Sample Problem Gauss Law
shows five charged lumps of plastic and an electrically neutral coin The cross section of a Gaussian surface S i
Five plastic objects each with an electric charge and a coin which has no net charge A Gaussian surface shown in cross section closes three of the plastic indicated What is the net electric flux through the surface if.
The Key Idea here is that the net flu through the surface depends on the net charge enclosed by surface S. This that the coin and charges q and do not contribute to The coin does not contribute because it is neutral and thus contains equal amounts of positive and negative charge Charges and do not contribute because they are outside surface S Thus is q, + q2 + q) and gives us.
The minus sign shows that the net flux through the surface is inward and thus that the net charge within the surface is negative.