# Homework Help: HW help!

1. Apr 2, 2008

1. The problem statement, all variables and given/known data

[1] Sphere A has a radius of 10.0 cm and carries a charge of +5.0 x 10^(-8) C
while sphere B has a radius of 5.0 cm and carries a charge of -2.0 x 10^(-8) C.
The spheres are conducting hollow, and concetric.

A) calculate the magnitude and direction of the electric field (E) at a point
3.00 cm from the center of the arrangement.

B) Calculate E at 6.00 cm from the center.

C) Calculate E at point 12.0 cm from the center.

D) Calculate the potential difference between the outer surface of the inner
sphere and the inner surface of the outer sphere.

E)A very small particle [ m = 2 x 10^(-9) kg, q = 1.5 x 10^(-9) is released from
the outer surface of the inner sphere. Calculate the speed with which it will
strike the inner surface of the outer sphere.

2. Relevant equations

E = q / eo = òE·dA [not sure im using the right formula]

3. The attempt at a solution

I got an answer for a = 280
B) 70
C) 17.5
D) 15.7
E) no idea

But all of them are wrong. This problem is really irritating me, someone please help me out.
1. The problem statement, all variables and given/known data

[2] Two sheets od metal (A = 5.0 cm^2 are parallel and separated by 2.0 mm.
The potential difference between them is 1500V.
A) Calculate the electric field that exist between the plates.

B) Calculate the charge on the postive plate on the postive assuming the plates
have equal but opposite charges.

C) Calculate the work done in separating the plate to 3.0 mm apart.

2. Relevant equations

3. The attempt at a solution

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 2, 2008

### HallsofIvy

This looks more like a physics problem than a math problem so I am going to move it to the physics homework section.

And I'm no physicist (as Doc Al has pointed out) but I was under the impression that the field inside a conducting sphere would be 0- so (a) would be 0. Similarly, the field in (b), where the point is between the two spheres, depends only on the smaller sphere.

3. Apr 2, 2008

my bad i meant to post in physics section.

anyway thanks for the info. anyone else?

4. Apr 2, 2008

### HallsofIvy

What is E for a point charge of -2.0 x 10^(-8) C at distance 6.00 cm?

What is E for a point charge of -2.0 x10^(-8)+ 5.0 x 10^(-8) C= 3.0 x 10^(-8)C at a distance of 12.0 cm?

Just integrate.

After you have done D this is just a matter of "conservation of energy". The decrease in potential energy is the increase in kinetic energy.
(Doc Al is now welcome to correct all my mistakes.)

Last edited by a moderator: Apr 2, 2008