Positive work:direction of external force

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Discussion Overview

The discussion revolves around the concept of work done on an electron as it approaches a proton, particularly focusing on the relationship between potential energy, external forces, and the direction of displacement. The scope includes classical mechanics and the implications of attractive versus external forces in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asserts that as an electron approaches a proton from infinity, the decrease in potential energy indicates that the work done must be positive, questioning the directionality of the external force relative to displacement.
  • Another participant clarifies that in a classical context, the attractive force between the negatively charged electron and positively charged proton is in the same direction as the displacement, resulting in positive work.
  • A subsequent post reiterates the question about the external force and its direction, emphasizing that the focus is not on the attractive force but rather on the external force responsible for doing work.
  • Another participant argues that no external force is necessary for the electron to move towards the proton, stating that positive work is done as the electron accelerates due to the attractive force, which converts potential energy into kinetic energy.
  • This participant presents two scenarios: one without an external force, where the work done by the attractive force results in an increase in kinetic energy, and another where an external force opposes the motion, leading to a net work of zero and a corresponding change in potential energy.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and role of external forces in the context of work done on the electron. There is no consensus on whether an external force is required or how it interacts with the attractive force.

Contextual Notes

The discussion includes assumptions about classical mechanics and does not delve into quantum mechanics. The implications of external forces on potential energy and work done are not fully resolved, leaving open questions about the definitions and interactions involved.

gracy
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If an electron is brought near the proton from infinity,potential energy of the electron decreases so work done must be positive because the change in potential energy is the NEGATIVE of the work done .But how ?I mean for work done to be positive, force and displacement should be parallel to each other here
exter.png

so is the external force which is responsible for doing the work is in the same direction as of direction of displacement?
 
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I presume you are not talking about a quantum analysis of this, as the concept of an electron approaching a photon assumes an exact measurement of position.

So I imagine you're just talking about a classical negatively charged particle approaching a classical positively charged particle. In that case the force is attractive (opposites attract) so the force on the -ve particle is in the same direction as the displacement - both being towards the +ve particle.

That gives positive work.
 
gracy said:
so is the external force which is responsible for doing the work is in the same direction as of direction of displacement?
I am not talking about attractive force rather I am referring to the external force which is responsible for doing the work
 
There is no need for an external force. The -ve particle will fall towards the +ve one without an external force, and positive work will be done in the process. The work will emerge as kinetic energy.

If you want to introduce an external force you need to tell us what it is doing. Is it for instance pushing the -ve particle away from the +ve one in order to slow its descent? In that case you can no longer use a principle that ' the change in potential energy is the NEGATIVE of the work done' unless you also take into account the change in potential energy of the system providing the external force. So you have two options:

1. No external force. The PE net decreases by w. The work done by the attractive force is +w, and the KE of the particle increases by w.

2. An external force is pushing the -ve particle away. Then the external force does work -w, so the net work done is
+w (by the attractive force) + -w (by the external force) = 0
This matches the change in KE of the particle, which is zero.
The PE of the particle decreases by w, which is exactly offset by the increase in PE of the system providing the repulsive force (assuming no work is dissipated as heat). So the change in PE is zero, which matches the work done.
 

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