Question

What must the charge (sign and magnitude) of a particle of mass 1.44 g be for it to remain stationary when placed in a downward-directed electric field of magnitude 680 N/C

Answer 1

Q E = m g electric and gravitational forces balance - both E and g are taken as positive in the downward direction

Q = m g / E = .00144 kg * 9.8 m/s^2 / 680 N/C = 2.08E-5 coul

This is the charge required for the specified force

Therefore Q = -2.08-5 C for the charge to remain stationary