Electricity is a form of energy (Chapter 5, section on Forms of Energy), and it is produced from other forms of energy in a variety of ways. Kinetic energy produces electricity when, in a generator or alter-
nator, a coil is revolved in a magnetic field. Pressure on a suitable crystal (quartz, tourmaline, lead zirconate titanate) produces a spark by the ‘piezo-electric’ effect. Friction can cause particular materials to become charged with static electricity. Heat energy can produce electricity when applied to a
‘thermocouple’ or junction of two dissimilar metals. Light energy falling on a selenium cell or ‘photo-electric’ cell
produces electricity. Chemical energy in the form of dry batteries or primary or secondary
cells is a common source of electricity. All these‘sources’ofelectricityare infactsimplymeansofproducinga
flowofelectronswithin the material which forms the electric circuit.