Question about the motion charges moving within an electric field

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Discussion Overview

The discussion revolves around the motion of charges within an electric field, specifically whether charges follow electric field lines during their motion. Participants explore various scenarios, including initial velocities and the effects of mass on the trajectory of charges in both uniform and curved fields.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that if a charge starts with no initial velocity, it will follow the field lines, akin to how a dropped object falls vertically under gravity.
  • Others propose that if a charge has an initial velocity, its path will be influenced by both its original velocity and the force from the electric field, potentially resulting in a parabolic trajectory.
  • A participant questions whether a positive charge placed on a field line without initial velocity would remain on that line, suggesting that it might not leave the line if it starts at rest.
  • Another participant reflects on the implications of curved field lines, suggesting that once a charge is accelerated, its velocity may not remain parallel to the field lines, leading it to deviate from its original path.
  • There is a discussion about the relationship between the mass of the charge and its motion, with some suggesting that a charge with mass would gain kinetic energy and could oscillate between field lines when influenced by other charges.
  • One participant notes that in a conservative field, the total energy (kinetic plus potential) remains constant, allowing for the possibility of a charge moving "uphill" in terms of potential energy.

Areas of Agreement / Disagreement

Participants express differing views on whether charges follow field lines, particularly in cases of initial velocity and mass. The discussion remains unresolved, with multiple competing perspectives on the motion of charges in electric fields.

Contextual Notes

Participants reference specific scenarios involving straight and curved field lines, and the discussion includes assumptions about the nature of electric fields and the properties of charges that may not be fully explored.

Excalibur1152
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I've almost certain that I have read/ seen from MIT's OCW that the motion of a charge within an electric field does not follow the field lines.

Today my physics teacher said the opposite, that the path of motion of a charge within a field will follow a field line.

Who is right? And why?

EDIT: I meant to put an 'of' in between the 'motion' and 'charges' in the title.
 
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If the charge started off with no velocity then it will 'fall' along field lines just the same as a dropped object falls vertically on Earth. If the object was originally moving then it will follow a trajectory, accelerating in the direction of the field lines, and its velocity will always be the (vector) sum of its original velocity and the velocity in the direction of the force due to the field. For a uniform field, the shape of path will be part of a parabola.
 
edip2.gif




So if I were to place a (positive) charge on top of one of the field lines ^^^ without giving it an initial velocity, it would never leave the line? It's path would only exist upon the same field line?



(I know that there are an infinite # of lines, but as far as the picture is concerned)
 
Excalibur1152 said:
edip2.gif




So if I were to place a (positive) charge on top of one of the field lines ^^^ without giving it an initial velocity, it would never leave the line? It's path would only exist upon the same field line?



(I know that there are an infinite # of lines, but as far as the picture is concerned)

Right - that diagram has made me think again! I was considering, initially, straight field lines (point source or between plates).

If the lines are curved then you would expect the velocity of the charged object, once it had been accelerated at all, would not be parallel to the lines of force so it would move through them and not follow just one line. The degree that it would depart from the line it 'started on' would presumably depend upon the ratio of the charge of the object to its mass. If it had no mass then it would follow the line.
 
sophiecentaur said:
Right - that diagram has made me think again! I was considering, initially, straight field lines (point source or between plates).

If the lines are curved then you would expect the velocity of the charged object, once it had been accelerated at all, would not be parallel to the lines of force so it would move through them and not follow just one line. The degree that it would depart from the line it 'started on' would presumably depend upon the ratio of the charge of the object to its mass. If it had no mass then it would follow the line.

Indeed, if the field lines are straight, then the charge should only move along the field lines.

If the charge does have mass, it gains kinetic energy, and so when the field line curves, then work has to be done over a distance to "change" the path of motion? Wouldn't this distance be the distance that the charge moves out of the field line? (maybe not that simple, but hopefully you can still follow my logic).

As an example, if you put a positive charge with mass right between the two charges on the dipole I posted earlier (but imagine that both are positive charges, so three positive charges total), but moved it closer to one of the charges, wouldn't it oscillate between them? If it did that, then wouldn't it be moving against a field line?
Much thanks for responding to my posts.
 
It's a conservative field so kinetic plus potential energy will be constant, so yes it could go 'uphill' at times.
 

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