Electronic devices operate by the movement of electrons through conductors, e.g. wires, and electronic components. The quantity, rate of flow, and driving force for electrons are described by the following basic quantities.
Symbol: q, unit: coulomb (C)
Charge is an amount of electrons. One coulomb is the charge of 6x1018 electrons.
Symbol: I, unit: ampere or amp (A)
Current is the rate of flow of charge, i. e., the number of coulombs flowing past a point per second. One amp is equal to one coulomb per second.
Symbol: V or E, unit: volt (V)
Voltage (also called potential, potential difference, potential drop, or electromotive force - EMF) is the electronic potential energy between two points, and is the driving force that causes charge to flow. One volt is defined as the potential difference that requires one joule of energy to move one coulomb of charge.
Voltage is always measured relative to some other point in a circuit, e.g., the potential across a resistor. Voltage measurements made at a single point in a circuit are made relative to the earth (ground), which is assigned an "absolute" voltage of zero.
Symbol: Z, unit: ohm ()
Impedance is the degree to which an electronic component impedes the flow of current. In general it is a frequency-dependent quantity.
The impedance of a resistor is also called its resistance. The impedance of capacitors and inductors is also called their reactance.
The definitions of resistance, capacitance, and inductance are included with the discussion of resistors, capacitors, and inductors, respectively.