BASIC
ELECTRICITY-1
This
is one of the required topics in the Electrical Repairs Course.
We will cover only material that is needed to repair truck electrical
problems. We will not go into scientific detail beyond what you
need for understanding of truck electrical problems.
This
topic will cover direct current (DC) circuits only, because that
is what your truck battery provides to all your truck electrical
devices. Alternating current (AC) is used in your home, is much
more complicated, and will not be covered in these topics. Good
luck with your new knowledge; the rewards are well worth the effort.
Electric
Charge & Atoms
Electricity
is a physical phenomenon involving positive and negative charge.
When these charges are in motion they may produce heat, light
and magnetism. When charges are not in motion, static electricity
can manifest itself as a force such as clothes clinging to each
other when they are removed from a dryer. Static electricity in
clouds results in lightening strikes from cloud to cloud or from
cloud to earth.
All substances
are composed of atoms. Each atom has negative charges called electrons,
which orbit the atom nucleus, which contains positive charges
called protons. In most atoms, the positive and negative charges
are equal and balanced. If an atom has 10 electrons, it also typically
has 10 protons.
The electrons
orbit the atom nucleus, therefore, they can be easily torn away
by external energy. A charge potential is created when electrons
from one atom are moved to another atom. Some energy is required
to make that electron change atoms, and once the change takes
place, that charge is the stored energy which was required to
make the change and move that electron.
Friction
(rubbing) and chemical reactions (batteries) can create electrical
charge differences between two materials. In clouds on hot summer
afternoons, the friction of the air masses moving against each
other, rubs electrons from one air mass and deposits them onto
another air mass. Now the two air masses have an electrical potential
difference (charge difference). Dust or moisture in the air masses
increases the friction and increases the potential difference
build up. Lightening strikes are the result of a tremendous potential
difference. Batteries create a much smaller potential difference
between the two battery plates due to the chemical reaction of
the plates with the battery electrolyte (acid).
Conductors
Conductors
are used to move electrical current to another location. Most
metals have an abundance of free electrons, which allows electrons
to flow easily from one atom to the next atom. Some of these metals
are Gold, Silver, Copper, and Iron. Gold is the best conductor
and iron is the least conductor of these listed metals.
Conductors
need very little potential difference to make electrons move (current
flow (amps)) within them. Most truck wiring is made of copper,
which is cheaper than silver or gold. Copper is the best choice
for conductors on a truck because it is cheaper than gold and
silver, and has less resistance that iron wire.
The bigger
the diameter of the copper wire, the more atoms there are per
unit of distance to provide electron transfers. Therefore, the
larger the wire, the less resistance the wire has (smaller ohms).
Your truck's starter requires massive current to crank the engine
(about 500 amps), therefore the starter cables are very large
in diameter so that they can pass the massive quantity of electrons
with very little resistance. On the other hand, the dome light
requires very little current (about 1/2 amp), so it has a wire
which is very small in diameter.
Insulators
Insulators
are used to block electrical current. Most conductors are sheathed
in an insulation (insulator) material to prevent the electrical
current from taking an alternate path while traveling to its destination.
Insulator
substances have very few free electrons. This prevents easy electron
flow from one atom to the next atom. Glass, plastic, and rubber
are examples of good insulators. Other good insulators are air
and water! Yes, pure water is an insulator. Only the minerals
in the water make it conduct electricity. Think about it for a
moment, don't you put water in your battery? If it was a conductor,
wouldn't it short out the battery plates? This is why you are
supposed to use distilled water in batteries. Distilled water
has no minerals to short out the battery plates.
By the way,
that water in your bath tub has many minerals in it, so you don't
want to place any electrical appliances near by!
Stored
Energy
Batteries
provide a way to store electrical energy in chemical form. When
electrical energy is required, the battery converts chemical energy
into electrical energy as needed. The lead acid battery, is composed
of lead plates and acid electrolyte, and is used for all automotive
and trucks applications.
Each truck
battery cell produces 2 volts; therefore 12-volt batteries have
6 cells. The two plates of the battery cell are labeled plus (+)
and minus (-). As the lead acid battery is charged, the charging
energy is stored  within
the electrolyte (acid) of the battery, and a 2-volt potential
difference exists between the two battery plates. When the battery
is in use (or discharged), the electrolyte and the battery plates
work together to provide free electrons to do the battery's work.
In other words, the energy stored within the battery is converted
chemically into electron potential difference between the two
plates, which performs work outside of the battery.
Conventional
Current & Electron Current
Back
in the early days of electrical study, scientists decided that
electrical current flowed from the positive battery plate to the
negative battery plate. This concept is still accepted today,
and is referred to as conventional current theory.
When
scientists finally learned the atom's construction, it became
quite obvious that the electron (negative) charge is the part
to break away from the atom, since the proton (positive) charge
is contained within the nucleus of the atom. Electron flow is
the true electrical current. So this current concept was aptly
named electron current theory. Both theories abound today, so
check the book you are reading to see if they are using conventional
theory or electron theory.
Like
electrical charges repel and unlike electrical charges attract.
So the excess electrons at the negative plate of the battery cell
would just love to get over to the positive plate where there
is a shortage of electrons. All they need is some path to make
the jump between the two plates. Once these electrons have a path,
electrical work is done, and eventually the battery will become
discharged and neutralized of its stored energy (same charge on
both plates).
The
following illustration shows how electrons pass from one atom
to another and the resulting electron comes out the other end
of the wire. The electron passes through the wire from atom to
atom, much like the energy of the pool stick passes from one ball
to the next, with the 8 ball flying off the end. When the first
ball is struck by the pool stick, another ball pops out the other
end of the string of balls.
