Thus to collect a charge of one coulomb, from a body from which electrons move out every second, we will need approximately years. One coulomb is, therefore, a very large unit for many practical purposes. It is, however, also important to know what is roughly the number of electrons contained in a piece of one cubic centimetre of a material. A cubic piece of copper of side cm contains about .
× electrons. Example . How much positive and negative charge is there in a cup of water? Solution Let us assume that the mass of one cup of water is g.
The molecular mass of water is 18g. Thus, one mole (= . × molecules) of water is g. Therefore the number of molecules in one cup of water is ( / ) × .
× . Each molecule of water contains two hydrogen atoms and one oxygen atom, i.e., electrons and protons. Hence the total positive and total negative charge has the same magnitude. It is equal to ( / ) × .