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# MCQ Physics Unit 4 - Charges in an electric field! watch

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!
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!

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 times smaller so that when it is squared, you then that factor of 2 at the bottom. Hence, you do (2sf).

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