Electric Potential Difference Problem

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Homework Statement


Two charges, A (-3 C) and B (+4 C), are placed 2.40 m apart. Point X is 30 cm from A, and Point Y is 30 cm from B. What is the electrical potential difference going from X to Y?

Homework Equations


W = △Vq

The Attempt at a Solution


Teacher gave answer as 6.9 x 10^4 V

I am not sure whether or not to use electric force or electric potential energy in this problem. I've tried both but obviously I cannot apply it appropriately to get the answer above.
 

Answers and Replies

  • #4
haruspex
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Two charges, A (-3 C) and B (+4 C), are placed 2.40 m apart. Point X is 30 cm from A, and Point Y is 30 cm from B. What is the electrical potential difference going from X to Y?
That's not enough information. You need to be able to work out how far X is from B and how far Y is from A as well. Have you stated the whole question exactly as given to you?

You do not need to consider forces or energy. It is entirely to do with electric potential (which is not the same as electrical potential energy).
Once you have all the distance information, there are just two other things you need to know:
- the formula for the potential caused by a point charge q at distance r from it
- that potentials add, i.e. the net potential at a point in space is the sum of the potentials there caused by all the charges in the system
 
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Is this a 1D problem or a 2D problem? You need to draw a sketch of the locations under consideration.
 
  • #6
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If charges A and B are 2.40 m away from each other then point X is 1.8 m away from B and point Y is 1.8 m away from A.

Here's a diagram if this helps:
````````````````2.40 m`````````````
|---------------------------------|
A_____x________________y_____B
``30 cm````````````````````````30 cm``


Ignore the ` it is just used for spacing. Hopefully this helps.

Also:

V = kQ / d
 
  • #7
So it seems like it is a one dimensional problem. Have you been introduced how to calculate electrical fields yet ?
 
  • #8
haruspex
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then point X is 1.8 m away from B and point Y is 1.8 m away from A.
Not according to the diagram. Try calculating those again.
Once you have distances right, use the formula you quote to find the potential that charge A creates at X and the potential that charge B creates at X.
 
  • #9
haruspex
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Have you been introduced how to calculate electrical fields yet ?
No need for fields here, just potentials.
 
  • #10
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So you calculate the potential at X due to both the charge at A and the charge at B. Do the same calculation at Y. You can then readily see the difference in potential between points X and Y.
 
  • #11
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Yeah my teacher wrote the wrong answer apparently. Turns out I did it correctly by calculating the total potential difference at point X and Y then to find the change in potential difference I subtracted the X value from the Y value and got the potential difference for a point to go to X to Y. Your comments were helpful though. I appreciate your input and it did help me figure out the problem.
 

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