Berkeley Review Physics, Ch. 8, Problem 8.4a

[lgirl60]

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Here's the question: Which of the following electric field configurations describes the lines of force in an electric field between two parallel plates of opposite charge?

I know that it's B and not D, but I know that just from memorization/recalling it in physics. Why are the lines in the middle straight and not curvy? Why are the lines towards the edges curvy? BR didn't do a great job of explaining it lol.
Thank You!

 

[BerkReviewTeach]

Here's the question: Which of the following electric field configurations describes the lines of force in an electric field between two parallel plates of opposite charge?

I know that it's B and not D, but I know that just from memorization/recalling it in physics. Why are the lines in the middle straight and not curvy? Why are the lines towards the edges curvy? BR didn't do a great job of explaining it lol.
Thank You!

If you turn back one page to Figure 8-6, they have a simplified version of the field in question 8.4a. If you consider all six force vectors acting on the charge in the middle, you'll notice that the y-component vectors cancel (up is balanced perfectly by down). The result is a net force vector that is straight from the left plate to the right plate. That vector becomes a field line. Question 8.4a is the same concept, but with several more charges and it's rotated by 90 degrees. If you were to put a charge directly in the middle of that field, then the charges on its left side would perfectly offset the charges on its right side, resulting in no left or right electrostatic force (only a net vertical force). The net force on a cation would be straight down. This is essentially true throughout most of the field until you move so far to the right or left that the charges on one side no longer offset (balance out) the charges on the other side. This is why the field lines start to curve as you reach the edges of the region between the plates.

 

[lgirl60]

If you turn back one page to Figure 8-6, they have a simplified version of the field in question 8.4a. If you consider all six force vectors acting on the charge in the middle, you'll notice that the y-component vectors cancel (up is balanced perfectly by down). The result is a net force vector that is straight from the left plate to the right plate. That vector becomes a field line. Question 8.4a is the same concept, but with several more charges and it's rotated by 90 degrees. If you were to put a charge directly in the middle of that field, then the charges on its left side would perfectly offset the charges on its right side, resulting in no left or right electrostatic force (only a net vertical force). The net force on a cation would be straight down. This is essentially true throughout most of the field until you move so far to the right or left that the charges on one side no longer offset (balance out) the charges on the other side. This is why the field lines start to curve as you reach the edges of the region between the plates.

Thank you so much BerkReviewTeach, your explanation makes so much sense. I am very grateful that you are active on SDN's MCAT forums!