Two point charges Q and –3Q are placed at some distance apart. If the electric field at the location of Q is $\overrightarrow{\mathrm{E}}$, the field at the location of –3Q is:

Step 1: E is electric field due to charge -3Q at the location of Q.

$\mathrm{E}=\frac{1}{4\pi {ϵ}_0}\frac{3Q}{{\mathrm{x}}^2} (\mathrm{directed} \mathrm{from} \mathrm{Q} \mathrm{to} -3\mathrm{Q})$   ……. (1)

Step 2: E’ is electric field due to charge Q at the location of -3Q.

${\mathrm{E}}^{\mathrm{\prime }}=\frac{1}{4\pi {ϵ}_0}\frac{\mathrm{Q}}{{\mathrm{x}}^2} (\mathrm{directed} \mathrm{from} \mathrm{Q} \mathrm{to} -3\mathrm{Q})$    …….. (2)

From equation (1) and (2), we get:

$\overrightarrow{{\mathrm{E}}^{\mathrm{\prime }}}=\frac{\overrightarrow{\mathrm{E}}}{3}$