BR physics chapter 8 ex 8.6B and 8.5A

[2010premed]

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Which of the following configurations of dipoles would produce the GREATEST electrostatic potential energy?

A. +- -+
B. -+ +-
C. - -
+ +


D. - +
+ -

Answer: C... why?

8.5A
An electron gun usually consists of 2 electrode plates, one more negatively charged than the other. If an electron of mass m is allowed to accelerate between the two electrodes separated by a oltage V, which equation BEST represents the final speed of the initially stationary electron upon striking the more positive electrode?

A: v = sq root 2eV/m

Exp:
Ei + W on object = Ef

0 + eV = 1/2 mv^2

what equation does eV come from??

 

[lorenzomicron]

Initial energy is zero. But, initial potential energy U (W to be done) is eV since U=qV

 

[golgiapparatus88]

More electric potential energy is gained when like-charges are close to each other. Without using an equation you know that if you try to take two of the same charges and bring them close to each other, they will want to repel. To give an example, ATP has so much energy because PO4(2-) is added to ADP which already had negatively charged phosphate groups. When the phosphate , and Dis used to phosphorylate a compound, a ton of energy is released.

For this example, A and B have the same potential energy and D has the least amount of potential energy. Even without knowing why, C has to be the right answer. The 2 positive charges and 2 negative charges are both close to each others charge creating a lot of potential energy.

Lemme know if that made sense.

 

[golgiapparatus88]

Sorry didn't see the second question.

electric potential energy = qV (q is the same thing as e, it's just the charge) and kintetic energy is always 1/2mv^2

solve for v = sqrt(2qV/m) or sqrt (2eV/m)

Edit: For the potential energy = qV, V can also be written as the greek letter phi Φ = k(q/r) so PE = qΦ

 

[sangria1986]

More electric potential energy is gained when like-charges are close to each other. Without using an equation you know that if you try to take two of the same charges and bring them close to each other, they will want to repel. To give an example, ATP has so much energy because PO4(2-) is added to ADP which already had negatively charged phosphate groups. When the phosphate , and Dis used to phosphorylate a compound, a ton of energy is released.

For this example, A and B have the same potential energy and D has the least amount of potential energy. Even without knowing why, C has to be the right answer. The 2 positive charges and 2 negative charges are both close to each others charge creating a lot of potential energy.

Lemme know if that made sense.

Actually...while your explanation is correct the flaw in the question itself is that a dipole by definition has two charges of same magnitude but necessariliy opposite charge. Thus you cannot have a -- dipole (by definition that is not a dipole).