Download Section 1.3 BASIC ELECTRICAL CIRCUIT

BASIC ELECTRICAL CIRCUIT
In order to have a complete electrical circuit
three main parts must be present.
a power_source,
a load, and
a conducting pathway.
A Power Supply
►
A Power Supply is a device that provides
potential difference so as to force electrons
to flow from a negatively charge cell to a
positively charged cell.
► Potential
Difference, as discussed in our
earlier chapter is the imbalance in nature
where negative electrons are separated
from atoms that are able to receive atoms.
► Potential
Difference can be created by either
chemical or electromechanical processes.
► The
chemical process of creating a potential
difference is by inserting differing metals in an
acidic solution where the metals interact with the
acid to, with one metal, produce excess electrons
and the other metal produces atoms that are
lacking electrons. This process is known in the
general public as a battery.
► The
electromechanical process is to pass
wire conductors through magnetic fields,
this induces electrons to move through the
wire, creating a current flow.
When the battery terminals are connected
together all of the electrons flow through
the wire from the negative cell with excess
electrons to the positive cell that contains
positive atom that are lacking electrons
(these are know as holes).
► When all electrons have found a partner
hole the battery becomes neutral and, as
we say, is dead (or neutralized).
►
► These
terminals are the connection points
for each cell in the battery. The positive
cells are marked with a plus sign (+) and
the negative cells that has too many
electrons and is marked with a negative sign
(—).
two markings are referred to as polarity
markings. All power supplies have polarity but not
all electrical devices have polarity. Whenever
polarity is a critical issue, it will be marked on the
device.
► These
► The
technology behind solid-state components is
based on the fact that current can be directed to
flow in a single direction, if current is allowed to
flow in the wrong direction components can be
damaged. The proper polarity must be followed to
avoid damage to equipment and/or people.
Throughout the history of electricity /
electronics there has emerged two theories
that describe electricity flow. In the early
years scientist thought electricity flowed
from positive charge toward negative
charge (similar to water flowing from a full
bucket to an empty one).
When scientist became aware of atoms and
the negative charge of an electron they
began to describe current flow as negative
to positive. These two theories are known
as conventional and electron current flow.
It is not of so much importance that you
know which theory is used but which
polarity electrical components are connected
in.
Load
► The
second aspect a circuit must include is
a load. A load is created in a circuit when
the electrical energy produced in a circuit is
converted to some other form of energy
such as heat, light, or magnetism. A lightbulb is a load that produces light. When
designing a circuit it is important that the
power supply rating and the load rating
should match.
►
If the lamp is rated at 6 volts, then the
battery should only supply 6 volts of
potential difference. If the battery is rated
at a lower voltage rating, the lamp will
appear dim or will not light. If the battery is
rated at a much higher voltage, the lamp
will be damaged by the excess electrical
energy.
► Often
times a resistor can simulate load in a
circuit. No work is done just the decrease
in current.
► If
a circuit has no resistance the electricity
will flow too fast and out of control and this
will damage components. This situation is
know as a short circuit.
► The
two most common ways Resistors are
created either by carbon material
resistance or by wire wound into coils that
by means of electromagnet fields resist
current flow. When a resistor has a high
resistance less current can flow through it
which allows the circuit builder to control
current flow.
Conductors
► The
third aspect that must be included to
create a complete circuit is a conductor. A
conductor is a length of material that will
allow electrons to move through it. Most
often you will work with copper wire as the
conductor. The atomic structure of copper
and its availability in nature makes this
material an excellent choice for electrical
circuit conductors.
► Most
conductors are covered in a plastic
coating to provide an insulating cover
around them. This makes the conductor
pathway safe to work with. When
components are connected just enough of
the isolative cover is stripped from the ends
to connect to the components and the rest
of the wire is protected from a short circuit.
► Although
a complete circuit is create when it
includes a power source, a conductor, and a
load, almost all circuit also include a control
device know as a switch to start and stop
current flow. There are many different
types of switches and we will learn about
these later in this lesson.
► To
help facilitate the development of electric
and electronic circuits a system of drawings
was developed known as schematics. A
schematic uses symbols to represent circuit
components and can show all connection in
a circuit before you actually solder them
together.
►A
simulator software goes one step further
by animating schematics to work just as real
circuits would to show outcomes without
damaging components or causing human
injuries.
A Simple Example
An example of a simple electrical circuit we
see quite often is a flashlight. It consists of
a battery (sometimes two or three) as the
source of electrical energy, a small light-bulb
as the electrical load, and wires as the
conductors connecting the battery to the
light-bulb.
In the power supply of this circuit, the battery,
which uses chemical reactions, produce
ionization. Ionization is the process of
producing an excess of electrons (negative
charge) in one cell and a depletion of
electrons (positive charge) in the other. As
the electrons flow through the wire from to + through the light-bulb it changes the
electron flow into another form; light.
► Any
electrical circuit that has a power
source pushing electrons through a
conductive pathway to an electrical device
that converts electricity flow into other
forms of energy can be quantified by three
key forces; voltage, current and resistance.
Power Supplies produce
Voltage
► Voltage
is the unit of measurement that
quantifies the potential difference a power
supply produces. What is voltage? Voltage is
the force behind electron flow. Voltage is
often supplied by chemical reactions in a
battery or electro-mechanical actions such
as generators and turbines.
► Voltage
is measured in units known as
volts. volts are abbreviated by V is the
electrical unit used to express the amount
of electrical pressure present, or the amount
of electrical force produced from the present
imbalance.
