# Electrostatics question, involving mass?

1. May 2, 2013

### hangingwire

1. The problem statement, all variables and given/known data
One electron has a mass of 9.11x10^-31 KG. Find the force 1.0 gram of electrons would exert on another 1.0g of electrons from 1.0 kilometer away

F = ?
Melectron = 9.11x10^-31 kg --> (0.01kg of electron = 9.11x10^-33 kg)
Electron Coloumb = 1.6x10^-19 C (ignoring the negative sign b/c it is a vector)
R = 1000m

2. Relevant equations

F=(kQ1Q2/R^2)
Since they are the same mass

F = kQ^2/R^2

3. The attempt at a solution

K = 9x10^9
R = 1000^2

What is the value for Q? I am not sure because any value I try to enter is invalid.

Answer is 2.8x10^20N and I am getting a negative power answer when I try. Can someone steer me in the right direction please?

2. May 2, 2013

### SammyS

Staff Emeritus
What is the number of electrons need so that you have a mass of 1 gram of electrons?

3. May 2, 2013

### hangingwire

1/(9.11x10^-28) = 1.1x10^27 electrons per gram.

if one electron is 1.6x10^-19c

(1.1x10^27)(1.6x10^-19) = 1.76x10^8 is one charge of a gram of electrons

Plugging it in...

(9x10^9)x(1.76x10^8)^2
-------------------------
(1000m)^2

answer = 1.584x10^12 CORRECTED! --> 2.78784x10^20 which is correct

Still off :|

Edit: NVM! Correct answer. Stupid Casio fx-300ES calculator got it mixed up somewhere. Hate how I have to keep pressing Shift to get the answer in decimals -.- Thank you!

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