There are two ways of connecting components:
so that each component has the same current.The battery voltage is divided between the two lamps
so that each component has the same voltage.Both lamps have the full battery voltage across them.
Most circuits contain a mixture of series and parallel connections
The terms series circuit and parallel circuit are sometimes used, but only the simplest of circuits are entirely one type or the other. It is better to refer to specific components and say they are connected in series or connected in parallel.
For example: the circuit on the right shows a resistor and LED connected in series (on the right) and two lamps connected in parallel (in the centre). The switch is connected in series with the two lamps.
Lamps in Series
If several lamps are connected in series they will all be switched on and off together by a switch connected anywhere in the circuit. The supply voltage is divided equally between the lamps (assuming they are all identical). If one lamp blows all the lamps will go out because the circuit is broken.
Christmas Tree Lights
The lamps on a Christmas tree are connected in series.
Normally you would expect all the lamps to go out if one blew, but Christmas tree lamps are special! They are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one ‘fuse’ lamp which blows normally.
If there are 20 lamps and the mains electricity voltage is 240V, each lamp must be suitable for a 12V supply because the 240V is divided equally between the 20 lamps: 240V ÷ 20 = 12V.
WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!)
Lamps in Parallel
If several lamps are connected in parallel each one has the full supply voltage across it. The lamps may be switched on and off independently by connecting a switch in series with each lamp as shown in the circuit diagram. This arrangement is used to control the lamps in buildings.
This type of circuit is often called a parallel circuit but you can see that it is not really so simple – the switches are in series with the lamps, and it is these switch and lamp pairs that are connected in parallel.
Switches in Series
If several on-off switches are connected in series they must all be closed (on) to complete the circuit.
The diagram shows a simple circuit with two switches connected in series to control a lamp.
Switch S1 AND Switch S2 must be closed to light the lamp.
Switches in Parallel
If several on-off switches are connected in parallel only one needs to be closed (on) to complete the circuit.
The diagram shows a simple circuit with two switches connected in parallel to control a lamp.
Switch S1 OR Switch S2 (or both of them) must be closed to light the lamp.
(Article from the Electronics Club)