What is a series and parallel circuit?

A series circuit is constructed so that when energy flows through the circuit, it has to pass through each element one at a time, and in sequence. The energy has only one pathway to return to where it was (since circuits imply a closed pathway)

(e.g. if you have a battery, and a closed switch, and a light bulb, and you wire them in series, then the energy has to pass through the battery, then the closed switch, then the bulb, and back to the battery. There are no pathways to bypass any of the elements).

A parallel circuit is arranged so that the energy can flow through one element, OR through another element, just as easily. For a number of components X, if the energy has X different pathways to travel to return to where it started, then all X of the parts are in parallel with each other.

(e.g. if you wire a battery, a lightbulb, a coffeemaker, and a desk fan in parallel, the energy from the battery has a separate pathway to go through each of the bulb, coffeemaker, and fan, before it goes back to the battery)

A series circuit is useful when you want to control a number of components with a single switch, or when you need to know if ALL of the components work. If one component is broken, it breaks the entire circuit and no energy can flow, so NONE of the components receive any energy. Think of an (old) string of holiday lights. Series circuits can implement a logical function called AND.

Parallel circuits are useful when you need to connect a bunch of components up independently of each other. For example, if you wanted to connect up a light that shines in one room, and a light that shines in another room, then you'd wire them in parallel. That way, if one lamp broke, the other would still work (since the energy would still have a pathway to travel).

(Think of the cigarette lighter plug in your car -- this outlet is in parallel with the other devices -- the fan, the starter motor, the headlights. If they were in series, they'd all need to be on at the same time to work.)

Parallel circuits can implement a logical function called OR.