The following are some very basic electronics concepts. My hope is to whet your appetite for electronics so you will follow up with a real electronics textbook.
Electrical Charge
The symbol in formulas for electrical charge is Q, sometimes written as q. A proton and electron have the same quantity of charge, written as e, but different polarities (positive and negative). By convention, an electron is considered to have a -e charge while a proton has +e.
The coulomb is the basic unit of electric charge, named after Charles-Augustin de Coulomb and represented by the letter C. A coulomb represents a charge of about 6.24 × 1018 e. Physicist Robert Millikan determined the charge of an electron or proton was 1.6 × 10-19 C.
Voltage
A charge has the potential of doing work by moving another charge through attraction or repulsion. When you separate positive and negative charges, they have a natural tendency to move towards each other to restore neutrality. This force between charges is a potential difference called an electromotive force (EMF).
The unit of measuring a potential difference is the volt abbreviated v, named after Alessandro Volta. The symbol for a potential difference is V for voltage. Sometimes the letter E for EMF is used instead.
Electrical potential is defined with respect to some common point. The negative terminal of a car battery is connected to the metal car body. Thus the twelve volts measured in the battery are with respect to the metal body.
Voltage polarity (negative or positive) determines the direction in which electrical current will flow. Voltage is like a water pump. The pump doesn't move through the pipes, but it pushes the water through.
Current
Electrical current is the flow of charge through a material (usually a conductor such as copper wire). In the water pump analogy above, the current would be the flow of water moving through the pipes.
Current measures the intensity of moving charges in units called amperes (A) or amps, named after physicist and mathematician André-Marie Ampère. One ampere represents the flow of one coulomb (6.24 x 1018) of charge per second. The electrical symbol for current is I.
Current flow requires a closed circuit. If there is a break in the current path (such as an open switch), then the current will not flow.
There are two competing ways to analyze current flow in a circuit. The first method is called Electron Flow. Here, current is considered to be electrons moving from a negative potential to a positive potential.
The second method is called Conventional Current and is primarily used in electrical engineering. Here, current flow is considered to be positive charges moving from a positive potential to a negative one. The charges and direction of current flow are the reverse of electron flow. Either method can be used correctly to analyze a circuit. In the tutorials on this Website, the electron flow method will be used unless otherwise noted.
Resistance
Resistance is the opposition to current flow. In theory, a conductor should provide no resistance to the flow of electricity. In reality, even the highest quality conductor will provide a small amount of resistance.
Resistance is measured in units called ohms, named after physicist and mathematician Georg Simon Ohm and represented by the Greek letter Omega (Ω). The electrical symbol for resistance in formulas is R.
Conductance
Conductance is simply the reciprocal of resistance. The unit of conductance is the seimen (S), named after Ernst Werner von Siemens, and its electrical symbol in formulas is S. (Older texts will use the unit mho and the symbol G.)
Ohm's Law
How do all these concepts relate to one another? The most basic formula in electronics is Ohm's Law derived by Georg Simon Ohm in 1827 and expressed as: V = IR Ohm's Law shows that in a circuit, the voltage (V) is equal to the current (I) multiplied by the resistance (R). If a current of one ampere flows through a circuit with ten ohms of resistance, the value of the voltage would be ten volts (1 ampere × 10 ohms).
Ohm's Law isn't limited to solving for voltage. With a little algebraic manipulation, you can use it to solve for current or resistance: I = V/R R = V/I Both of these equations are simply Ohm's Law rewritten.
Power
Power is the amount of energy dissipated in a circuit. Power is measured in watts (w), named after engineer James Watt, and its symbol in formulas is P. A watt is the work done by one volt while moving one coulomb of charge for one second. Since one coulomb of charge per second is one ampere, a watt can also be defined as a volt-ampere, thus power is calculated by multiplying voltage by current. P = I × V or sometimes written as P = IE
When current flows through a resistor, some power is dissipated as heat. This power can be expressed in terms of resistance using Ohm's Law. Take the power formula P = IV and substituting IR for V gives: P = I (IR) or P = I2R. The power formula could also be written as P = V2 / R.