# Homework Help: Floating Cork Ball.

1. Feb 4, 2005

### Bailey

not sure why i keep getting it wrong.............maybe caz i misunderstood some concept or something. anhyoo.......here is it:

A cork ball of mass 5.20 g is placed between two very large horizontal plates. The bottom plate has a uniform charge density of +0.22E-06 C/m2, whereas the upper plate has a uniform charge density of -0.06E-06 C/m2. The cork ball, which carries an unknown charge, is placed between the plates and is observed to float motionlessly. What is the charge on the ball?

how i plan to solve it: since it give a mass, so i can find the force its acting downward due to gravity, which is "mg".

n since the ball is not moving up or down, there must be a force equal in magnitude but opposite in direction pointing upward). thus the charge of the ball should be positive.

i used E = (surface charge density) / permittivity of free space (8.85x10^-12)

then use F = qE ----> q = F/E

i ended up with 2.01E-16 C .............but its wrong.
anyone know why?

2. Feb 4, 2005

### Bailey

omg.lol. never mind. our prof emailed us witha hint on this quesiton -_-"

the magnitude of the electric field = 1/2(sigma/epsilon)

epsilon = permittivity of free space.

well i ended up with 3.22x10^-6 C , which is correct. (but wasted so much time on it)