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  1. #1 electric field 
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    Find the electric field at a point midway between two charges of + 3.0 x 10^-9 C and + 60 x 10^-9 C seperated by a distance of 30 cm.


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  3. #2 Re: electric field 
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    Quote Originally Posted by european SENS
    Find the electric field at a point midway between two charges of + 3.0 x 10^-9 C and + 60 x 10^-9 C seperated by a distance of 30 cm.

    Find the electric field for each one at the desired distance and use superposition.

    Cheers


    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  4. #3  
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    what is superposition.
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  5. #4  
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    so the equation for electric field is E = [k*q] / r^2

    so we have

    [9 x 10^9 * 3.0 x 10^-9] / 30^2

    and

    [9 x 10^9 * 60 x 10^-9] / 30^2

    Now what do i do with them
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  6. #5  
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    Quote Originally Posted by european SENS
    so the equation for electric field is E = [k*q] / r^2

    so we have

    [9 x 10^9 * 3.0 x 10^-9] / 30^2

    and

    [9 x 10^9 * 60 x 10^-9] / 30^2

    Now what do i do with them

    Firstly, I think the distance you want is half-way, right?

    Then imagine the directions of the fields and they will either add or subtract. That is superposition.


    Cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  7. #6  
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    so yes they should be halfed...

    [9 x 10^9 * 3.0 x 10^-9] / 15^2

    and

    [9 x 10^9 * 60 x 10^-9] / 15^2

    and i think that they would face opposite directions since the same charges repel one another. does that mean i subtract the two?
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  8. #7  
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    Quote Originally Posted by european SENS
    and i think that they would face opposite directions since the same charges repel one another. does that mean i subtract the two?
    Sounds reasonable....
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  9. #8  
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    do i subtract the top from bottom or bottom from top to get my final answer?
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  10. #9  
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    Quote Originally Posted by european SENS
    do i subtract the top from bottom or bottom from top to get my final answer?

    Either way, but try to imagine which direction the result will point.
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  11. #10  
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    0.12 - 2.4 = -2.28

    and

    2.4 - 0.12 = 2.28

    im thinking that the negative answer is correct since they repel and dont face one another.
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  12. #11  
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    Quote Originally Posted by european SENS
    0.12 - 2.4 = -2.28

    and

    2.4 - 0.12 = 2.28

    im thinking that the negative answer is correct since they repel and dont face one another.

    Well, 2.28 is the magnitude (assuming the math is correct - I'm too lazy to check).

    But what you want to imagine is, if you placed a positive test charge at that location, which way would it move?



    Generally, there are two ways to do these type of problems;
    1. Keep track of all the signs and make sure you do everything correctly (e.g., adding instead of subtracting). This, I would call the meticulous approach. For me, personally, this gets tedious. But that's just me.

    or
    2. Play fast and loose, use intuition, and imagine the final result. This, as you might have guessed, is what I usually do.


    Cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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