And they have a lot of loops. Although it is hard to read above, in this figure many of the wires are labeled with a specific potential . The potential of those certain wires usually has some meaning - it represents a signal from one part of the circuit to another, or it is a common voltage source for a lot of components. We can analyze any segment of a circuit using these rules, similar to how we related velocities in segments of mechanical systems. These are voltages relative to ground.
Ground is the symbol at the bottom, and the voltage of that wire is of course zero.
Finally, notice that I named too many voltages. So it would have been more efficient just to call it V bus , and not bother to name V 3 . Here is another example - we will examine this circuit, writing Kirchoff's Laws, element constitutive laws, and start to think about how we would get state equations for the system. This circuit looks pretty complicated. Five resistors, two capacitors, and two busses with named voltages.