Circuit Theory/Variables and Units

Electric Charge (Coulombs)

Electric charge is a property of some subatomic particles. Electric Charge is measured in SI units called "Coulombs", which are abbreviated with the letter capital C. A Coulomb is the total charge of 6.24150962915265×10E18 electrons, thus a single electron has a charge of −1.602 × 10E−19 C. The variable used to represent a quantity of charge is the "q" (lower-case Q). Electric charge is the subject of many fundamental laws, such as Coulomb's Law, and Gauss' Law. However, Charge is not important for the study of electric circuits, and so this wikibook will not make much use of it after this.

For further information about electric charge, Coulomb's Law or Gauss' Law, see the wikibook Modern Physics.

Current (Amperes)

Current is a measurement of the flow of electricity. Current is measured in units called Amperes (or "Amps"). Technically, an ampere is measured in terms of "coulombs per second" although in reality, the coulomb is actually defined in terms of the ampere. Amperes are abbreviated with an "A" (upper-case A), and the variable most often associated with current is the letter "i" (lower-case I). In terms of coulombs, an ampere is:

$i=\frac{dq}{dt}$

Because of the wide-spread use of complex numbers in Electrical Engineering, it is common for electrical engineering texts to use the letter "j" (lower-case J) as the imaginary number, instead of the "i" (lower-case I) commonly used in math texts. This wikibook will adopt the "j" as the imaginary number, to avoid confusion.

Voltage (Volts)

Voltage is a measure of the work required to move a charge from one point to another in a electric field. Thus the unit "volt" is defined as a Joules (J) per Coloumb (C).

$V=\frac{W}{q}$

W represents work, q represents an amount of charge.

Voltage is sometimes called "electric potential", because voltage represents the potential of a forcing function to produce current in a circuit. More voltage means more potential for current. Voltage also can be called "Electric Pressure", although this is far less common. Voltage is not measured in absolutes but in relative terms compared to a reference point. The reference point against which all voltages are measured in a given circuit is known as ground.

Energy

Energy is measured most commonly in Joules, which are abbreviated with a "J" (upper-case J). The variable most commonly used with energy is "w" (lower-case W). This book will not talk much about energy, although the Modern Physics wikibook will. Refer to that book for more information.