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: How much energy is needed to place 4 positive charges

  1. Aug 1, 2012 #1
    Can someone just confirm if I'm doing these two problems correctly ...

    1. How much energy is needed to place 4 positive charges, each of magnitude +5mC, at the vertices of a square of side 2.5cm

    2. Choose one way of assembling the charges and calculate the potential at each empty vertex as this set of charges is assembled.

    For #1, I used the formula PE = kQQ/r

    PE = 9.0 x 106J

    and because there are 6 sets (4 sides and 2 diagonals) with similar lengths and charges, I added the PE values together, giving me a total of PET = 5.4 X 107J

    For #2, I'm not sure what exactly its asking me to do, but I used the formula V = kQ/r

    For the r, I used Pythagoreom Theorom to find the distance to the middle

    V = 2.55 x 109V

    and I'm not really sure, but there are 4 sets this time?

    So Total Potential = VA1+VA2+VA3+VA4?
  2. jcsd
  3. Aug 1, 2012 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Hi, shashaeee.

    For problem 1, did you treat the "diagonal" pairs as having the same distance r as the "side" pairs?

    For problem 2, it appears to me that you want to think of assembling the square one charge at a time and at each step calculate the potential at the remaining empty vertices. So, suppose you started by placing one charge at the upper left corner of the square. You would then have 3 empty vertices. So, you should calculate the potential at each of those three empty vertices. Then suppose you add the second charge at the upper right corner. You would be left with 2 empty vertices. So, calculate the potential at those 2 empty vertices, and so on.
  4. Aug 1, 2012 #3
    Oh, no I didn't! Thank you!

    So for #2 ... basically ...

    If my box was arranged as

    1 _____3
    2 _____ 4

    V1 = V2+V3+V4

    V12 = V3+V4

    V123 = V4
  5. Aug 1, 2012 #4


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    My interpretation of #2 is just to calculate the potential at each empty corner, but don't add them together. So, when charge 1 is in place, you'll calculate V2, V3, and V4. When charges 1 and 2 are in place, you'll calculate new values for V3 and V4. When charges 1, 2 and 3 are in place, then you'll recalculate V4.

    At least that's my reading of the problem. Hope I'm not misinterpreting it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook