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What Is Electricity?
You use it every day, but what exactly is electricity? To understand it, we must go down to the very building blocks of matter itself: the atom. Without getting too complicated (and going down to the level of quantum mechanics), matter is comprised of atoms. Atoms are made up of three basic particles: protons, which are positively-charged particles; electrons, which are negatively-charged particles; and neutrons, which have no charge. An atom has its neutrons and protons clustered together at its center, called the nucleus, while the electrons orbit around the nucleus.
Electrons are negatively-charged, and protons positively-charged. In a stable substance, therefore, the charges balance out and the item comprised of that substance will have no net charge. This is why an item at rest will not spontaneously generate electricity: it is stable. Neutrons and protons normally remain static within the nucleus of most (non-radioactive) substances. Electrons, on the other hand, can easily be removed from atoms and can move between those atoms and adjacent ones. This is most readily observable through the phenomenon of static electricity. In the classic experiment that you can do at home, a balloon is is rubbed on a child's head. When this is done, electrons are stripped from atoms in the hair, and moved to the balloon (or is it the other way around? :^) ). Once the two are separated, one is left with a positive charge and the other with a negative charge. Iif the balloon is then moved near that child's head again, the hair will stick up as the remaining, charged atoms are attracted to those in the balloon.
What causes electrons to move? It is two things: first, the attraction of the negatively-charged electrons towards a positive charge; and second, a means of allowing the electrons to flow. Electricity is then at its heart, a flow of electrons. Whether it is the powerful discharge of a lightning bolt, or the flow of electricity in an appliance, or even the small flow of electricity in a battery-powered wristwatch, they are all at their core the same.
Electricity flows readily in some materials but not in others. What differentiates materials is primarily the atomic structure of the matter that comprises them. Some conduct electricity readily; they are of course called conductors. Typical good electrical conductors include copper, aluminum, gold and other metals, and water. Materials that do not conduct electricity are called insulators. Common insulators include wood, glass, plastic, and air (though many people don't think of air as an insulator, it is actually one of the best.) Some materials conduct electricity better than others because some atoms hold on more tightly to their electrons than others. However, any substance, even one that normally insulates so well as to stop the flow of electricity, will conduct electricity if the charge is strong enough--even the air.
This brings us to the concept of a circuit. A circuit is any combination of objects that allows electricity to flow. A battery by itself is not a circuit; there is no flow of electricity (this is sometimes called an open circuit. Attach a wire from one end of the battery to the other though, and you have a closed circuit and electricity will immediately flow.