Electric charge (called charge from now on) is a concept defined rigorously in electrostatics in terms of the properties that matter exhibits when placed in an electromagnetic field. The movement of charge results in electric current. The SI unit of charge is the coulomb (C), which is equal to the negative the charge of approximately $\frac{1}{1.602\times10^{-19}}~=6.24\times10^{18}$ electrons.

An electric current is a stream of charged particles, such as electrons or ions, moving through a region in space. The moving particles are called charge carriers, which, in the case of electric circuits, are electrons moving through an electron conductive material (usually wire or component). The SI unit of electric current is the ampere or amp, which is the flow of electric charge across a surface at the rate of one coulomb per second. Electric current is measured using ammeters.

The conventional symbol for current is $I$ (see Fig. 1), which originates from the French phrase intensité du courant, (current intensity). The $I$ symbol was first used by André-Marie Ampère, after whom the unit of electric current is named.

A current in a wire can flow in either of two directions. When defining a variable $I$ to represent the current, the direction representing positive current must be specified, usually by an arrow on the circuit schematic diagram. This is reference direction of the current. When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown until the analysis is completed. Consequently, the reference directions of currents are often assigned arbitrarily. When the circuit is solved, a negative value for the current implies that the actual direction of current through that wire is opposite that of the chosen reference direction.

The electric potential, also known as electrostatic potential (or, in this webbook, potential), is a concept defined rigorously in electrostatics in terms of the amount of work energy needed to move a unit of electric charge from a reference point to the specific point (often taken at infinity) in an electric field. In the case of electric circuits the reference point is the ground node.

The SI unit of electric potential is the volt (V).

Notice that the electric potential is defined at a single point in space. In elecric circuits, the electric potential is defined at each node; note that this definition is not ambiguous because the potential of an ideal wire or a collection of wires connected to each other (all wires in this course will be assumed ideal) is the same at all points of the wires.

Voltage, also known as (electric) potential difference, is the difference in electric potential between two points in space. Notice that, the voltage is always measured with respect to two points (or nodes in an electric circuit) and, since it is the difference between two electic potentials, its SI unit is the volt (V). If the two points are denoted by $A$ and $B$, we have $$\begin{equation}V_{AB}=-V_{BA}\end{equation}$$ If the potential of node A is $V_A$ and the potential of node B is $V_B$, we have $$\begin{equation}V_{AB}=V_{A}-V_{B}\end{equation}$$ To simplify the notations on circuit diagrams, we often denote point $A$ (the first point) with $+$ and point $B$ (the second point) with $-$ (see Fig. 1). In this case, voltage $V$ is understood to be the voltage from the node denoted with $+$ to the node denoted with $-$.