Resistors are examples of so-called passive devices. We call them passive because they always dissipate energy. Active circuit elements actually generate energy. Examples of active circuit elements include independent voltage sources and independent current sources.
An independent voltage/current source is an idealized circuit component that fixes the voltage or current in a branch, respectively, to a specified value. Remember that the state of a circuit is given by the voltage across and current through each branch of the circuit. If a branch is a resistor, then we know that the current and voltage are related via Ohm's law. If that branch is an independent voltage source, then we know that the voltage across the branch has a fixed value, but the current is free. If the branch is an independent current source, then the voltage is free and the current through the branch is fixed.
Figure 8 shows the symbols for three
independent sources. The lefthand symbol depicts an
independent voltage source. The symbol is a
circle with the voltage polarities marked on them
and the voltage value 
. The righthand symbol
depicts an independent current source. The symbol is a
circle with the current direction denoted by an arrow
in the middle of the circle and the value or magnitude
of the current 
. The middle symbol is the symbol
for a specific type of independent voltage source known
as a battery. A battery is a physical
realization of an independent voltage source. Physical
realizations for independent current sources are
often specially built transistor circuits (an important 3-terminal device that we'll introduce later).
