Question
There exists a uniform electric field $E=4 \times 10^5 \,Vm ^{-1}$ directed along negative $x$-axis such that electric potential at origin is zero. Acharge of $-200 \,\mu C$ is placed at origin, and a charge of $+200 \,\mu C$ is placed at $(3 \,m , 0)$. The electrostatic potential energy of the system is ...........$J$

Answer

a
(a)

$V_3=4 \times 10^5 \times 3=12 \times 10^5$

$U=\frac{k q_1 q_2}{r}+q_1 V_2+q_2 V_2$

$U=\frac{-9 \times 10^9 \times 200 \times 200 \times 10^{-12}}{3}+(-200)(0)+200\left(12 \times 10^5\right) \times 10^{-6}$

$U=-120+240$

$U=120 \,J$

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