# Homework Help: External force physics

1. Feb 7, 2009

### phy112

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

How much work does it take for an external force to set up the arrangement of charged objects in the diagram on the corners of a right triangle? Assume that q = -5.2 µC, that d = 19 cm, and that the three objects are initially very far away from each other.

2. Relevant equations

W=Fed =qEd

3. The attempt at a solution
I have no clue how to start. I figured out the potential to be -1.15, I just need a nudge in the right direction

2. Feb 7, 2009

### Phrak

Re: work

Bring the charges in from infinity, one at a time?

'd' does not completely specify a right triangle.

3. Feb 7, 2009

### phy112

Re: work

one of the sides is 12 cm (y axis). The other side (d) is 19 cm (x axis). The hypotenuse is 22.5. I couldnt load the pic onto this website

4. Feb 7, 2009

### phy112

Re: work

also charge 1 is 5.5e-6C charge 2 is -5.2e-6C and charge 3 is 2.5e-6 C

5. Feb 7, 2009

### Phrak

Re: work

Find the work by integration

$$W =\int{eE(r)dr}$$

To start, put one charge at its vertex, find E(r) for the charge. Bring another charge to it's vertex, integrating the work.

For the last charge, the work done is the superposition due to the other two charges.