EK Physics 771

[KoalaT]

I'm confused on this question. Can someone explain?

771. A particle with charge q experiences a force F when placed a distance d from a point charge. At distance d, the electric field has a strength E and a potential V. Which of the following represents the potential energy of the charge?

A. Fd
B. Eqd
C. Vq
D. Ed

The answer is C. And I understand how it is, but why are A and B incorrect?

F = KQq/r^2
V = Kq/r
U = KQq/r
E = Kq/r^2

The way I see it U should not only equal Vq but should also equal Fd (Fr) and Eqd. Why not?

The answer explanations states this: "If we multiply times the charge, we have potential energy. Choice A is incorrect because F changes with the distance d, so a constant force was not applied over d. Choice B is incorrect for the same reason because the electric field due to a point charge is not constant. Choice D has the wrong units."

Why can the answer not be Fd or Eqd? I get it, if d changes so will F. Is it simply because they are describing F as the value rather than the actual equation? It just seems like kind of a stupid question and I want to make sure I'm not missing an important fundamental here.

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[KoalaT]

I feel like I'm applying the equations too rigidly and am missing out on conceptualizing electrostatics appropriately.

 

[KoalaT]

Pretty sure I've already answered my own question. F is the set numerical value of the force, so the answer can't be Fd because it changes the value of F. Same with answer choice B. If it was simply the formulas being manipulated, then yes those do show potential energy of a charge.

 

[Dr. Trenb]

I'd like to bring to your attention, in case you are unaware, that examkrackers has a public forum where you can look up explanations of questions previously asked by members or post a new one if you can't find what you're looking for. Just search "MCAT Forum - Examkrackers"