The importance of electric current in daily life. A simple electric circuit
Electric current is used for lighting, heating, running machines and tools, communication, computers and medicine. An electric circuit is a closed path made of a current source, a consumer (a bulb, heater, motor), connecting wires and a switch. Current flows only in a closed circuit with a source; when the switch is opened the circuit is broken and the current stops. Quantities describing the current: the current I is related to the amount of charge passing through the circuit, its unit is the ampere (A), it is measured with an ammeter connected in series; the voltage U shows how strongly the source (or a part of the circuit) drives the current, its unit is the volt (V), and it is measured with a voltmeter connected in parallel to the part measured. The units are named after the Italian Volta and the Frenchman Ampère. Consumers are connected in series (one wire, one path) or in parallel (branched): bulbs and sockets at home are in parallel, so if one goes out the others keep working. A circuit diagram is drawn with conventional symbols.
Experiment (safe): build a circuit from a 4.5 V battery (or two 1.5 V dry cells), a small torch bulb and a switch; close and open the switch. Then add a second bulb in series, then in parallel, and compare brightness. Safety: use only a low-voltage battery (no more than 6 V), never use a socket, never connect the battery terminals directly with a wire (it heats up), and have an adult nearby.