# AAMC Physics QPack #36

#### 77deuce

##### Full Member
5+ Year Member If the equation for electric potential energy is U=kQq/r, then wouldn't "r" (on average) be smallest during phase "B" in the diagram. Therefore, electric potential energy would be highest at that point.

I understand the hooke's law explanation that potential energ y is largest at maximum displacement. However, why would they use the term electrical potential energy if they didn't want us to us that equation?

Is it because they said electrical potential energy instead of electric potential energy?

#### Doogie.Howser

##### Full Member
2+ Year Member
It says that it's an oscillating system and therefore you can infer that you need to use the equation 1/2kx^2. Even if you didn't know any formulas, you can knock out answers. A and C can be struck out because you can see that the two are the same. If the gray box denotes electrons that are nearby the positive charge of a proton, then the most stable will be at time B because it's stable- strike out B. The only answer left is D, but lets make sure it makes sense. You can imagine that the sea of electrons in A and C will oscillate to be like B and therefore have a higher (and equal) potential energy, so the answer is D.

#### babyoreo

##### Full Member
There's nothing wrong with using U=kQq/r; you are simply misinterpreting it.
Q and q are the two charges of interest. Think about the differences between a +Q and a +q vs a +Q and a -q. Remember that this question is referring to protons in a sea of electrons. When r is small, U is big... but also negative.

I think a more intuitive way of thinking about this problem is KE + PE = total energy. Where KE is lowest is where PE is greatest. The oscillations are slowest at A and C.

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#### 77deuce

##### Full Member
5+ Year Member
There's nothing wrong with using U=kQq/r; you are simply misinterpreting it.
Q and q are the two charges of interest. Think about the differences between a +Q and a +q vs a +Q and a -q. Remember that this question is referring to protons in a sea of electrons. When r is small, U is big... but also negative.

I think a more intuitive way of thinking about this problem is KE + PE = total energy. Where KE is lowest is where PE is greatest. The oscillations are slowest at A and C.

I like you. Thanks!

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