1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Another Electric Field Question

  1. Jan 7, 2005 #1
    So the question reads:

    "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?"

    The correct answers are:
    (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. :smile:
  2. jcsd
  3. Jan 7, 2005 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

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

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook