What is the unknown charge (magnitude and sign)?
What is the magnitude of the force that the unknown charge exerts on the charge of magnitude -0.600 micro-Coulombs?© BrainMass Inc. brainmass.com September 25, 2018, 8:47 pm ad1c9bdddf - https://brainmass.com/physics/coulombs-law/forces-two-charged-particles-each-other-201965
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First of all, the charges attract each other. Hence they have opposite signs.
Since the known charge (q1) is negative, the unknown charge (q2) must be positive.
The formula for the magnitude of the Coulomb force is
F = (k *q1 * q2) / ...
The solution shows how to apply Coulombs law to this electrostatic system. The unknown charge with the magnitude and signs are determined.