Kaplan Electrostatic Review Problem 8

[StarryNights]

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So in the Physical Sciences review notes (Chapter 6, electrostatics) I cannot understand number 8! A negative charge (-1 uC) goes from y = -5 to y = +5 with two charges (both +5 C) on both sides of y = 0. The question asks for the work required to move the negative charge from
y =-5 to =5. The answer is supposed to be work = zero since the force changes direction as the elctron crosses y = 0 but I just do not see it. This is definitely a problem I will screw up on later because I don't see it as work canceling out. If anyone understands this, help! It'd be better if they solved it quantitatively, but they didn't. I hate Kaplan. That's all for my random rant.

 

[not respect]

okay well quantitatively, think about the potential caused by the two positive charges. it is the same at both the points you mentioned, so the work should be 0.

 

[spyderracing32]

Just think about this in terms of potential energy. If the charges are equidistant from the center, then you lose PE as you go from -5 toward 0 but you gain it all back again by moving from 0 to +5 with your test charge. The work is the difference in energy and since there is none, there is no work done.