Two Tiny Grains of Sand Having Charges Of $4.0 \mu \mathrm { C }$

Multiple Choice

Two tiny grains of sand having charges of $4.0 \mu \mathrm { C }$ and $- 4.0 \mu \mathrm { C }$ are situated along the $x$ -axis at $x _ { 1 } =$ $2.0 \mathrm {~m}$ and $x _ { 2 } = - 2.0 \mathrm {~m}$ . What is electric potential energy of these grains relative to infinity? $( k = 1 / 4$ $\pi \varepsilon _ { 0 } = 9.0 \times 10 ^ { 9 } \mathrm {~N} \cdot \mathrm { m } ^ { 2 } / \mathrm { C } ^ { 2 }$ )

A) $0 \mathrm {~J}$
B) $- 72 \mathrm {~mJ}$
C) $- 36 \mathrm {~mJ}$
D) $36 \mathrm {~mJ}$
E) $72 \mathrm {~mJ}$