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Homework: Coulomb's Law

  1. Jan 14, 2014 #1
    1. The problem statement, all variables and given/known data
    A point charge q1 = -3.9 μC is located at the origin of a co-ordinate system. Another point charge q2 = 7.3 μC is located along the x-axis at a distance x2 = 7.8 cm from q1.
    1) What is F12,x, the value of the x-component of the force that q1 exerts on q2?


    2. Relevant equations
    F=(k*q1*q2)/r^2


    3. The attempt at a solution
    [(-3.9*10^-6 C)*(7.8*10^-6 C)*(9*10^9 C)]/(0.078^2) m= -45 N

    but the answer is wrong I converted everything into metric units.
     

    Attached Files:

  2. jcsd
  3. Jan 14, 2014 #2

    Simon Bridge

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    Converting to SI units can simplify things, but if you are frequently using different units it can work out better to convert the constants instead.

    k=89.876N.cm2/μC2

    I think you just had some rounding issues ... I'm getting a slightly different number off the same calculation.
     
  4. Jan 14, 2014 #3
    when I used your k value with no conversions I got -42.1 N. I guessed the program wanted a more accurate solution thanks.
     
  5. Jan 15, 2014 #4

    Simon Bridge

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    When I used yours - I got 42.115N and mine got me 42.057N ...

    With conversion the calculation works out as:

    90*3.9*7.3/7.8^2 = 42.115
     
  6. Jan 15, 2014 #5
    must have been the order with my calculator . I have a ti-30x IIS which is harder to track parenthesis and slightly longer operations. thanks again
     
  7. Jan 17, 2014 #6

    lightgrav

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    depends on whether q2 = 7.3 μC , or 7.8 μC .
     
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