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Homework Help: Electricity/Electric Field Questions

  1. Apr 19, 2005 #1
    I've attached the questions from a worksheet.


    For starters, how do i do the first one? i know Fe = q*E, but what is the charge? is it just 2 x 1.6x10^16 (because there are two protons?)

    for the rest, here are the answers i got, could anyone confirm any? i'm not sure about these.

    2) a) Fe = 0.45 N
    b) E = 4.5 x 10^9 N/C
    3) E = 250 N/C
    4) Q = 3.0 x 10^-6 C
    5) E = 5.4 x 10^10 N/C to the right

    Thanks again!

    Attached Files:

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    Last edited: Apr 19, 2005
  2. jcsd
  3. Apr 19, 2005 #2
  4. Apr 19, 2005 #3


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    1.[itex] q_{p}=e\simeq 1.6\cdot 10^{-19}C [/itex]

    Please show us how u did them.Numbers are least important when solving a problem.

  5. Apr 20, 2005 #4

    Andrew Mason

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    1. check your value for e. The charge of an alpha particle is 2e
    2. a) correct: F = kqQ/r^2
    b) correct: E = F/q
    3. correct: E = F/e
    4. correct magnitude but check the sign.
    5. incorrect. check distance value.

  6. Apr 20, 2005 #5
    Andrew, why is the charge of an alpha particle 2e? I thought an alpha particle was electron-less.
  7. Apr 20, 2005 #6


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    It is.It's +2e,because of the 2 protons inside the nucleus...

  8. Apr 20, 2005 #7
    Ok. The plus sign makes it all better :)
  9. Apr 20, 2005 #8
    ok, so

    1) Fe = 2*1.6x10^-19*75
    and i get 2.4 x 10^17 N

    4) isn't the sign + ? if the field is directed away from it, doesn't it mean its a positive charge in Q?

    5), i used the radius of 1, when it should be 1/2. using 1/2, i get
    2.16 x 10^11 N/C right

    thanks again for all the help!
  10. Apr 20, 2005 #9

    Andrew Mason

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    Just a small slip on the exponent: 2.4 x 10^-17 N (but a huge difference). What is Fe? Shd be: F = qE where q = 2e.
    A trick question. The first sentence is irrelevant and designed to confuse - it succeeded. You are right. The field direction is given by the direction of motion of a + charge, which is away from Q so Q is +.


  11. Apr 20, 2005 #10
    thanks for all this help, i was able to do all the problems from today (and get them right). :biggrin:
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