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Raspberry Pi For Dummies by Mike Cook, Sean McManus

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Discovering What a Circuit Is

The first thing you have to understand is that a circuit is something where electricity can flow; it is a path, or a conduit. It is continuous; that is, it’s a loop with no dead ends. If you have a dead end, you don’t have a circuit. Electricity has to be able to flow. So let’s be more specific in what we mean by electricity. It can be complex stuff and because it’s invisible, we have to do a bit of imagining to appreciate what is going on. There are two aspects of electricity: current and voltage.

Current is what actually flows. Voltage is what forces the current round a circuit. Voltage can’t flow and current doesn’t exist in the absence of a voltage. However, voltage can exist in the absence of current. You’ve no doubt felt the effects of static electricity, which is the build-up of voltage that occurs when insulators (materials that don’t normally conduct electricity) are rubbed together.

It’s kind of like how rubbing a balloon on wool can make the hairs on the back of your hand stand up. You can feel it, but only because you feel your hairs being lifted. You aren’t feeling the electricity itself. You only feel static electricity when it stops being static and a current flows. At a very high voltage, a little current can hurt a lot. You’ve probably felt the static discharge shock of touching a metal object after walking over a nylon carpet.

Understanding the nature of electricity

So what is electric current? It is a flow of electrons past a point, ...

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