Work required to assemble charged particles

[SilverGirl]

Homework Statement

How much work is required to assemble eight identical charged particles, each of magnitude q, at the corners of a cube of side s?

Homework Equations

W=deltaU

delta U = kQq/r

The Attempt at a Solution

I've come up with those equations, and was trying to plug the information into delta U. However, all the charges are identical, so there are not two different values for q. But would it be delta U = kq^2/s ??

 

[Almanzo]

I take it that the charges come from infinity.

The work required to bring them in from infinity is the sum of the electrostatic energies of the pairs of charges. With 8 charges there are 8*7/2 = 28 such pairs. These can be viewed as the 12 sides of the cube, its 12 surface diagonals and its 4 body diagonals. Be sure to remember that the energy is inversely proportional to the distance, not (like the force) to the distance squared.

 

[SilverGirl]

I take it that the charges come from infinity.

The work required to bring them in from infinity is the sum of the electrostatic energies of the pairs of charges. With 8 charges there are 8*7/2 = 28 such pairs. These can be viewed as the 12 sides of the cube, its 12 surface diagonals and its 4 body diagonals. Be sure to remember that the energy is inversely proportional to the distance, not (like the force) to the distance squared.

Yeah it doesn't say where the charges are coming from.

8*7/2 ... where does the 7/2 come from? How are you getting 28 pairs?

12 sides of a cube? Aren't there only 6?

 

[SilverGirl]

Anyone?

 

[SilverGirl]

Does anyone know how they got the 28 pairs?