Calculating Force Between Two Conducting Spheres

[8parks11]

Homework Statement

Two conducting spheres of the same size have charges q1=-4x10^-6
and q2 = 8x10^-6C,and their centers are separated by 3.0cm

a) what is the force between them?
b) what would be the force if the spheres were touched together and again separated by the same distance

Homework Equations

well we could use F=kq1q2/r^2

The Attempt at a Solution

for a) I plugged in 9x10^9 x -4x10^-6 x 8x10^-6 all divided by .09 which gives -3.2 but the answer is -320N.can someone explain my why?

for b) i don't know how it would start.
i tried doing several things. use the -320 as the separated force and
get the force of the one together and add them together.

something like that but i don't know how it gives 40N.

 

[8parks11]

any help.
i still can';t figure this basic problem out

 

[twotaileddemon]

Well... for a
F = kq1q2/r^2.. like you said. I plugged it in my calculator, and did
(9x10^9)(4x10^-6)(8x10^-6)/(.03)^2 and got 320 N... perhaps you did a calculation error, but that is how you set it up. I don't remember in class directly putting the negative into the equation, but perhaps the force is negative because you have opposite charges? (I'm not sure on direction)

As for b, if both spheres touch each other, afterwards they must have the same charge. If the spheres touch, then they will receive the same charge (2 x 10^-6 C). Therefore, just apply the new values of q to the equation.. and you should get something like
F = kq1q2/r^2 = (9x10^9)(2x10^-6)(2x10^-6)/(.03)^2 = 40 N

so a is a) 320 N (not sure about direction) and b is 40 N (also not sure of direction) ... do you see why?

 

[twotaileddemon]

Oh.. and the reason the spheres each receive 2 x 10^-6 charge is because that is the amount of charge that is in the middle of both charges... notice how the first has -4 x 10^-6 and the second has 8 x 10^-6.. well the middle of -4 and 8 is 2, so that's how much each charge has! Just remember that when two spheres touch each other (of opposite charge), they must end up with the same charge.. so just take the middle of it!

 

[8parks11]

thanks for the great explanation.

the directions are just attractice and repulsive respectively.

 

[twotaileddemon]

Oh okay ^^. You're welcome.. I hope it helped!