# Homework Help: 3 excess electrons calculate the radius

1. Feb 18, 2010

### Mccleaves

3 excess electrons...calculate the radius...coaxial cable...

1. The problem statement, all variables and given/known data
An oil droplet with 3 excess electrons is held stationary in a field of 4.24x104N/C.
What is the radius of the oil drop?( The density of the oil is 824 kg/m3,e=1.60x10-19C)

2. Relevant equations
F=qE=mg
m=(density)(volume)

3. The attempt at a solution
well i assumed that the droplet was a sphere and so the volume would be (4/3 $$\pi$$r3. So i set F=qe=mg because the droplet is stationary so the net force is zero. And the mass = density x volume so i just plugged every thing in and solved for r ...i got 8.43 x 10-7....can some one verify this ..i am not really confident lol

1. The problem statement, all variables and given/known data
The cross section of a long coaxial cable is shown, with radii as given. The linear charge density on the inner conductor is-10nC/m and the linear charge density on the outer is -40nC/m. The inner and outer cylindrical surfaces are respectively denoted A,B,C and,D ad shown. The linear charge densities on surfaces C and D, in nC/m, are closest to:
A) +10 and -40
B) -10 and -30
C) 0 and -40
D) +10 and -50
E) -30 and -10

2. Relevant equations
I'm not exactly sure what to do...
3. The attempt at a solution
The only thing i could think of was that the system is a conductor so the inside charges must be equal to zero and so i thought that what ever the inside charges added up to then the outside charge would be the same magnitude as the sum of the inside charges but opposite in sign. I don;t even know if that is conceptually correct ...please help ...i don;t think i understand the underlying concept of this question...let alone actually solving it haha.

Last edited: Feb 18, 2010