net force and net torque on the dipole, proposing that both are zero.
  • Another participant introduces the concept of dipole moment, indicating that if the dipole moment is parallel to the external electric field, the dipole is in a stable position.
  • A different viewpoint describes the dipole as being in an unstable equilibrium, where any perturbation will create a rotational force that could lead to a stable configuration with increased charge separation.
  • One participant expresses confusion about the implications of the external electric field, suggesting that it would cause the positive charge to be repelled and the negative charge to move forward.
  • Several participants agree that if the charges are held rigidly, they will rotate to align with the field, but question the net force on the charges overall.
  • There is a discussion about the nature of equilibrium, with some asserting that the net force should be zero while acknowledging the presence of torque that could lead to a shift into a stable equilibrium position.

Areas of Agreement / Disagreement

Participants generally agree that the net force on the dipole is zero, but there is contention regarding the net torque and the stability of the equilibrium position. The discussion remains unresolved regarding the dynamics of the dipole's movement in the electric field.

Contextual Notes

The discussion highlights the complexity of the dipole's behavior in an electric field, with references to unstable and stable equilibrium positions, but lacks consensus on the implications of these states.

annamal
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Will this dipole rotate or change position? The external electric field is in black. Two charges with their electric fields are drawn in orange.
Screen Shot 2022-04-20 at 11.48.38 PM.png
 
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Calculate the net force on the dipole and the net torque (with respect to the center of the dipole) , you will find them both zero.

Another way to solve this, is via the concept of dipole moment. If the dipole moment is parallel to the external Electric Field then the dipole is in stable position.
 
So, the e-field arrows go from + charges (potential, really) towards minus. This will exert a coulomb force to push/pull the charges. But, it's stable like a pencil standing on it's point (really, an unstable equilibrium) any perturbation in alignment, which will always happen somehow, will create a rotational force to make them switch positions. Then it really will be stable (a stable equilibrium, once things settle down), but with the charges separated a bit more than if there was no field. The concept is simple, the + charge wants to move with the arrows, the minus charge wants to move the other way.
 
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This confuses me a bit because the external electric field in black is as though we placed a positive charge to the very left of the image, right next to the blue +q, which means, the blue +q should be repelled and move away with the red -q moving forward.
 
annamal said:
which means, the blue +q should be repelled and move away with the red -q moving forward.
That's what @DaveE just said. And if the two charges are held rigidly at their current separation (hence, a diople), they will be rotated by those forces until they align with the field. But since the E field is constant in magnitude in the area that you show, what is the "Net" force on the two charges overall?
 
berkeman said:
That's what @DaveE just said. And if the two charges are held rigidly at their current separation (hence, a diople), they will be rotated by those forces until they align with the field. But since the E field is constant in magnitude in the area that you show, what is the "Net" force on the two charges overall?
Shouldn't it be 0 in this unstable equilibrium position?
 
Shouldn't what be zero? The net force and torque? I suppose so, but it's a very unstable equilibrium as drawn, and will experience a torque fairly soon that flips it into the stable equilibrium position (after a period of oscillations that will depend on any damping that is present).
 
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berkeman said:
Shouldn't what be zero? The net force and torque? I suppose so, but it's a very unstable equilibrium as drawn, and will experience a torque fairly soon that flips it into the stable equilibrium position (after a period of oscillations that will depend on any damping that is present).
You asked about the net force. The net force should be 0.
 
annamal said:
You asked about the net force. The net force should be 0.
Correct, but not the net torque. You were asking about how the dipole would move in this situation.
 

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