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This section will let tell you the basics of what you need to know in order to understand how a circuit works.  

Physics:
Substances are made up of atoms.   An atom has three parts: protons and neutrons make up the nucleus, and electrons that orbit the nucleus in shells.   The outer shell (called it's valence shell) determines the atom's conductivity.   The atom's conductivity depends on the number of electrons in the valence shell.   When an atom has only one electron in it's valence shell, it is almost a perfect conductor.   When an atom has eight electrons in it's valence shell is said to be complete and is an insulator (can't share electrons).   Therefore electrical conductivity decreases with an increase in the number of valence electrons   An atom of an electrical conductor can, temporarily, give up (or share) their valence electrons when placed under electrical pressure (voltage applied to the conductor).   The shared electron hops from one atom to another (why?,).   This hopping of electrons, from one atom to another, creates a flow or electrical current.   Take a look (below) at an atom of copper (29 electrons) or an atom of gold (79 electrons) each has only one electron in it's valence shell meaning they are good conductors.  

Copper atom	Gold atom

Electronics:
An electrical current is a flow of electrons.
Electrons always flow from negative to positive.
Current (flow of electrons) is determined by 3 factors: Electromotive force or voltage (E), resistance (R), and current (I).  
E (electromitive force) measured in Volts, is the pressure pushing electrons through a conductor.
R (resistance) measured in Ohms, is a device's resitance to the flow of electrons. Every kind of electrical device has resistance, even a battery.
I (current) measured in Amps, is the amount of electrons flowing through the conductor.

A circuit contains devices that allow a flow of electrons (a current).
A power source is a device that provides a voltage and a current.
A power source has two sides, usually positive an negative.
A circuit begins at one side of the power source source, flows through other devices in the circuit, then returns to the opposite side of the source.
This is a DC (Direct Current) circuit, we'll talk about AC (alternating current) later.
There are many kinds of electrical devices: wires, batteries, resistances, capacitors, inductors, transistors, and diodes, to name the most common.
In order to be called an electronic circuit, it must have at least one device that can conrotl the flow of electrons.
Transistors and diodes are thought of as active devices, the others are passive.
A diode only allows a current to flow in one direction.
A transistor is a 3 termnal device that controls the flow of electrons.
A capacitor stores electrons, and resists a change in voltage across it.
An inductor stores energy and resists a change in the flow of current through it.
There are numerous kinds of transistors, usually determined by the process or the material used to make the transistor.   For now were only going to talk about junction transistors, not field effect transistors, and certainly no vacuum tubes.
In order to calculate things in an electrical circuit we use Ohm's Law: E = IR, voltage is equal to current times resistance.   Since this is algebra, we can also say: I = E/R, or R = E/I.
A voltage (E) will develop across anything, in an electrical circuit, which resists (R) the flow of electrons (I).   This is called an IR drop since E = IR.

Proceed to Class 2.

Why Current:
A complete atom has an equal number of protons (positive) and electrons (negative) so the atom has a neutral charge.   When an electron hops out of an atom, the atom is left with a slight positive charge.   A charged atom is an ion.   When an electron hops out of an atom the electron is said to be 'free'.   An atom that has lost an electron has a slight positive charge and attracts nearby 'free' electrons.