# DC and AC Voltages

 by friction between two materials was one source of DC voltage. Early machines generating (very low levels of) electric power such as electrostatic machines, inductance machines and belt generators as well as the batteries developed during this period produced DC voltages as well, i.e. they gave rise to direct current when resistive loads were connected. Today, DC voltages are mainly used to supply electronic circuits, e.g. in radios, pocket calculators and PCs. DC voltage and current are defined as follows:  A DC voltage is a voltage of constant magnitude and direction. For loads of constant resistance, this type of voltage gives rise to direct current (DC).

The following animation provides an example of a direct current characteristic. Large amounts of electrical energy are commonly produced by converting mechanical (kinetic) energy into electrical energy by means of electromagnetic induction using generators. The same principle is employed on a smaller scale in bicycle dynamos, for instance. In this case, the generator's rotation produces an AC voltage,the magnitude and direction of which change over time. Applying this voltage to a load like a bicycle lamp causes an alternating current to flow through the lamp. AC voltage and current are defined in general as follows:  An electrical potential, the magnitude and direction of which change over time, is termed an AC voltage. The current produced by such a voltage in a load is termed alternating current (AC).

The animations below show two examples of alternating current. Whereas the current in the upper example has a somewhat irregular characteristic, the lower current has a periodic, rectangular characteristic. In this case, the current's amplitude remains constant but its direction (or sign) changes periodically. During this course, any current which changes over time (AC), will be represented in lower case, as opposed to DC which will be indicated using a capital I.  Sinusoidal Alternating Quantities

Whereas in communications engineering and information technology, various shapes of alternating voltage (such as the square wave considered earlier) can prove useful, electrical energy technology is dominated by sinusoidal alternating voltage and current, usually produced by generators and used for transmitting energy via high-tension transmission lines. Such voltages and currents follow a sine-wave curve as shown in the animation below. 