Atoms
Atomic Composition
In order to understand how circuits work and how to design them, we must first understand some basic physics. Electrical theory and application comes directly from the structure and forces of atoms.
Physical things we interact with in the world are made of molecules, which are then composed of atoms.
Most molecules are very small, atoms are very, very small. To give an idea of the size of an atom; a cup of water has about 8.36 x 1024 water molecules. Water's chemical symbol is H2O, meaning that it is a molecule made of Hydrogen (x2) atoms, and one Oxygen atom. Therefore, there are three times as many atoms as molecules in a glass of water, or 25.08 x 1024 or 25,080,000,000,000,000,000,000,000 atoms.
Protons, Neutrons, and Electrons
Atoms are composed of three more elementary particles; protons, neutrons, and electrons.
Protons and neutrons are incredibly dense and make up the core, or nucleus of an atom. In fact, they're so dense, that a teaspoon of them packed together would weigh many tons on earth. However, an atom is mostly empty space, because while the protons and neutrons are tightly clustered in the center of the atom, electrons orbit a distance around the nucleus:
Atoms are classified on the periodic table by the number of protons they possess, which is also their atomic number. So, for example, hydrogen has (1) proton, so therefore its atomic number is 1. Helium has 2 protons, so its atomic number is 2, lead has 82, and so forth.
Electrical Charge
Both protons and electrons have an attraction/repelling force, or "charge." Protons and electrons have the same amount of charge from a quantity standpoint, but they are qualitatively different, in that protons and electrons exert opposite forces, which cause them to attract each other. Two electrons will repel each other; similarly, two protons will also repel each other. The amount of charge a proton or electron has is known as a unit electrical charge, often denoted as e, and was once thought to be the smallest amount of electrical charge possible (we now know that quarks have an even smaller electrical charge).
Protons are said to have a "positive charge," neutrons have no charge (neutral), and electrons have a "negative charge." However, the terms positive and negative in this context have no intrinsic meaning, instead, their naming has been chosen arbitrarily, in that they could have just as easily been called up and down charges, or protons could have been said to be negative and electrons positive. The important part here is that they are opposites.
Ions
Because protons and electrons have the same amount but opposite type of charge, the number of electrons in a balanced atom (in which the net electrical charge is zero), must be equal. So a hydrogen atom (atomic number of 1) should have one proton and electron. Iron, which an atomic number of 26, should naturally have 26 protons, and 26 electrons.
However, atoms can gain or lose electrons, causing them to have a net positive charge (if there are more protons) or net negative charge (if there are more electrons). These atoms are called ions. The process of becoming ionized is ionization.
Conductors
Atoms often combine together by sharing electrons to form molecules or in a repeating lattice to form crystals and metal:
In some materials, such as silver or copper, electrons move easily between atoms. Materials that readily share electrons like this are said to be conductors or conductive.
Insulators
Materials that don't readily let electrons flow are said to be insulators. Materials such as glass and wood are good insulators.
Metal oxides (metals that have oxygen atoms bound to them) often insulate, even though the same metal without oxygen will conduct. Gasses usually make good insulators because the atoms are too far apart to exchange electrons.