In the study of electric circuits, we deal with the fundamental phenomenon of the movement of electrically charged particles or simply charged particles. The fundamental quantities that are used to describe how rapidly charged particles move in a circuit and in what way they do so in the circuit are current and voltage.
Current is sometimes referred to as the ``through''
quantity and voltage as the ``across'' quantity. In
the physical context, current is the flow of electric charge through
a component or apparatus, whereas voltage is the potential difference
between two points in a circuit. Current flows from high potential to
low potential. In particular we define the current, I, flowing in a
component or apparatus as the amount of charge passing through that
component or apparatus per unit of time. Denoting charge by q,
we may write current I as
Two other important quantities that are frequently used in describing
physical systems are power and energy. If a small
quantity of electric charge 
is displaced from a point A to
a point B, then the change in its potential energy (or
work done) is equal to 
, where V denotes the voltage
between A and B. In order to measure how rapidly work is done, we
consider the amount of work done per unit of time. This quantity is
called power, and is usually denoted by P .
The unit of current is the ampere (A), that of voltage
is the volt (V), that of energy is the joule (J),
and that of power is the watt (W). Prefixes are often used to
emphasize the significant figures when the magnitudes are too large
or too small. Common prefixes and their corresponding multipliers are
shown in Table 1.1. For example, 1ns denotes
1
10
s, and 6kV denotes 610
V.
