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Homework Help: Calculate the speed of a particle?

  1. Apr 25, 2013 #1
    1. The problem statement, all variables and given/known data
    If you have an electron in the following electrostatic field:

    ------------------------- (+)
    (-) B 2m from origin

    (-) A 1m from origin

    ------------------------- (-) Origin

    Okay, so; the force field is pushing the electron from point A to point B.

    It has more potential energy at point A than point B, meaning point B is moving faster than point A.

    So I have to find the velocity of the electron at point A assuming that its HALF of the speed at point B

    2. Relevant equations
    F = kq1q2 / r^2
    E = kq1q2 / r

    Ek = 1/2 m v^2
    Et1 = Ek + Ep
    Et1 = Et2

    3. The attempt at a solution
    If we move an electron from point A to point B, we do negative work on it. How is this possible? I don't understand this at all.

    Also, if it gains 10 J of potential energy from point B to A, and we have to calculate its speed.. How is this possible?

    We have the equation

    Ek ---> transfer to ----> Ep

    So our change in kinetic energy is now negative.

    But we can't take the square root of a negative value, so we can't solve for velocity
    Last edited: Apr 25, 2013
  2. jcsd
  3. Apr 26, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Who is this "we" that you speak of?
    You have worked out that the electron has negative work done on it - by what?

    If you physically pushed the electron from B to A, you would have to do positive work on it right?

    How about doing it the other way around?

    Note - the negative KE is a negative change in KE.
    It just means you end up with less than you started - which is find because you know that the speed at A is less than the speed at B.
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