Potential energy of a system of point charges, or work to change its spacing

1. A.
   $-\frac{q^2}{8\pi\varepsilon_0 r}$
2. B.
   $-\frac{3q^2}{8\pi\varepsilon_0 r}$
3. C.
   $-\frac{5q^2}{8\pi\varepsilon_0 r}$
4. D.
   $-\frac{7q^2}{8\pi\varepsilon_0 r}$

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1. A.
   1.4 J
2. B.
   $\ 2.7\text{ }J$
3. C.
   $\ 4.1\text{ }J$
4. D.
   8.2 J

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1. A.
   $\dfrac{q^2}{4\pi\epsilon_0x}$
2. B.
   $\dfrac{-2q^2}{4\pi\epsilon_0x}$
3. C.
   $\dfrac{3q^2}{4\pi\epsilon_0x}$
4. D.
   $\dfrac{-4q^2}{4\pi\epsilon_0x}$

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1. A.
   $\dfrac{-9q^2}{8\pi\epsilon_0r}$
2. B.
   $\dfrac{9q^2}{8\pi\epsilon_0r}$
3. C.
   $\dfrac{-7q^2}{16\pi\epsilon_0r}$
4. D.
   $\dfrac{7q^2}{16\pi\epsilon_0r}$

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1. A.
   $\dfrac{2q^2K}{a}$
2. B.
   $\dfrac{q^2K}{2a}$
3. C.
   $\dfrac{qK}{a^2}$
4. D.
   zero

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1. A.
   $-\dfrac{q^2}{8\pi\epsilon_0a}$
2. B.
   $-\dfrac{3q^2}{8\pi\epsilon_0a}$
3. C.
   $-\dfrac{5q^2}{8\pi\epsilon_0a}$
4. D.
   $-\dfrac{7q^2}{8\pi\epsilon_0a}$

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