F = q1q2/r^2, what is r?

[mmcoffee]

Shouldn't it be r = 0.5 here? Anyone know why they used 1? I thought r = radius not distance b/w points.


Thanks

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[Cawolf]

r is the distance between the points - these are point charges, they have no radius and are infinitely small.

Both point charges are 1 m from the test particle.

Their answer is right, but I don't like their explanation.

I would personally calculate the force from each particle separately (or the total electric field strength if you prefer) and then sum them to show that the interaction with q1 is a force of +2k and the interaction with q2 is a force of +6k.

Either way, the distance at t = 0 between each point is 1 m and this is value you should use for r.