Basic Electrical Theory

CHASSIS ELECTRICAL
UNDERSTANDING AND
TROUBLESHOOTING
ELECTRICAL SYSTEMS

Basic Electrical Theory
For any 12 volt, negative
ground, electrical system to
operate, the electricity must
travel in a complete circuit.
This simply means that current
(power) from the positive (+) terminal of the battery must
eventually return to the
negative (-) terminal of the
battery. Along the way, this
current will travel through
wires, fuses, switches and components. If, for any reason,
the flow of current through the
circuit is interrupted, the
component fed by that circuit
will cease to function properly.

Perhaps the easiest way to
visualize a circuit is to think of
connecting a light bulb (with
two wires attached to it) to
the battery — one wire attached to the negative (-)
terminal of the battery and the
other wire to the positive (+)
terminal. With the two wires
touching the battery terminals,
the circuit would be complete and the light bulb would
illuminate. Electricity would
follow a path from the battery
to the bulb and back to the
battery. It's easy to see that
with longer wires on our light bulb, it could be mounted
anywhere. Further, one wire
could be fitted with a switch
so that the light could be
turned on and off.

The normal automotive circuit
differs from this simple
example in two ways. First,
instead of having a return wire
from the bulb to the battery,
the current travels through the frame of the vehicle. Since the
negative (-) battery cable is
attached to the frame (made
of electrically conductive
metal), the frame of the
vehicle can serve as a ground wire to complete the circuit.
Secondly, most automotive
circuits contain multiple
components which receive
power from a single circuit.
This lessens the amount of wire needed to power
components on the vehicle.

HOW DOES ELECTRICITY
WORK: THE WATER ANALOGY
Electricity is the flow of
electrons — the subatomic particles that constitute the
outer shell of an atom.
Electrons spin in an orbit
around the center core of an
atom. The center core is
comprised of protons (positive charge) and neutrons (neutral
charge). Electrons have a
negative charge and balance
out the positive charge of the
protons. When an outside force
causes the number of electrons to unbalance the charge of the
protons, the electrons will split
off the atom and look for
another atom to balance out. If
this imbalance is kept up,
electrons will continue to move and an electrical flow
will exist.

Many people have been taught
electrical theory using an
analogy with water. In a
comparison with water
flowing through a pipe, the
electrons would be the water and the wire is the pipe.

The flow of electricity can be
measured much like the flow
of water through a pipe. The
unit of measurement used is
amperes, frequently
abbreviated as amps (a). You can compare amperage to the
volume of water flowing
through a pipe. When
connected to a circuit, an
ammeter will measure the
actual amount of current flowing through the circuit.
When relatively few electrons
flow through a circuit, the
amperage is low. When many
electrons flow, the amperage
is high.

Water pressure is measured in
units such as pounds per
square inch (psi); The electrical
pressure is measured in units
called volts (v). When a
voltmeter is connected to a circuit, it is measuring the
electrical pressure.

The actual flow of electricity
depends not only on voltage
and amperage, but also on the
resistance of the circuit. The
higher the resistance, the
higher the force necessary to push the current through the
circuit. The standard unit for
measuring resistance is an
ohm. Resistance in a circuit
varies depending on the
amount and type of components used in the circuit.
The main factors which
determine resistance are:

* Material — some materials have more resistance than others.
Those with high resistance are
said to be insulators. Rubber
materials (or rubber-like
plastics) are some of the most
common insulators used in vehicles as they have a very
high resistance to electricity.
Very low resistance materials
are said to be conductors.
Copper wire is among the best
conductors. Silver is actually a superior conductor to copper
and is used in some relay
contacts, but its high cost
prohibits its use as common
wiring. Most automotive
wiring is made of copper.

* Size — the larger the wire size being used, the less resistance
the wire will have. This is why
components which use large
amounts of electricity usually
have large wires supplying
current to them.

* Length — for a given thickness of wire, the longer the wire,
the greater the resistance. The
shorter the wire, the less the
resistance. When determining
the proper wire for a circuit,
both size and length must be considered to design a circuit
that can handle the current
needs of the component.

* Temperature — with many materials, the higher the
temperature, the greater the
resistance (positive
temperature coefficient). Some
materials exhibit the opposite
trait of lower resistance with higher temperatures (negative
temperature coefficient).
These principles are used in
many of the sensors on the
engine.

OHM'S LAW
There is a direct relationship
between current, voltage and
resistance. The relationship
between current, voltage and
resistance can be summed up
by a statement known as Ohm's law. Voltage (E) is equal
to amperage (I) times
resistance (R): E=I x R Other
forms of the formula are R=E/I
and I=E/R

In each of these formulas, E is
the voltage in volts, I is the
current in amps and R is the
resistance in ohms. The basic
point to remember is that as
the resistance of a circuit goes up, the amount of current that
flows in the circuit will go
down, if voltage remains the
same.

The amount of work that the
electricity can perform is
expressed as power. The unit
of power is the watt (w). The
relationship between power,
voltage and current is expressed as: Power (w) is
equal to amperage (I) times
voltage (E): W=I x E This is only
true for direct current (DC)
circuits; The alternating current
formula is a tad different, but since the electrical circuits in
most vehicles are DC type, we
need not get into AC circuit theory.


Credit:
http://www.freeautomechanic.com/wiringproblems.html