# Homework Help: Charge and electric field

1. Sep 1, 2008

### ibaraku

1. The problem statement, all variables and given/known data
Two particles with equal mass have charges 1.0uC and 1.0nC, respectively. What must be their mass if their mutual gravitational attraction is to overcome their Coulomb repulsion?

2. Relevant equations

F = kQ1Q2/r^2

and maybe

n = Mbody/M

3. The attempt at a solution

At first I thought I could use the first formula, where k = 9 x 10^9 Nm^2/C^2, Q1 = 1uC and Q2 = 1nC
but then i realized that
1) I don't have the distance and
2) I have no clue what the values of "u" and "n" are.

So then I thought, to use the second since I DO need the mass, but I do not know thw values of n and M.

I am lost, any help is much appreciated!!!
Thanks

2. Sep 1, 2008

### johnj7

First, what are you referring to with
n = Mbody/M?

There are two kinds of force equations you need
describing
(1) gravitational attraction
(2) electrostatic attraction

you have the second one, ie coulombs law...
what is the equation for gravitational attraction?
ie... F=.....

when you get that, you can equate the equations to see what masses you need to overcome the electrostatic repulsion

oh also, the "u" and "n" denote powers of 10
just like cm = 10^-2 and mm = 10^-3
google for these values, you should learn them, they will help for all your science classes

3. Sep 1, 2008

### ibaraku

I was going through my book, and saw this formula to obtain the number of water molecules in an object, I thought it could be helpful since mass is involved

I see, you are talking about
F=G(mass1)(mass2)/d^2

how many power of 10 does "u" represent???
and "n"???

Thanks for your help, I took physics about 2 years ago and I am having problems with this class

4. Sep 1, 2008

### johnj7

here you go, this should help
http://www.csudh.edu/oliver/chemdata/prefix.htm [Broken]

u = micro
n = nano

Last edited by a moderator: May 3, 2017
5. Sep 1, 2008

### Redbelly98

Staff Emeritus
Often, "u" is used to represent the Greek letter mu when you are limited to standard keyboard text characters. So micrometers are written um, microcoulombs are written uC, etc.

edit: johnj7 beat me to it

6. Sep 1, 2008

### ibaraku

Thanks a lot!!!

Last edited by a moderator: May 3, 2017