Suppose that a charged particle of diameter 1.00 micrometer moves with constant speed in an electric field of magnitude 1.00*105 newtons per coulomb while acted upon by a drag force of 7.25*10-11 newtons. what is the charge q1 on the particle? ignore the effects of gravity.

It's a bit of a trick question, had the same one on my homework. You're given an electric field strength (1*10^5 N/C for mine), a drag force (7.25*10^-11 N) and the critical info is that it's moving with constant velocity (the particle is in equilibrium/not accelerating).

All you need is F = (K*Q1*Q2) / r^2

Just set F=the drag force and the electric field strength is (K*Q2) / r^2, plugging those values in gives you

(7.25*10^-11 N) = (1*10^5 N/C) * Q1 - - - > Q1 = 7.25*10^-16 C