1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Potential difference in between positive and negative charge

  1. Jan 3, 2018 #1
    1. The problem statement, all variables and given/known data
    Two point charges Q1= +5.00 nC and Q2 = -3.00 nC are seperated by 35.0cm. What is the electric potential at a point midway between the charges?

    2. Relevant equations
    V = Ke(q/r), PE = Vq


    3. The attempt at a solution
    This question is from Serway textbook. The answer is 103J/C. I understand the answer was obtained by Ke(5 nC- 3 nC)/ 17.5cm. But I wonder why do we subtract the two? Shouldn't we add them? For example, if +1charge were put at the mid point, the charge would feel both repelled and attracted by positive and negative charges, hence the summative force of the two. But the answer tells that the charge would have the potential energy that is partially cancelled out.
     
    Last edited by a moderator: Jan 3, 2018
  2. jcsd
  3. Jan 3, 2018 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That does not mean anything. You can have the potential at the midpoint (taking potential at infinity to be zero), or the potential difference between two points,
    Please show both your calculations.
     
  4. Jan 3, 2018 #3

    Doc Al

    User Avatar

    Staff: Mentor

    How did you do your calculations? To calculate a potential difference, you need two points. Perhaps they are asking for the potential at the midpoint, not the potential difference?
     
  5. Jan 3, 2018 #4
    say, for example, +1 charge were near negative charge and to bring the +1 charge to the midpoint the work needed is the sum of the absolute value of 5x10^-9 and -3x10^-9 charges times Ke/r. But to solve for V, we actually take out the absolute value and subtract them. How is it that voltage is not the sum of the abaolute value of the two when the +1charge will feel summative force from the two?
     
  6. Jan 3, 2018 #5
    The two points are 5 nano charge and -3 nano charge and 35cm apart. The question asked the potential difference at the midpoint.
     
  7. Jan 3, 2018 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    How near? At the negative charge the potential would be -∞, and it would take infinite work to move it anywhere.
    No, that's wrong. What standard equation are you relying on for that?
     
  8. Jan 3, 2018 #7

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    As Doc Al and I have independently commented, that makes no sense. I would assume they want the potential at the mid point, taking the potential at infinity to be zero. Are you translating from the original?

    As requested, please post all of your calculations in detail. From your brief descriptions, it is quite hard to understand what you are doing.
     
  9. Jan 3, 2018 #8
    The equation I used was PE = Vq
     
  10. Jan 3, 2018 #9

    Doc Al

    User Avatar

    Staff: Mentor

    It also might be helpful to post the complete problem, literally word for word. (Is it a textbook problem?)
     
  11. Jan 3, 2018 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Ok, but you need to be clear what the variables mean. That gives you the work done in moving a charge q up a potential difference of V. E.g. you could be moving the charge from infinity to a point at potential V. There is nothing there about absolute values.
    Now please show how you are using this equation in your calculations.
     
  12. Jan 3, 2018 #11
    1. The problem statement, all variables and given/known data
    Two point charges Q1= +5.00 nC and Q2 = -3.00 nC are seperated by 35.0cm. What is the electric potential at a point midway between the charges?

    2. Relevant equations
    V = Ke(q/r), PE = Vq


    3. The attempt at a solution
    This question is from Serway textbook. The answer is 103J/C. I understand the answer was obtained by Ke(5 nC- 3 nC)/ 17.5cm. But I wonder why do we subtract the two? Shouldn't we add them? For example, if +1charge were put at the mid point, the charge would feel both repelled and attracted by positive and negative charges, hence the summative force of the two. But the answer tells that the charge would have the potential energy that is partially cancelled out.
     
  13. Jan 3, 2018 #12

    Doc Al

    User Avatar

    Staff: Mentor

    You had posted the above restatement of the problem in a new thread. I moved it to this one. (Don't create multiple threads on the same topic.)

    In this revised version, you are asked to find the electric potential, not the potential difference, at the midpoint. That makes sense.
     
  14. Jan 3, 2018 #13
    If there were a charge of +1 at the midpoint, would it not mean 1C × 103J/C = 103J?
     
  15. Jan 3, 2018 #14

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes, but that is force, not energy. It relates to the potential gradient, not the potential "height".
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted