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Electric Field components

  1. Jan 27, 2004 #1
    I'm having trouble getting started on a problem assigned to me in my College Physics class. First here's the problem:

    Four charges occupy the four corners of a square(q1 is in the upperleft hand corner, q2 bottom left, q3 bottom right, q4 upper right). q1 = q3 = -q and q2 = q4 = +q , where q = 8 C. The sides of the square are all 2.2 meters long. Calculate the x and y electric field components at the point M. (M is between q2 and q3 on the line)

    Ok now what I've come up with in the hours I've been staring at this problem:
    What I have drawn is that there are three Electric fields coming off of this point M. One goes due east and is a combination of the electric field from q2 and q3. Another electric field goes Northwest towards q1. The last one goes Southwest in a line away from q4.

    I have all these words and definitions in my head and I'm completely confusing everything to do with electric fields. Can someone please set me straight and help me through this problem? Hints, words of wisdom??
     
  2. jcsd
  3. Jan 27, 2004 #2
    If u got struck Try doing this

    Write each vector piece wise i.e the components along East & North

    Apply the pythagoras to get the magnitude of the resultant and trigono for calculating the respectivr direction
     
  4. Jan 27, 2004 #3
    Ok, But first are my electric field vectors correct the way I have them set up? Should I figure out forces for each of those vectors?
     
  5. Jan 27, 2004 #4

    chroot

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    Yes, Ashley, the directions are correct. Now, find the magnitude of each vector (use Coulomb's law). Finally, add the vectors together.

    As himanshu says, it's easiest to add vectors when you select a basis and represent each vector with components in that basis. Adding the vectors then just involves adding the components.

    - Warren
     
  6. Jan 28, 2004 #5
    Since M is the midpoint between the charges 2 and 3 I'm thinking the net electric field at M is zero because making rough digrams of the vectors they make a triangle. Am I on the right track or am I totally off base?
     
  7. Jan 28, 2004 #6
    Yes if M is the Centre of Square
     
  8. Jan 28, 2004 #7
    Well M is the midpoint of the line between charges 2 and 3 not the center of the square
     
  9. Jan 28, 2004 #8

    chroot

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    No, the total field at M is not zero.

    Go ahead and represent each of the four vectors in the (east, north) basis and add them together.

    - Warren
     
  10. Jan 28, 2004 #9
    No its not 0 Can we have ur sol how u got 0
     
  11. Jan 28, 2004 #10
    Ok, how I see it looking at the electric field vectors coming from this point M, electric field vectors point away from positive charges and toward negative charges. SO From M there is a vector pointing east away from q2 and another one pointing east toward q3 so those two vectors can be easily added together because they both go east on the same line. So together that is one vector.
    Now the q1 is negative so there is a vector from M going towards q1 making a second vector going northwest.
    Last there is a vector going southwest away from q4 because it is positive.
    The last two vectors when I draw them to add all the vectors i get a triangle.
    SO yea I'm completely confused
     
  12. Jan 28, 2004 #11

    chroot

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    Ashley,

    Do what we continue to ask you to do. Express each vector as a pair of numbers.

    The angle between the point M and the point q1 is 63.43 degrees, or (180 - 63.43) if you're counting from the east axis.

    The angle between the point M and the point q4 is similarly 63.43 degrees, from the east axis.

    First, verify these numbers. It's just basic geometry.

    - Warren
     
  13. Jan 28, 2004 #12
    Ah forget it
     
  14. Jan 28, 2004 #13

    chroot

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    We're trying to help you understand how to do this problem. Drawing the vectors, while useful for visualizing the problem, is not going to help you find the answer.

    How did you teacher explain these problems?

    - Warren
     
  15. Jan 28, 2004 #14
    That's the problem he hasn't explained how so I'm trying to teach myself.
     
  16. Jan 28, 2004 #15

    chroot

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    Well, we're trying to help. Do you understand how to represent a vector with components?

    - Warren
     
  17. Jan 28, 2004 #16
    Vaguely, it's been about two years since I took physics 1
     
  18. Jan 28, 2004 #17

    chroot

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    Okay. The first step is to figure out the angle and direction of the each vector that sticks out of the point M. There are four vectors (although you can easily combine two of them, as you said).

    First, list the angles (counted clockwise from the east direction) and magnitudes of each vector. Can you do this? It's just basic geometry, solving triangles.

    - Warren
     
  19. Jan 28, 2004 #18
    ok I got 63.43 from the east
     
  20. Jan 28, 2004 #19

    chroot

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    List the ANGLE and MAGNITUDE of EACH of the FOUR vectors.

    - Warren
     
  21. Jan 28, 2004 #20
    Vector 1: 0 East
    Vector 2: 0 East
    Vector 3: 63.43 North of East
    Vector 4: 63.43 South of East
     
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