It is found experimentally that the electric field in a certain region of the Earth’s atmosphere is directed vertically down. At an altitude of 300 m the field is and at an altitude of 200 m it is . We have to find the net amount of charge contained in a cube of 100 m on edge located between 200 and 300 m. We can neglect the curvature of the Earth.

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As the electric field in the region is directed vertically down ward, the outward normal to the Gaussian surface of the cube of 100 m on edge at the height of 300 m will be in direction opposite to the electric field, and the outward normal to the Gaussian surface of the cube at the height of 200 m will be in the direction of the field.

Therefore, the total flux of the electric field through the Gaussian surface of the cube will be

According to Gauss’ law

, where q is the charge enclosed by the closed Gaussian surface.

Therefore, the net amount of charge contained inside the cube will be