How Many Excess Electrons or Protons Does a Suspended Water Drop Have?

[arod2812]

Homework Statement

A small drop of water is suspended motionless in air by a uniform electric field that is directed upward and has a magnitude of 10800 N/C. The mass of the water drop is 3.99 x 10-9 kg. How many excess electrons or protons reside on the drop?

Homework Equations

which equation do i use? Do i need to use the mass of a proton to find the number of excess protons?

The Attempt at a Solution

F=ma; E=F/q

 

[blochwave]

The force from gravity acting on the rain drop, Fg=mg, will need to equal the force from the electric field, F=E*q, for the drop to stay motionless

 

[arod2812]

And how do I solve for how much protons are present?

 

[blochwave]

(Or electrons)

Well you're going to solve for q, so you'll know the NET charge on the raindrop. You know the charge of a SINGLE proton or electron