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Homework Help: Electric Fields Question

  1. Jan 8, 2008 #1
    1. The problem statement, all variables and given/known data

    Use Coulomb's law to determine the magnitude and direction of the electric field at points A and B in Fig. 16-57 due to the two positive charges (Q = 8.0 µC) shown. Is your result consistent with Fig. 16-31b?

    16_41.gif
    Figure 16-57
    16_29b.gif
    Figure 16-31b

    A)Find the magnitude of the electric field at point A and direction.
    B)Find the magnitude of the electric field at point B and direction.

    2. Relevant equations

    E=kQ/r^2


    3. The attempt at a solution

    A)
    Let the left Q charge be Q1
    Let the right Q charge be Q2

    I started by finding the distance vector between Q1 and A, which is the same for Q2 and A.

    sqrt((0.1^2)+(0.05^2))=.111m

    I then found the electrical field between Q1 and A, which is the same for Q2 and A, but in the opposite direction.

    E=(9e9)(8e-6)/(.111^2)=5.84e6

    I then split the electrical field into x and y components. Used law of sines to find the angle.

    sin(90)/(.111)=sin(x)/(.05)

    x=26.8 degrees

    Splitting into x and y components:

    Q1 to A.
    5.84e6*sin(27)=2.651e6 up
    5.84e6*cos(27)=5.203e6 right

    Q2 to A.
    5.84e6*sin(27)= up
    5.84e6*cos(27)=5.203e6 left

    The x-components cancel due to opposite directions and the y-components are combined.

    2.651e6 + 2.651e6 = 5.3e6 N/C 90 degrees above the horizontal.

    This is apparently the wrong answer and I do not know what I am doing wrong...

    B)
    I'm not going to attempt B until I get part A right...
     
  2. jcsd
  3. Jan 9, 2008 #2

    mjsd

    User Avatar
    Homework Helper

    interesting, I don't see a problem with your workings either
     
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