It
is the same with electron flow in a conductor (wire). As each
electron is stuffed into an atom at one end of the wire, another
electron is forced out of the wire from another atom at the other
end. Note the positive charge on the left side of the wire and
the negative charge on the right side of the wire. This charge
difference, or potential difference, or voltage difference, (they
all mean the same thing) is what forces the electrons to pass
through the wire. Electrons have a negative charge, so they rush
away from the like negative charge and travel through the wire
to get to the unlike positive charge. When the electrons are forced
through the wire, work is done. It took energy to force the electrons
through all those atoms. Therefore the wire has some Resistance
(opposition to current flow).
.
The above
illustration shows the difference between conventional current
flow and electron current flow. All references to electrical current
at this website will refer to electron current flow, because that
is how it really works. In either case, the light bulb still lights!
Electrical
Work (Power)
Work is done
by electricity when electrons are forced to move through some
resistance to discharge a potential difference. In the above battery
and light bulb example, electrons were forced to move through
the wire and through the light bulb. This flow of electrons is
called current, and is measured in amperes. The wire's opposition
to the electron flow and the bulb's opposition to electron flow
is called resistance, and is measured in ohms.
When current
is forced through a resistance, work is said to have been done.
Electrical work is measured in Watts and the power is represented
by "P" for power. Approximately 760 Watts equals one
horsepower.
Any device,
which uses electrical power, is called an electrical load, so
a headlamp, which is illuminated, is an electrical load. Electrical
power is electrical work being done. Just like it takes power
to move your truck, it also takes power to light that light bulb.
Current must be pushed from the negative battery terminal, through
the bulb, and back to the positive terminal of the battery, and
through the electrolyte, to make the bulb light.
Electrical
Symbols & Ohm's Law
The electrical
symbol for power is "P". The electrical symbol for voltage
is "E" (E for voltage??). The electrical symbol for
resistance is "R" (they got one right). The electrical
symbol for current is "I" (go figure!).
A man named
Charlie Ohm spent most of his weekends studying electricity and
came up with a set of rules which are called Ohm's Law. This rules
are used to evaluate any electrical circuit. His rules define
the relationship between voltage, resistance, current, and power.
His rules are really quite simple. They are:
I = E / R
(current = voltage divided by resistance)
E = I * R ( voltage = current multiplied by resistance)
R = E / I ( resistance = voltage divided by current)
P = E * I ( power = voltage multiplied by current)
P = (E * E) / R (power = voltage squared divided by resistance)
P = I * I * R (power = current multiplied by current multiplied
by resistance)
Simple
Series Circuit
All electrical
circuits have three elements. A source of voltage (potential difference),
an electrical load (something which takes current (amps)), and
a path for the current to flow through (conductors). The following
electrical circuit consists of a 12-volt battery, a headlamp bulb,
and the wires (conductors) which connect the battery terminals
to the bulb.
Two things
determine the power consumed by an electrical load, the potential
difference (voltage) of the source, and the current (amps) delivered
into the load resistance.
The voltage
of the circuit shown below is a 12-volt truck battery. This circuit
is called a series circuit because the same current passes through
all components of the circuit. The electrical resistance of this
load is the filament within the head lamp light bulb.
In the circuit above, notice the Voltmeter and the Ammeter. The
voltmeter measures the potential difference across two points.
In this example, the voltmeter is measuring the voltage across
the light bulb. The ammeter measures the current passing through
the ammeter. Detailed theory of meters can be found in our meter
theory topic, and proper use of meters can be found in our
topic called using multimeters
.
The filament
within the light bulb is made of a special metal, which passes
current easier than an insulator, but harder than a conductor.
This special metal is called tungsten. The 12-volt potential difference
of the battery, forces electrons through the tungsten, but the
tungsten resists this current flow, resulting in work being done
by the battery to overcome the tungsten resistance. This work
is converted into heat which heats the tungsten until it gets
white hot, and gives off light. This heating of the tungsten requires
power from the battery, and the unit of electrical power is watts.
Notice that the light bulb rating is 36 Watts.
The resistance
of this light bulb is specified as 4-ohms. Let us use Ohm's Law
and determine what current the amp meter will indicate.
I = E / R
(therefore, current "I" = 12-Volts divided by 4-Ohms
= 3-Amps)
There, that
wasn't so hard was it. We know that the bulb is rated at 36 Watts,
so lets see how much power the bulb is using in this circuit.
P = I * I
* R (therefore, power "P" = 3-Amps multiplied by 3-Amps
multiplied by 4-Ohms = 36-Watts)
The bulb
is operating at its designed power rating in this circuit. What
is the voltage reading of the voltmeter?
E = I * R
(therefore, voltage "E" = 3-Amps multiplied by 4-Ohms
= 12-Volts.
You have
just seen how Ohm's Law is used in real life. Any time you know
two circuit parameters, you can calculate the third circuit parameter,
using Ohm's Law.
This concludes
the basic electricity topic. We have covered a lot of material
which will be used in the next topic. We avoided much scientific
detail because you don't need that detail to fix truck electrical
problems. You do need an understanding of each area that we presented
here. If you are unsure about anything in this topic, reread that
area until it makes sense. Everything else electrical is based
upon the material presented in this topic.
We suggest
that your next topic be Basic Electricity-2,
where you will learn the other important concepts of electricity.
If you have any problems or comments, feel free to contact webRider.
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