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Homework Help: Magnitude and direction of Electric field

  1. Feb 6, 2014 #1
    1. The problem statement, all variables and given/known data

    Calculate the magnitude of the electric field E = (-11i^+14j^)N/C

    ∣∣E⃗ ∣∣ = _______ N/C

    Calculate the direction (relative to the +x-axis ) of the electric field E = (-11i^+14j^)N/C.

    θ =_________∘ counterclockwise from the +x-axis

    2. Relevant equations

    3. The attempt at a solution

    So magnitude would be SQRT(-11^2 + 14^2) = SQRT(317) = 17.8 N/C

    For direction you'd take the inverse tan so:

    theta = ATAN(14/-11) = -51.8 degrees

    But that means the angle is 51.8 degrees BELOW the x-axis, so counter-clockwise it'd be

    360-51.8 = 308.2 degrees
    BUT I'm getting this part wrong and I don't know why? what am I doing wrong
  2. jcsd
  3. Feb 6, 2014 #2
    From the expression for the field vector in component form, it looks like the angle lies in the second quadrant.
  4. Feb 6, 2014 #3
    So should i just subtract 51.8 from 180 degrees?
  5. Feb 6, 2014 #4
  6. Feb 7, 2014 #5
    Thank you!
  7. Sep 10, 2015 #6
    How do you know to find theta using arctan(14/11)? and then to subtract that from 180?
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