1. Not finding help here? Sign up for a free 30min 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!

My brain has been shocked by this problem dealing with charges and Coulombs law

  1. Jan 16, 2013 #1
    My brain has been shocked by this problem dealing with charges and Coulombs law :(

    Physics_zps5a4fa021.png

    This is what I did...

    F = k*q1*q2/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
    F = 0.000000264 / 0.087025
    F = 0.0000003034
    F = 3.034 * 10^-7 N

    F = k*q1*q3/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.295)^2
    F = 0.000000132 / 0.087025
    F = 0.0000001519
    F = 1.519 * 10^-7 N

    Resultant: √ (3.034 * 10^-7)^2 + (1.519 * 10^-7)^2 =√ ( 1.1488262 * 10^-13) = 3.39 * 10^-7

    I got it wrong though :(

    Please help
     
  2. jcsd
  3. Jan 16, 2013 #2
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    This looks like an online quiz. What is the class policy on seeking help vs. working independently for unsupervised quizzes?
     
  4. Jan 16, 2013 #3
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    Check your value for r in the second calculation.
     
  5. Jan 16, 2013 #4
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    This isn't an online quiz. It's an online homework assignment that's been given to my class through Webassign.
     
  6. Jan 16, 2013 #5
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    Oh!
    Ok give me a sec. Imma try to change r in the second equation to 0.100...
     
  7. Jan 16, 2013 #6
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    F = k*q1*q2/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
    F = 0.000000264 / 0.087025
    F = 0.0000003034
    F = 3.034 * 10^-7 N

    F = k*q1*q3/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.100)^2
    F = 0.000000132 / 0.01
    F = 0.0000132
    F = 1.32 * 10^-5 N

    Resultant: √ (3.034 * 10^-7)^2 + (1.32 * 10^-5)^2 =√ ( 1.743 * 10^-10) = 1.32 * 10^-5 (?)


    "INCORRECT. Your incorrect answer may have resulted from roundoff error. Make sure you keep extra significant figures in intermediate steps of your calculation."

    WHAT????
     
  8. Jan 16, 2013 #7
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    EDIT

    F = k*q1*q2/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(6.0 * 10^-9) ] / (0.295)^2
    F = 0.000000264 / 0.087025
    F = 0.000003037 (there was one extra 0 that i took off here)
    F = 3.037 * 10^-6 N

    F = k*q1*q3/r^2
    F = [ (8.99 * 10^9)(4.9 * 10-9)(3.0 * 10^-9) ] / (0.100)^2
    F = 0.000000132 / 0.01
    F = 0.0000132
    F = 1.32 * 10^-5 N

    Resultant: √ (3.037 * 10^-6)^2 + (1.32 * 10^-5)^2 =√ ( 1.834 * 10^-10) = 1.35 * 10^-5

    Is that the answer?
     
  9. Jan 16, 2013 #8
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    What about direction?

    This is what I did...

    arctan(0.0000132/0.000003037) = 77.04

    I submitted that answer and I got this -.-

    "INCORRECT. Examine the diagram and determine which quadrant contains the angle of the net force."

    The answer isn't -77 either. :(
     
  10. Jan 16, 2013 #9

    gneill

    User Avatar

    Staff: Mentor

    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    Your force value for the first charge is off by a power of ten. You'd probably be better off sticking with exponential notation than going through fixed point notation; it's too easy to miscount decimal places.

    The problem statement says it wants the direction angle measured counterclockwise from the positive x-axis. So a negative value is NOT okay. Sketch the force vectors and the resultant vector and calculate accordingly.
     
  11. Jan 16, 2013 #10
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    Oh wait...the answer is -101 according to webassign o_O
    Idk how they got that
     
  12. Jan 16, 2013 #11
    Re: My brain has been shocked by this problem dealing with charges and Coulombs law :

    Rise....
    you can look at the charges and figure out which quadrant the resultant is in,right??
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: My brain has been shocked by this problem dealing with charges and Coulombs law
Loading...