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Math Help - A bit more physics help

  1. #1
    Member Jones's Avatar
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    A bit more physics help

    Hi, again...
    I have another problem im unable to solve =,(
    The task is to calculate the elecrical field in the point P. I do know we need to use coloumbs law
    but i don't how the force from the two charged particles affects P.
    An image i made to make it more legible

    Last edited by Jones; September 16th 2006 at 10:50 AM.
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Jones View Post
    Hi, again...
    I have another problem im unable to solve =,(
    The task is to calculate the elecrical field in the point P. I do know we need to use coloumbs law
    but i don't how the force from the two charged particles affects P.
    An image i made to make it more legible

    The formula for the electric field is:
    E = (kq)/r^2
    Where k is the Coulomb constant 9.0 x 10^9 Nm^2/C^2, q is the charge at some point away, and r is the distance between the charge and the observation point.

    Now, E is a vector, so what you need to do is determine the magnitude and direction of E due to each charge, then add the E values vectorally. The rule for assigning the direction to E is simple: The E field lines always radiate outward from a positive charge, and radiate inward to a negative charge. (So E due to the charge at point A is downward, and E due to the charge at B is to the left.)

    I'll leave it here for now. If you have problems adding the vectors (most students have forgotten most of what they know about adding vectors by the time they start electric fields) just let me know.

    -Dan
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  3. #3
    Member Jones's Avatar
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    Quote Originally Posted by topsquark View Post
    The formula for the electric field is:
    E = (kq)/r^2
    Where k is the Coulomb constant 9.0 x 10^9 Nm^2/C^2, q is the charge at some point away, and r is the distance between the charge and the observation point.

    Now, E is a vector, so what you need to do is determine the magnitude and direction of E due to each charge, then add the E values vectorally. The rule for assigning the direction to E is simple: The E field lines always radiate outward from a positive charge, and radiate inward to a negative charge. (So E due to the charge at point A is downward, and E due to the charge at B is to the left.)

    I'll leave it here for now. If you have problems adding the vectors (most students have forgotten most of what they know about adding vectors by the time they start electric fields) just let me know.

    -Dan

    And that would give us something like this:

    But how long should E1 and E2 be?
    Last edited by Jones; September 16th 2006 at 01:01 PM.
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  4. #4
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by Jones View Post
    And that would give us something like this:

    But how long should E1 and E2 be?
    E1 = k(q1)/(r1)^2 and Ex = k(q2)/(r2)^2.

    And E1 is in the direction of your "other" left. It's a negative charge so the field lines point toward it, not away from it.

    Specifically: E1 = (9.0 x 10^9)(36 x 10^(-6))/(6.0 x 10^)-2))^2 N/C
    E2 = etc.

    -Dan
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  5. #5
    Member Jones's Avatar
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    Quote Originally Posted by topsquark View Post
    E1 = k(q1)/(r1)^2 and Ex = k(q2)/(r2)^2.

    And E1 is in the direction of your "other" left. It's a negative charge so the field lines point toward it, not away from it.
    -Dan
    I know that, just wanted to make sure you knew....

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