A charged particle of mass $2 mg$ remains freely in air in an electric field of strength $4 \mathrm{N} {\mathrm{C}}^{-1}$ directed upward. Calculate the charge and determine its nature $\left(\mathrm{g}=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$.

Three charges of respective values $-\sqrt{2}\mu C, 2\sqrt{2}\mu C$ and $-\sqrt{2}\mu C$  are arranged along a straight line as shown in the figure. Calculate the total electric field intensity due to all three charges at the point $P$.