Work Step by Step
At a certain location, the electric field due to the excess charge on earth's surface points downwards. The electric field line depicts the path taken by a positive charge when left in the field. This means that if a positive charge is let loose on that particular location, it will be drawn towards earth. We already know that like charges repel and unlike charges attract. The positive charge is attracted to earth. This means the sign of excess charge on earth is negative.