Source and linear circuit elements are
ideal circuit elements. One central notion
of circuit theory is combining the ideal elements to describe
how physical elements operate in the real world. For example,
the 1 kΩ resistor you can hold in your hand is not
exactly an ideal 1 kΩ resistor. First of all, physical
devices are manufactured to close tolerances (the tighter the
tolerance, the more money you pay), but never have exactly their
advertised values. The fourth band on resistors specifies their
tolerance; 10% is common. More pertinent to the current
discussion is another deviation from the ideal: If a sinusoidal
voltage is placed across a physical resistor, the current will
not be exactly proportional to it as frequency becomes high, say
above 1 MHz. At very high frequencies, the way the resistor
is constructed introduces inductance and capacitance
effects. Thus, the smart engineer must be aware of the frequency
ranges over which his ideal models match reality well.
On the other hand, physical circuit elements can be readily
found that well approximate the ideal, but they will always
deviate from the ideal in some way. For example, a flashlight
battery, like a C-cell, roughly corresponds to a 1.5 V
voltage source. However, it ceases to be modeled by a voltage
source capable of supplying any current
(that's what ideal ones can do!) when the resistance of the
light bulb is too small.
"Electrical Engineering Digital Processing Systems in Braille."