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    I really need help on how to do these types of questions, I'm assuming that it requires use of ratios, perhaps using the following equation:
    F = kQ1Q2/r^2
    I'm really unsure on what to do... Any help would be greatly appreciated!
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    (Original post by 1017bsquad)
    Screenshot link: http://prntscr.com/aljqyv
    I really need help on how to do these types of questions, I'm assuming that it requires use of ratios, perhaps using the following equation:
    F = kQ1Q2/r^2
    I'm really unsure on what to do... Any help would be greatly appreciated!
     F=\frac{kQ_1Q_2}{d^2}

    You know that at 20mm, the force is 0.5F. You want to double your force, so you would need your denominator to decrease by a factor of 2 (if your denominator is twice as small, your fraction will be two times larger, which you want).

    A common mistake would be to say, well, let's just half 20 and that will reduce the denominator of our fraction by a factor of 2. However, because of the squared term, you'd decrease your fraction by a factor of 4, which is not what you want.

    What you need is for your distance to be  \sqrt{2} times smaller so that when it is squared, you then that factor of 2 at the bottom. Hence, you do  \frac{20}{\sqrt{2}} = 14 (2sf).
 
 
 
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