What is the difference between positive work and negative work? Because the definition of work is the dot product of the force and displacement vectors, I can see that work is negative when the force acts in the "opposite" direction of the displacement. Also, I see that W = -(delta)PE. I ask this question as a result of encountering two concepts that seem to contradict these statements. The first:(adsbygoogle = window.adsbygoogle || []).push({});

(1) "In a system of two positively charged particles, PE is positive, and positive work must be done to bring like charges together. In a system of two charges opposite in sign, PE is negative, and negative work must be done to bring opposite charges together. Energy is released!"

The second:

(2) "Oppositely charged parallel plates are separated by 5.33 mm. A potential difference of 600 V exists between the plates. How much work must be done on an electron to move it to the negative plate if it is initially positioned 2.90 mm from the positive plate?"

In (1), I can see that the PE is positive when the charges are positive, but shouldn't the work necessary to bring the charges together be negative (since the charges would naturally repel, it's intuitive to me to think that since the displacement works in a direction opposite the electric field force, the work is negative; also, W=-(delta)PE)?

In (2), the answer is 4.38 x 10^-17 J. However, I do not understand why it is a positive answer. Shouldn't the work be negative if the electron is being displaced against the electric field force?

I have a feeling that my questions stem from a misunderstanding of the concept of work. I'm sorry I didn't use the template; I didn't feel that it applied to my questions.

My first post on this forum, yay! I'm so glad I found this site. I'm trying to self-study for the AP Physics B test and I feel this site will help clear up a lot of confusion.

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# Homework Help: Positive vs. Negative Work

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