where W is energy in joules, Q is charge in coulombs, and V is the resulting
voltage in volts.
Note carefully that voltage is defined between points. For the case of the
battery, for example, voltage appears between its terminals. Thus, voltage
does not exist at a point by itself; it is always determined with respect to
some other point. (For this reason, voltage is also called potential difference.
We often use the terms interchangeably.) Note also that, although we
considered static electricity in developing the energy argument, the same
conclusion results regardless of how you separate the charges; this may be
by chemical means as in a battery, by mechanical means as in a generator, by
photoelectric means as in a solar cell, and so on. In equation form,