Circuit Theory Circuits

Circuits (also known as "networks") are collections of circuit elements and wires. Wires are designated on a schematic as being straight lines. Nodes are locations on a schematic where 2 or more wires connect, and are usually marked with a dark black dot. Circuit Elements are "everything else" in a sense. Most basic circuit elements have their own symbols so as to be easily recognizable, although some will be drawn as a simple box image, with the specifications of the box written somewhere that is easy to find. We will discuss several types of basic circuit components in this book.

Ideal Wires

For the purposes of this book, we will assume that an ideal wire has zero total resistance, no capacitance, and no inductance. A consequence of these assumptions is that these ideal wires have infinite bandwidth, are immune to interference, and are — in essence — completely uncomplicated. This is not the case in real wires, because all wires have at least some amount of associated resistance. Also, placing multiple real wires together, or bending real wires in certain patterns will produce small amounts of capacitance and inductance, which can play a role in circuit design and analysis. This book will assume that all wires are ideal.

Ideal Nodes

Schematic representation of wire crossing with (a) and without (b) connection between the wires. The third style is prefered.

Like ideal wires, we assume that connecting nodes have zero resistance, et al. Nodes connect two or more wires together. On a schematic, nodes are frequently denoted with a small filled-in black dot. When 2 wires cross on a schematic, but they do not physically intersect (for instance if one wire lays on top of another wire), there is no node drawn.

The diagram on the right shows three ways of drawing the interesection of two wires with connection (a) and no connection (b). The modern convention is to use the third style.

In real life, nodes are often connected together, either by wire nuts, or solder, or other connectors. These connectors can have a certain amount of associated resistance, capacitance, or inductance associated with them. This book will not, however, take this interference into account, as it is usually negligible.

Active vs Passive

The elements which are capable of delivering energy are called "Active elements".The elements which will receive the energy and dissipate or store it are called "Passive elements".

Voltage and Current Generators are examples of active elements that can deliver the energy from one point to some other point. These are generally considered independent generators of electric energy. From a system point of view, this is not an accurate depiction since the energy output will be directly related to the energy put into the system or stored in the system previously. Some examples of these generators are alternators, batteries etc...

A previous definition stated; "A dependent source will generate current or voltage but the energy output will depend on some other individual parameter(may be voltage or current) in the same circuit, whereas an independent source will generate regardless of the connections of the circuit."

From a localized perspective, this definition can still be useful. This definition can be used to differentiate a power source ("independent source") from an active power control device, or amplifier ("dependent source"). It is probably more useful to think of "dependent sources" as "energy amplifiers" or "active devices".

The three linear passive elements are the Resistor, the Capacitor and the Inductor. Examples of non-linear passive devices would be diodes, switches and spark gaps. Examples of active devices are Transistors, Triacs, Varistors, Vacuum Tubes, relays, solenoids and piezo electric devices.