Electricity is the result of electrons being separated temporarily from protons, thereby creating a difference in electric potential (or voltage) between the area with excess electrons and the area with a shortage of electrons . When electrons find an electrically-conductive path to move along, electric current flows. The first electric applications made use of Direct Current (DC) power, whereby the electrical charge from the electron flow is unidirectional. DC is produced by batteries, photovoltaic (PV) solar cells and generators. The alternating current (AC) used, for example, to power offices and workshops and to make standard, fixed-speed motors rotate, is generated by an alternator. It periodically changes magnitude and direction in a smooth, sinusoidal pattern. Voltage as well as current magnitude grows from zero to a maximum value, then falls to zero, changes direction, grows to a maximum value in the opposite direction and then becomes zero again. The current has then completed a period T, measured in seconds, in which it has gone through all of its values. The frequency is the inverse of the period, states the number of completed cycles per second, and is measured in Hertz. f=1/T f = frequency (Hz) T = time for one period (s). Magnitudes of current or voltage are usually indicated by the root mean square (RMS) value over one period. With a sinusoidal pattern, the relation for the current and voltage root mean square value is: root mean square = (peak value) / V2.