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Homework Help: Electric Field and Force

  1. Jan 17, 2012 #1
    1. The problem statement, all variables and given/known data

    Refer to the figure below. Find the magnitude of the electric field if the string with a 7 gram ball carrying a charge of 4 x 10-9 C forms an angle of 35o with the vertical.


    http://www.montereyinstitute.org/courses/AP%20Physics%20B%20II/course%20files/assignments/lesson31selfcheckquiz_files/image001.gif

    a. 1.41 x 106 N/C
    b. 1.20 x 107 N/C
    c. 2.30 x 107 N/C
    d. 6.25 x 108 N/C

    2. Relevant equations

    Fg=mg
    E=F/q


    3. The attempt at a solution

    Force due to electric field=Force due to gravity:

    F=mgcos(θ)
    F=7*10^-3kg*9.81m/s^2*cos(35)
    F=Eq
    E=F/q
    E=(7*10^-3kg*9.81m/s^2*cos(35))/(4x10^-9)C
    E=1.41x10^7 N/C

    which is not an answer...the correct answer in the key is b, but I dont know why?
     
  2. jcsd
  3. Jan 17, 2012 #2
    The force due to gravity is not equal to the force due to the electric field. It would help to draw the forces acting on the object.
    There is a tension in the string and TCos35 is the vertical force = weight of ball
    T Sin35 is the horizontal force = force due to electric field.
    I got T = 0.084N which gave horizontal Force = 0.048N
    Which gives (b) as the answer.... see if you can check this out
     
  4. Jan 17, 2012 #3

    I like Serena

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    Homework Helper

    Hi physgrl! :smile:

    How did you get the formula F=mgcos(θ)?
    I'm afraid it is not the horizontal force due to gravity.
     
  5. Jan 17, 2012 #4
    ohh, how can i get the tension force? I thought id't be the hypotenuse of downward weight but that would be .007*9.81/cos(35)=0.084
     
  6. Jan 17, 2012 #5

    I like Serena

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    This is correct.
    That is the tension force.
     
  7. Jan 17, 2012 #6
    That is correct.... that is what I got
    so the horizontal component of this tension force is TSin35 = 0.084Sin35 = 0.048N
     
  8. Jan 17, 2012 #7
    Ohh ok thanks!! :)
     
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