How much work is done when moving a charge?

[iceman2048]

Homework Statement

In Figure 24-60, we move a particle of charge +2e in from infinity to the x axis. How much work do we do? Distance D is 6.30 m.

http://img216.imageshack.us/img216/6987/fig2460.th.gif

Homework Equations

U = kQq/r
U=-W

The Attempt at a Solution

Ok since there's two stationary charges (2e and e), I figured you could use the superpositioning principle and claculate the work done to move the 2e from infinity to the x-axis against the stationary 2e plus the work done to move it against the stationary e charge. Summing them would give the answer.

U1= (k)(2e)(2e)/(2d) = 7.233E-29 J

U2 = (k)(e)(2e)/d = 7.23E-29 J

U(total) = 7.23E-29 + 7.23E-29 = 1.4466E-28 J

U = -Work
1.4466E-28 = -Work
Work = -1.4466E-28 J

Is my assumption that the superpositioning principle can be used false? Thank you in advance.

 

[Doc Al]

Your use of superposition is perfectly correct (although I didn't check your arithmetic). One error is in the sign of the work done: The work done equals the change in energy, in this case W = ΔU (not -ΔU).

 

[iceman2048]

Thank you very much Doc, you've saved the day once more! I just have one more question left that I'm struggling with but I'm going to try it some more. If I need help, I'll start a new thread to make everything more organized.

Thank you once again Doc!