16 A lamp, an electric motor, or a computer can be circuit loads. When a short circuit occurs, electrical current meets little or no resistance because its path has been diverted from its normal direction of flow. This produces excessive heat and may damage loads and switches and cause wires to catch fire. This type of circuit was employed in old-style strings of Christmas tree lights where, when even one of the light bulbs burned out or was removed, the entire string became inoperable. Open circuit Closed circuit ELECTRIC CIRCUITS What is a circuit? An electric circuit is the path, or loop, along which current flows. A simple electric circuit mainly consists of some electrical components, such as: a source of energy, such as a battery or a generator; a load, or resistor, which is a device that uses the current flowing in the circuit to work ; one or several wires or cables, which connect all the components and along which electricity flows; a switch, which can be closed to let current flow, or opened to interrupt the flow of current. Open and closed circuits A circuit where all the components are connected to the wire and electricity flows through them is called a closed circuit. On the contrary, when a gap occurs in the circuit, for example when the circuit switch is opened, the flow of electricity is interrupted and the circuit is defined an open one. When the current flowing through the wires does not follow the correct path but takes a wrong direction, a short circuit occurs . Short circuit to affect: influenzare gap: spazio load: carico loop: anello, circuito source: sorgente switch: interruttore 50 elecTriciTy R1 R3 R2 R1 R2 SWITCH R3 BATTERY SWITCH Parallel circuit BATTERY Series circuit Series and parallel circuits Circuits can be divided into two main categories depending on the number of paths that electric current must follow. A series circuit has only one path to connect the source to the loads; the current flows in the wire and through each of the circuit components, which are placed one after the other. If one of the components in a series circuit does not work, the current stops flowing, so the whole circuit turns off . In parallel circuits, current flows along several paths and each of the loads is connected to the source by its own path, which makes it independent from the other loads. In this way, if one of the loads turns off or breaks down, this will not affect the functioning of the other components of the circuit; for this reason, household appliances in a house are always wired in parallel, to avoid the functioning of each of them to be affected by faults in the other ones.