|MadSci Network: Engineering|
Reference: Mad Science Archives Question: A Question about electric circuits.
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.
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
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