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Old Sep 21st 2016, 08:53 PM   #1
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Electric potential energy of the system of three charges

1:

The electric potential energy of two point charges approaches zero as the two point charges move farther away from each other.

If the three point charges (two positive, one negative) lie at the vertices of an equilateral triangle, the electric potential energy of the system of three charges is...

Answer: negative.


2:

The electric potential energy of two point charges approaches zero as the two point charges move farther away from each other.

If the three point charges (one positive, two negative) lie at the vertices of an equilateral triangle, the electric potential energy of the system of three charges is...

Answer: negative.



Why would this be the case?
Equation for electric potential energy is, U = K( SUM[qi/ri] )
=> U = (q1/r1) + (q2/r2) + (q3/r3) ).
By this equation, I understand that the second question would be negative by U = (+q1/r1) + (-q2/r2) + (-q3/r3) ) = -q2/r2.
But Looking back at the first question, this doesn't hold.
Applying the first line of the question, will leave negative(question1) and positive(question2), so this also doesn't work.
What am I missing?

Last edited by johnwill8258; Sep 21st 2016 at 09:00 PM.
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Old Sep 22nd 2016, 09:02 AM   #2
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#1 ... one negative, two positive charges

$\dfrac{k\color{blue}{q}\color{red}{q}}{r} + \dfrac{k\color{blue}{q}\color{red}{q}}{r} + \dfrac{k\color{red}{q}\color{red}{q}}{r}$

$\color{blue}{negative} + \color{blue}{negative} + \color{red}{positive}$

#2 ... two negative, one positive charge

$\dfrac{k\color{blue}{q}\color{blue}{q}}{r} + \dfrac{k\color{blue}{q}\color{red}{q}}{r} + \dfrac{k\color{blue}{q}\color{red}{q}}{r}$

$\color{red}{positive} + \color{blue}{negative} + \color{blue}{negative}$
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