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Homework Help: Finding charge - problem

  1. Mar 24, 2015 #1
    1. The problem statement, all variables and given/known data

    Charge 1 (0.15 kg) in the following diagram is hanging from a light insulating thread, as shown. Charge 2 is brought close and pulls charge 1, deflecting the string away from the vertical axis. The charges have the same magnitude, but charge 1 is negative.

    A) Find the tension in the thread

    B) Find the charge on charge 2. Is it positive or negative? Explain your reasoning.

    I'm satisfied with my answers, however, I'm almost certain I haven't "listed" everything correctly. (example, putting FT in the wrong place where a different notation should be listed...)

    I'm taking notes from the tips that I've been given in previous threads (just want to make it clear that I'm actually storing this information in order to avoid having to scramble every time a new problem arises).


    2. Relevant equations

    Fg = m g
    Fe= K q2 / r2

    3. The attempt at a solution


    Ft = m g
    Ft = (0.15) (9.8) / cos 40
    Ft = 1.9

    The tension in the thread is 1.9 N


    Ft = k q2 / r2
    Fe = (9.0 x 109) q2 / 0.42
    q = √ (0.402) (1.9 sin40) / 9.0 x 109
    q = 4.7 x 10-6 C

    Charge 1 (given as negative) attracts Charge 2, thus charge 2 is positive.

    Attached Files:

  2. jcsd
  3. Mar 24, 2015 #2


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    Science Advisor
    Homework Helper
    2017 Award

    I agree: reading
    Ft = m g ​
    on one line, and then
    Ft = (0.15) (9.8) / cos 40​
    on the next makes my skin crawl. The 1/cos(40) seems to come out of the blue sky. Something like
    Fty = m g
    | Ft | = (0.15) (9.8) / cos 40​

    is more understandable. You can then continue with
    Ftx = | Ft | sin(40) = k q2 / r2 etc​
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