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From what i know, when a charge moves in the direction of the electric field, work is done by the charge and its potential energy decreases. So similarly, when a charge move against the electric field, work is done on the charge.


Okay here are the textbook questions: Determine if work was done on the field or by the field or no work is done for each of the following charged objects in a uniform electric field.

a) An electron moves towards a positive plate.

answer: by the field.

Shouldnt this be work done on the field by the charge since the electron is going with nature?


c) A proton moves towards a positive plate.

answer: on the field.

Shouldnt this be the opposite instead? the proton is going against nature?? and work is done to increase its potential energy?

Am i missing something?

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  • $\begingroup$ Really I mean we assume the potential at infinity =0,so when we move the charge from infinity to a given point it, s potential energy increases, secondly system wether it is of a single charge or plate has it, s own field, so you will never heard the word work on the field because it is the field that does work and depends upon magnitude we says which body field able to influence more. $\endgroup$ Commented Feb 4, 2020 at 14:10
  • $\begingroup$ A charged object in an electric field is subject to a force. If the object moves, then the field does work, but it can be positive or negative. Your question seems to imply that only positive work is “done by the field”. $\endgroup$
    – R.W. Bird
    Commented Feb 4, 2020 at 18:55

3 Answers 3

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An electron in electric field has tendency to move opposite to the electric field. If it does so the work done by the field force is positive as the force on electron and direction of its movement is in same direction. More simply If the electron is in electric field the field pushes it towards the positive plate

From what i know, when a charge moves in the direction of the electric field, work is done by the charge and its potential energy decreases. So similarly, when a charge move against the electric field, work is done on the charge. This statement is wrong as-

1.No work can ever be done by the charge. Work is either done by the electric field or some external force.

  1. Electron in an electric field moves opposite to its direction and proton would move in the direction of electric field. That is in an electric field electron has tendency to move towards positive plate and proton would move towards negative.

I have already answered the electron part above.Coming to the second part as I said the proton should move towards negative plate, but since it moves towards the positive plate some external force must have done work against electric field. Hence the work is done on electric field.

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Suppose I pull an object across a surface and it moves towards me. I exert a force and the object moves in the direction of the force, so I do (positive) work on the object.

Now suppose I push on the object but the object still moves towards me. The force and distance moved are in opposite directions so I do negative work. Or, put another way, the object does positive work on me (by Newton's third law).

Does that help?

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The charge does no work. Either the field and/or an external agent does work.

Consider the charge in an electric field the same as a mass in a gravitational field. Does the mass in a gravitational field do work when it falls in the gravitational field? Does the mass do work when you lift it up in a gravitational field?

Hope this helps.

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