Electrostatics

There are two kinds of charge, positive and negative, and their rule of interaction is that like charges repel and unlike charges attract. The charge on an electron is taken as negative, the charge on a proton as positive, and the magnitude they share is the elementary charge $e = 1.6 \times 10^{-19}\ \text{C}$. A measurement of the force between charges, made by Charles Coulomb in the nineteenth century, is the experimental foundation on which the whole chapter rests.

Charge is quantised. The total charge on any object is an integer multiple of the elementary charge, so the smallest possible charge is $1.6 \times 10^{-19}\ \text{C}$ and no object can carry a smaller isolated amount. This is a useful filter for a multiple-choice question: if an option gives a charge that is not a whole-number multiple of $e$, it can be dismissed without any calculation. Counting electrons works the same way: dividing a stated charge by $e$ tells you how many elementary charges it holds.

The unit of charge, the coulomb, is fixed by the definition of current rather than by charge itself. One coulomb is the charge transported by a current of one ampere in one second, so the coulomb is an ampere-second. This gives a route between the two quantities: a current and a time convert into a charge, and the number of electrons carrying that charge follows on division. The electrostatic force between two charges is independent of the temperature of the charges, a point the examiner tests when it lists temperature among the quantities that do affect the force.