MadSci Network: Engineering |
Greetings Michael:
Reference: Mad Science Archives Question: A Question about
electric circuits.
981476651.Ph
Historical Background
There has been confusion about the direction of flow of electricity
in an electrical circuit
for more than 200 years. During the late 1700s, when it was
discovered that electricity had a
polarity (+ and -), scientists such as Benjamin Franklin proposed
that electricity flowed from
positive potentials to negative potentials. Thus during the
development of electrical power
systems and rail systems during the 1800s electrical devices were
marked with arrows to show
the direction electrical current is flowing from positive to negative
in the devices and systems.
Around 1900 negatively charged electrons were discovered to be the
actual source of electrical
current flow and they flowed from negative potentials and were
attracted to positive potentials.
However, there was so much electrical equipment already in the world
by 1900 marked with the
wrong direction of current flow and so many electricians that were
taught about current that way,
the marking system has been continued to this day to be in the old,
wrong direction. Even today
electrical and electronic devices such as diodes and transistors and
their symbols are marked with
arrows in the opposite direction of electron current flow! This is
one of the confusing problems
we face when we study electrical engineering today.
Because many devices today control the direction of electron flow I
find it is best to use the
true direction of flow when analyzing circuits; however, most text
books still stick with the old
convention when analyzing electrical circuits. Either way works and
gives the correct answers as
long as you stick to one current direction and do not mix the two
concepts up.
Answers to your questions
I will use direct current (DC) systems for this example because most,
but not all automotive, aircraft
and ships use DC electrical circuits. DC Electrical power sources
such as batteries, generators, solar
cells etc. can be considered to be electron pumps that build up a
large supply of atoms with excess
electrons at the negative terminal and large number of atoms missing
electrons at the positive
terminal. We measure the electrical pressure between the power
source's terminals as voltage. When
we provide an external conducting path, a circuit (circle) for
electrons to flow from the negative
terminal of the power source to the positive terminal, we measure the
current flow in amperes (a given
number of electrons flowing per second).
Most electrical systems connect all devices, such as light bulbs, in
parallel across the power source
so that each device has the same voltage (electrical pressure) across
it and if one device burns out or
fails the other devices continue to operate.
GROUND
In the early 1800s one of the first electrical systems was the
electrical telegraph or lightning wire as
it was popularly called. Thousands of miles of telegraph wire was
strung across the continents and under
the ocean between continents. It was found out early that only one
telegraph wire need to be placed on
poles or in insulated undersea cables because the earth was a good
conductor to replace the return wire
to the telegraph battery and complete the circuit. Thus GROUND
became known as a universal conductor for
millions of circuits (In England they use the term Earth in place
of ground).
As electrical circuits were placed on vehicles for lighting and other
purposes the metal frame of an
automobile, the metal fuselage of aircraft and ship's hulls became a
moving ground wire to complete the
many parallel circuits that controlled lighting, motors etc. on
vehicles.
Your statement: "why do the instructions always say to disconnect
the negative battery terminal during
repairs�" is incorrect. Which terminal to disconnect is determined
by the automobile manufacturer. While
most automobiles manufactured in North America connect the negative
battery terminal to the frame, many
imported automobiles connect the positive battery terminal of the
battery to the frame! In these cars you
should disconnect the positive terminal first.
Why is first disconnecting the terminal connected to the frame
important? Most automobile batteries are
tightly packaged inside of the engine compartment and using metal
tools such as wrenches and sockets will
probably come in contact with the metal frame or a device connected
to the metal frame during the removal
of the battery cable. If this was the wrong terminal and not the
grounded terminal connected to the frame,
this accidental contact would cause a short circuit of hundreds of
amperes across the battery, producing
an arc, a flash, a burn or perhaps a battery or gas fume explosion!
Disconnecting the grounded terminal
is at the same potential as the battery terminal and an accidental
short circuit will not occur if the
tools make accidental contact with the frame during battery
removal.
A new problem is that today's automobiles have so much electronic
equipment running even when the car is
parked, the manufacture's books, which many people do not read, tell
the owners to disconnect
the grounded battery terminal if the car engine will not be run for a
week or longer. I have seen many people
at airport parking lots finding their auto batteries dead when they
return from long trips. For this reason
I always carry jumper cables to start the engines of these cars.
It does not matter which way you use the frame as a wire in the
circuit as long as you keep the current
flowing in the correct direction for direction sensitive devices.
Thus ordinary light bulbs do not care
which way current flows through then; however devices such as motors and
light emitting diodes (LEDs) are
current-direction sensitive. DC motors change their direction of
rotation by changing the current direction.
As to your question about fuses. Many devices in automobiles
have
separate fuses in series with each device.
Thus if the device fails the fuse prevents short circuits across the
battery and the other parallel circuits
are not affected. Because the frame is used to complete the circuits
for all of the devices in the car a
fuse in the terminal connected to the frame might open up all of the
device circuits even if only one device
failed. So fuses are placed in the power cables to the devices from
the battery terminal not connected to
the frame, which ever polarity the manufacturer has not grounded to
the frame.
I go into more detail about electric circuits in the Referenced
question in the Mad Science Archives.
Thank you for your interesting quetion.
Best regards, Your Mad Scientist
Adrian Popa
Try the links in the MadSci Library for more information on Engineering.