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This chaper introduces electrostatics starting from very basic ideas of charging by friction. The PhET simulation illustrates some of the phenomena mentioned in Section 15.1.

The understanding of electric current as a flow of electric charge is illustrated in the  Physlet simulation Current physlet (select  under DC circuits in the menu on the left). In this simulation, charge is represented by discrete charge carriers (a sort of 'microscopic picture') rather than the continuous fluid mode (sometimes called the ') adopted in Understanding Physics. It also distinguishes betwen what it calls the 'Actual Current' — negative (blue) charge carriers in the metal wires, positive (red) charge carriers in the voltage source — and the 'Conventional Current' — positive charge throughout.

The suggestion at the end of the physlet "that we've got things backwards" as regards the convention for positive/negative is very unfair on Benjamin Franklin; the facts are that, while charge carriers in metals may be negative, in many cases positive carriers can also be responsible for electric currents (e.g., in semiconductors, electrolytes, ion beams, etc.). As with all conventions, the choice is arbitrary.