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Homework Help: Electric Fields

  1. Feb 10, 2005 #1
    An electron is placed in an electric field of strength 300 N/C. What is the magnitude of the force the electron experiences?
    1.6x10^13 N
    2.4x10^13 N
    3.2x10^14 N
    4.8x10^14 N
    6.4x10^14 N

    A particle of mass 0.005 kg is given a charge of +4.0 µC and is placed in an electrical field that is directed antiparallel to the earth's gravitational field. What is the field strength, expressed in N/C, if it balances the weight of the particle?
    1.6 x10^6
    2.4 x10^6
    3.2 x10^6
    4.4 x10^6
    4.7 x10^6

    Wen I tried to do these questions, I did not even come close to the figure in bold. I thought F = Eq or F = mg/q would be sufficient enough, but I guess not. Can anyone explain how to arrive at each answer?

  2. jcsd
  3. Feb 10, 2005 #2
    F = Eq is the correct equation to use for the first part, but all those answers listed are wrong.
    q is the charge of an electron = 1.6 * 10^-19 C,
    Just look at the exponents. You would have to have an E in the order of 10^33 N/C in order to come anywhere near those numbers. Are you sure you copied the question correctly?

    For the second, I think you mean E = mg/q not F = mg/q. Again, all the choices are wrong? Where did you get these questions?
  4. Feb 10, 2005 #3

    Andrew Mason

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    None of these answers are even close to being right. The force is qE. If E is 300 and q is 1.6e-19 Coulomb, qE = 4.8e-17 N.

    Again: F = qE, but in this case F also = mg, so qE = mg; E = mg/q

    Plugging in the numbers, E = 5e-3*9.8/4e-6 = 1.225 e4 N/C Again none of the answers fit.

    Where did you get these questions?

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