# Homework Help: Another Electric Field Question

1. Jan 7, 2005

### thursdaytbs

"A small drop of water is suspended motionless in air by a uniform electric field that is directed upward and has a magnitude of 8480 N/C. The mass of the water drop is 3.50 x 10^-9 kg. (a) Is the excess charge on the water drop positive or negative? Why? (b) How many excess electrons or protons reside on the drop?"

(a) Positive, so that the electrostatic force points upward.
(b) 2.53 x 10^7 protons

I really wasn't sure where to begin, so I wrote some equations down.
I tried using F=Eq, or E = F/q where the E = 8480, but I wasn't sure about F or q. There was an equation that said something = q/A but I don't have A or q.

I think the main part i'm missing is converting the 3.50 x 10^-9kg of H2O into how much charge it has. Can anyone give me some direction as to where to go for this problem? Any help is greatly appreciated, Thanks.

2. Jan 7, 2005

### Andrew Mason

You are correct that F=qE gives the upward force. What is the downward force (hint: think mechanics not electricity)?

AM