- #1
CaneAA
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I am having a difficult time determining the sign of work in electrostatic problems. I'm attempting to reason out a general rule based on the given examples in my book and to relate it to work done by gravity--but I'm getting two contradictory observations. I don't know if maybe there are different rules depending on whether the force doing the work is conservative or non-conservative...
For example: Consider an electron moving toward a region of higher potential (+ side; lower potential energy) in an electric field. From what I understand, the work done by the electric force is POSITIVE.
If, however, the electron is being pulled toward the negative plate [there has to be an external non-conservative force doing this, right?], then the work done by the electric force, which is opposing this motion, is NEGATIVE.
So, it would appear that in determining the sign of the work done by an electric force [a conservative force, like gravity], if the particle is moving to a region of lower potential energy (ex. toward + plate), the electric work is positive. And vice versa.
So, this would be the same as the work done by gravity. If an object is dropped (moving to a region of lower potential energy) --> gravity does positive work. And vice versa.
But now, if you want to move an electron toward the negative plate (higher potential energy), you need to apply an external non-conservative force. But the work done by this force (in moving the electron to a region of higher potential energy) is POSITIVE.
So, I guess my main source of confusion is that for one type of force, moving the electron to a region of higher potential energy results in positive work, and for another type of force (i.e. electric force), moving the electron towards a region of higher potential energy, results in negative work.
So, ultimately, my question is: How do you determine the sign of the work done? Are there 2 rules for determining the sign of work depending on whether the force doing the work is conservative or non-conservative--or am I missing a simpler connection between the sign and work?
Thank you!
For example: Consider an electron moving toward a region of higher potential (+ side; lower potential energy) in an electric field. From what I understand, the work done by the electric force is POSITIVE.
If, however, the electron is being pulled toward the negative plate [there has to be an external non-conservative force doing this, right?], then the work done by the electric force, which is opposing this motion, is NEGATIVE.
So, it would appear that in determining the sign of the work done by an electric force [a conservative force, like gravity], if the particle is moving to a region of lower potential energy (ex. toward + plate), the electric work is positive. And vice versa.
So, this would be the same as the work done by gravity. If an object is dropped (moving to a region of lower potential energy) --> gravity does positive work. And vice versa.
But now, if you want to move an electron toward the negative plate (higher potential energy), you need to apply an external non-conservative force. But the work done by this force (in moving the electron to a region of higher potential energy) is POSITIVE.
So, I guess my main source of confusion is that for one type of force, moving the electron to a region of higher potential energy results in positive work, and for another type of force (i.e. electric force), moving the electron towards a region of higher potential energy, results in negative work.
So, ultimately, my question is: How do you determine the sign of the work done? Are there 2 rules for determining the sign of work depending on whether the force doing the work is conservative or non-conservative--or am I missing a simpler connection between the sign and work?
Thank you!