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Homework Help: Gauss's Law and the electric field

  1. Mar 9, 2008 #1
    I'm stuck on a problem trying to determine charge, and I'm hoping someone can help.

    Suppose that a = 5.00 cm, b = 20.0 cm, and c = 25.0 cm. Furthermore, suppose that the electric field at a point 15.5 cm from the center is measured to be 3.70 *103 N/C radially inward while the electric field at a point 50.0 cm from the center is 2.50 *102 N/C radially outward.


    From this information, find the following charges. (Include the sign of the charges.)
    (a) the charge on the insulating sphere

    At first i worked out the whole EA=Q/Epsilon(0); and tried 4pi(r^2)*Epsilon*3.7e3 in order to solve for Q, but it didnt work out; what am I doing wrong?
  2. jcsd
  3. Mar 9, 2008 #2
    It would help to see more of your work. For example, what values do you get and what were you expecting?

    The only thing I can suggest, based on what you've told us, is that you be careful with your units. It looks like you're mixing cgs and mks units, so that could trip you up.
  4. Mar 9, 2008 #3
    Well for E, i used the 3.7x10^3 N/C, and A I used (3.1416*.05^2*4). Epsilon is a constant, so I figured it would be easy to solve for Q, but I get -1.03x10^-9 C, which is within 10%-100% off; so I know I'm somewhere in the range of the answer.
  5. Mar 9, 2008 #4
    How are you using Gauss's Law in this problem? Doesn't it require that the units all be cgs (cm, g, s) or mks (m, kg, s)? You have both N, which is MKS, and cm, which is cgs.

    This might not be your problem - the choice of units in E&M is always tricky - but without seeing what else you've done, it's hard for me to tell.
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