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Calculate the distance r where the electrostatic potential is zero between charges.

  1. Jan 13, 2013 #1
    1. The problem statement, all variables and given/known data
    photobucket4_zps0655b6ef.jpg
    The diagram shows two charges, +4 μC and –16 μC, 120 mm apart. What is the distance from
    the +4 μC charge to the point between the two charges where the resultant electric potential is
    zero?
    A 24mm
    B 40mm
    C 80mm
    D 96mm

    2. Relevant equations

    potential equation

    3. The attempt at a solution

    photobucket3_zps880c633e.jpg
    1. The problem statement, all variables and given/known data

    The correct answer is a 24mm
    I get that we could simply use a 1:4 ratio and therefore r must be a fifth of the way from the 4 micro coulomb charge
    However when I try and equate the formula and solve algebraicaly I seem to get the wrong answer if I put in the sign of the charges.

    So my question is why do we just use the magnitude of the charges?
     
  2. jcsd
  3. Jan 13, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: Calculate the distance r where the electrostatic potential is zero between charge

    You seem to have written U1 = U2 when you should have used U1 + U2 = 0.
     
  4. Jan 13, 2013 #3
    Re: Calculate the distance r where the electrostatic potential is zero between charge

    Thank you, I am a fool
    I must take a break and do some maths in between my physics
     
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