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: Equipotential Contours and Point Charge

  1. Jan 24, 2005 #1
    We haven't really covered this in class so I need some help.

    Assuming that these contours are those of a point charge at the origin, calculate the value of the charge in C.

    I tried using the equation kQ/R but that's wrong, I'm not even sure I'm using it right in fact. Any help? Thanks

    Attached Files:

  2. jcsd
  3. Jan 24, 2005 #2
    your equation is completely fine.... check your number.... Maybe your calculator needs a new battery... :tongue2:
  4. Jan 24, 2005 #3
    My calculator is just fine thanks, but i'm still getting it wrong. I'm using -7500 V as Q and then figuring out the radius by using the pythagoream theaorem, still getting it wrong. Help
  5. Jan 24, 2005 #4


    User Avatar
    Homework Helper

    Why are you using pythagorean theorem? Each contour is a circle... you can measure R right off the graph... You can use any contour, but I'd use the one at R=1m.
  6. Jan 24, 2005 #5
    Well both the x and y axis measure distance so i thought i'd have to find the radius of the circle by using the pythagoream theorem. Even by using r=1m i still get it wrong. :mad:
  7. Jan 24, 2005 #6
    write your number out and let us check for you...
    warning: what is the sign of charge?
  8. Jan 24, 2005 #7
    kQ/r=[(8.99e9 Nm^2/C^2)*(-7500 V)]/sqrt(.8^2+1.0^2)=-5.265e13. Is this completely wrong lol. We haven't covered it in class, and incidentally the homework is due tomorrow (my only problem left!), and our book seriously does not help.
  9. Jan 24, 2005 #8
    V is the voltage, and Q is the unknown charge you need to find......You mess up these two numbers
    you don't need the pythagoream therom to get the r, look carefully, the radii of the circle is same as the number @ x-axis
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook