Electricity is an invisible force that is used to transfer energy into heat, light, intelligence, or motion. Electricity is explained in terms of electrical charge, potential differ- ence (or voltage), electrical charge flow (or current), and resistance to current flow. Figure 1-1 graphically illus- trates electron flow through a conductor by comparing it with water flow through a pipe. The normal unit of current measurement is the ampere, whereas the normal unit of voltage measurement is the volt. The unit of opposition to current flow, or resistance, is the ohm.
