Electrical current is defined as the rate at which charge flows. When there is a large current present, such as that used to run a refrigerator, a large amount of charge moves through the wire in a small amount of time. If the current is small, such as that used to operate a handheld calculator, a small amount of charge moves through the circuit over a long period of time. The SI unit for current is the ampere (A), named for the French physicist André-Marie Ampère (1775–1836). An ampere is the flow of one coulomb of charge through an area in one second. A current of one amp would result from 6.25 × 1018 electrons flowing through the area each second. Most electrical appliances are rated in amperes (or amps) required for proper operation, as are fuses and circuit breakers.
Consider a simple circuit of a car battery, a switch, a headlight lamp, and wires that provide a current path between the components. In order for the lamp to light, there must be a complete
path for the current flow. The switch is there to control the circuit. A schematic is a graphical representation of a circuit and is very useful in visualizing the main features of a circuit. Schematics use standardized symbols to represent the components in a circuit and solid lines to represent the wires connecting the components. The battery is shown as a series of long and short lines, representing the historic voltaic pile. The lamp is shown as a circle with a loop inside, representing the filament of an incandescent bulb. The switch is shown as two points with a conducting bar to connect the two points, and the wires connecting the components are shown as solid lines.
When the switch is closed, there is a complete path for charges to flow, from the positive terminal of the battery, through the switch, then through the headlight and back to the negative terminal of the battery. The direction of conventional current is always represented in the direction that positive charge would flow, from the positive terminal to the negative terminal. For example, in metal wires, the current is carried by electrons, that is, negative charges. In ionic solutions, such as salt water, both positive and negative charges move.
