Electric Potential/Equilateral Triangle

[Siirous]

I'm stuck on this one, which I thought would be simple, hoping for a little nudge in the right direction.

The problem states an equilateral triangle, sides of .5mm (which I converted to .5E-3m), all equal charges of 4pC.

How much work must be done to move one charge to a point equidistant from the other two and on the line that joins them.

So here's how I went about figuring out how much work to move one point in between the other two. I set the triangle up with two points on the x-axis, .25E-3 meters from the origin, and the third point on the +ve y axis.

I said V=kq/r = (9E9)(4E-12)/.25E-3 = 144V from one charge on the x-axis, then multiplied by two, since the other charge is mirrored. So 288 Volts.

I then tried to move the charge on the yaxis into the origin by saying
U = qV so U = (4E-12)(288) =1152E-12 J = 1152 pJ

However, their answer is 576 pJ, half of what I got... where am I going wrong?

Thanks in advance,
Rob

 

[OlderDan]

Think about the potential difference between where the charge was initially placed and its final position.

 

[Siirous]

Ah ha, because I'm not bringing it in from infinity I can subtract the joules needed to bring it to it's current position, then leftover will be what I need, it works! Got the answer! Thanks a lot.