Chapter 22
DC Electric Circuits
Electronic components like batteries and resistors may be combined into closed loops calledcircuits.An
almost endless variety of such circuits may be used to create useful device: clocks, calculators, radios, etc.
In designing an electronic circuit, an engineer or electronics hobbyist will need to perform some simple
calculations to figure out how much current is going through each part of the circuit, and how much potential
difference there is across each component. We’ll look at some of the basic methods of analysis here, using
simple circuits consisting only of batteries and resistors.
22.1 Schematic Diagrams
To show how the components of an electronic circuit are connected together, we draw aschematic diagram.
Such a diagram uses symbols to represent the different components (as shown in Fig. 22.1), along with lines
to represent the connecting wires. When two lines in a diagram cross, a dot is used to indicate that the wires
are electrically connected at that point; the absence of a dot means that there is no electrical connection.
Note that in the battery symbol, the end with thelongline is thepositiveCterminal.Groundrefers to
a connection to a large conductor—traditionally to a copper pipe driven into the earth. You will often see
several parts of a circuit connected to a common ground, with theterminal of the battery or power supply
serving as the ground.
22.2 Kirchhoff Plots.
A good way to visualize what’s happening in an electrical circuit is a diagram that has been called aKirchhoff
plot(Ref. [17]). This is a three-dimensional plot in which one draws the circuit in thex-yplane; the potential
(voltage) at any point in the circuit is then plotted on the ́axis. (See Fig. 22.2.) The plot helps you to think
of voltage as analogous to elevation: batteries cause an increase in elevation, and resistors cause a drop in
elevation. And just as water always flows downhill, you can use the diagram to help visualize electric current
flowing from high potential to low potential.
22.3 A Simple Circuit.
As an example, let’s look at the simple circuit shown in Fig. 22.3, consisting of a battery and three resistors.
Conventional current flows around the circuit clockwise: from theCterminal of the battery, through the
resistors, then back into theterminal of the battery. (Remember that the electrons actually travel in the
opposite direction, counterclockwise.) The current flowing from the battery through resistorR 1 we labelI 1 ;