# Circuit Theory/Circuit Basics

## 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 Junctions or Nodes

!Nodes are areas where the Electromotive Force is the
same.

Nodes are also called \"junctions\" in this book in order to make a
distinction between Node analysis, Kirchhoff\'s current law and
discussions about a physical node itself. Here a physical node is
discussed.

A junction is a group of wires that share the same electromotive
force (not voltage). Wires
ideally have no resistance, thus all wires that touch wire to wire
somewhere are part of the same node. The diagram on the right shows two
big blue nodes, two smaller green nodes and two trivial (one wire
touching another) red nodes.

Sometimes a node is described as where two or more wires touch and
students circle where wires intersect and call this a node. This only
works on simple circuits.

One node has to be labeled ground in any circuit drawn before voltage
can be computed or the circuit simulated. Typically this is the node
having the most components connected to it. Logically it is normally
placed at the bottom of the circuit logic diagram.

Ground is not always needed physically. Some circuits are
floated on purpose.

Node Quiz

## Measuring instruments

**Voltmeters and Ammeters** are devices that are used to measure the
voltage across an element, and the current flowing through a wire,
respectively.

### Ideal Voltmeters

An ideal voltmeter has an infinite resistance (in reality, several
megaohms), and acts like an open circuit. A
voltmeter
is placed across the terminals of a circuit element, to determine the
voltage across that element. In practice the voltmeter siphons enough
energy to move a needle, cause thin strips of metal to separate or turn
on a transistor so a number is displayed.

### Ideal Ammeters

An ideal ammeter has zero resistance and acts like a short circuit.
Ammeters require cutting a wire and plugging the two ends into the
Ammeter. In practice an ammeter places a tiny resistor in a wire and
measures the tiny voltage across it or the ammeter measures the
magnetic field strength generated by
current flowing through a wire. Ammeters are not used that much because
of the wire cutting, or wire disconnecting they require.

## Active, Passive & ReActive

The elements which are capable of delivering energy or which are capable
to amplify the signal are called \"Active elements\". All power supplies
fit into this category.

The elements which will receive the energy and dissipate it are called
\"Passive elements\". Resistors model these devices.

Reactive elements store and release energy into a circuit. Ideally they
don\'t either consume or generate energy. Capacitors, and inductors fall
into this category.

## Open and Short Circuits

### Open

No current flows through an open. Normally an **open** is created by a
bad connector. Dust, bad solder joints, bad crimping, cracks in circuit
board traces, create an **open**. Capacitors respond to DC by turning
into **opens** after charging up. Uncharged inductors appear as
**opens** immediately after powering up a circuit. The word **open** can
refer to a problem description. The word **open** can also help develop
an intuition about circuits.

Typically the circuit stops working with opens because 99% of all
circuits are driven by voltage power sources. Voltage sources respond to
an open with no current. Opens are the equivalent of clogs in plumbing
.. which stop water from flowing.

On one side of the open, EMF will build up, just like water pressure
will build up on one side of a clogged pipe. Typically a voltage will
appear across the open.

### Short

A voltage source responds to a **short** by delivering as much current
as possible. An extreme example of this can be seen in this ball
bearing motor video. The
motor appears as a short to the battery. Notice he only completes the
short for a short time because he is worried about the car battery
exploding.

Maximum current flows through a **short**. Normally a **short** is
created by a wire, a nail, or some loose screw touching parts of the
circuit unintentionally. Most component failures start with heat build
up. The heat destroys varnish, paint, or thin insulation creating a
**short**. The **short** causes more current to flow which causes more
heat. This cycle repeats faster and faster until there is a puff of
smoke and everything breaks creating an **open**. Most component
failures start with a **short** and end in an **open** as they burn up.
Feel the air temperature above each circuit component after power on.
Build a memory of what normal operating temperatures are. Cold can
indicate a short that has already turned into an open.

An uncharged capacitor initially appears as a **short** immediately
after powering on a circuit. An inductor appears as a **short** to DC
after charging up. The **short** concept also helps build our intuition,
provides an opportunity to talk about electrical safety and helps
describe component failure modes.

A **closed** switch can be thought of as short.
Switches are surprisingly complicated.
It is in a study of switches that the term **closed** begins to dominate
that of **short**.