► The
term voltage is used to express the
amount of electrical force in much the same
way we use horsepower to express the
amount of mechanical force for an
automobile. Electrical pressure or voltage
can also be expressed as potential, potential
difference, or as electromotive force (emf).
For our purposes, these terms mean the
same thing. Voltage is usually represented
by the capital letter E or V.
Voltage Pressure causes
Current flow
► The
movement of electrons moving through
a conductor is known as current. The
amount of electrons flowing in a conductor
is rated in the electrical unit known as amps
abbreviated as A. The ampere is
represented in mathematical expressions as
the letter I just as voltage is represented as
V.
►A
coulomb is a somewhat large quantity of
electrons developed to help count the amount of
electrons that flow past a given point in a
conductor within a one second time frame. If 6.24
x 10^18 (one coulomb) passes a given point
within one a second time frame one Ampere of
current is flowing.
► Compare
a balloon filled with air to an electrical
battery. The amount of air molecules in the
balloon represents the amount of electrons or
coulombs. The amount of air pressure inside the
balloon is expressed as pounds per square inch
(PSI) of air pressure. In the battery, the amount of
electrical pressure inside the battery is expressed
as the voltage rating of the battery. The rate of air
flow out of the balloon is similar to electron flow,
or current, from the battery.
► The
current from the battery in an electrical
circuit is the volume of the electrons that
flow past a given point, and is rated in
amperes or amps. Just as the air will
continue to escape from the balloon until
the balloon is empty, the electron flow can
continue as long as there is voltage or
electrical pressure present in the battery.
Resistance
► The
third aspect that is present in a circuit that
can be measured and quantified is resistance.
Resistance is simply the opposition to the flow of
electrons and even in the most conductive
material there is always some amount of attraction
of electrons to protons that would cause
resistance to current flow. Resistance is measured
in ohms, and the electrical symbol for ohm is 
(the Greek letter omega).
► The
resistance values of elements and
compounds differ according to the atomic
structure of the material. A good conductor
of electricity is anything that permits the
free flow of electrons.
►A
poor conductor of electricity is a material that
will not permit the free flow of electrons.
Extremely poor conductors are referred to as
insulators.
►A
semiconductor is a material that limits the
flow of free electrons. A semiconductor is
considered neither a good conductor nor
poor conductor of electricity. Semiconductor
materials are at the very heart of modem
electronic applications and will be explored
in depth later in this course.
► So
to explain the basic happenings in a
circuit your would say, “When a voltage
across the ends of a resistor current begins
to flow, and when current is flowing through
a resistor heat is dissipated and cause a
load in the circuit”.
End Here
Current, AC and DC
► There
are two types of electrical current, DC
(direct current) and AC (alternating
current). The difference between these
currents is how they flow through an
electrical circuit.
► Direct
current flows in only one direction
through an electrical circuit. In a battery the
chemical reactions cause one cell to have a
surplus of electrons and another cell that
has a lack of electrons. The electrons flow in
one direction from negative surplus to
positive lack. The battery has a set polarity
(positive and negative terminals) and will
produce an electric current in only one
direction.
► Alternating
current, as its name implies,
flows in both directions. First it flows in one
direction, and then it reverses its flow to the
opposite direction.
► There
are no positive or negative polarity
markings in alternating current because the
polarity changes so rapidly in the typical ac
electrical circuit. The terms cycle and hertz
are used to describe how fast the current is
alternating or changing direction in the
circuit. A 60 cycle ac circuit (operating at 60
hertz) changes direction 120 times per
second. This is the standard for ac in the
USA.
Conventional Current Flow vs.
Electron Flow Theory
► Approximately
200 years ago, scientists
theorized that electricity had both positive
and negative polarities and at that time they
arbitrarily decided that electrical current
flowed from positive to negative. While it
was never actually proven as fact, this
theory was accepted for quite some time.
This theory is known as the conventional current
flow theory.
► As
our knowledge of science progressed, and with
the discovery of the atom and semiconductor
electronics, it became apparent that the
conventional current flow theory was incorrect. It
is widely accepted that it is the electrons that
actually move, and they flow from negative to
positive, not from positive to negative. This newer
theory is known as electron flow theory.
► The
emergence of this new theory caused a
controversy that is still in existence today.
For over 150 years all circuit designs had
been based upon the old, conventional
current flow theory.
► Many
circuits and devices still used today are
based on the conventional theory. This text uses
the convention that will make the concepts in each
example most easily understood. Most of the
figures in this text show electron flow. Regardless
of which theory is used to explain the phenomena
of electronics, the most important point is that the
correct polarity must be maintained when building
circuits with devices that require a definite polarity.
Examine each example for polarity markings.
Review Questions for Section 1.3
► 1.
What are the three main parts of an
electrical circuit?
► 2. The ____________ supplies the
electrons that will flow through the circuit.
► 3. The __________ provide a path through
which the electrical energy can flow.
► 4.
The ____________ is where the
electrical energy is converted to another
form of energy.
► 5. The source has __________ markings
that are identified with a ___ or ___
symbol.
► 6. The movement of electrons is known as
___________________.
► 7.
Opposition to current flow is called
► 8. Opposition to current is measured in
____________
► 9. Electrical pressure is measured in
____________
► 10.___________
theory states that
electrons flow from negative to positive.
► 11.__________ current flows only in one
direction while __________ current
constantly changes direction.
► 12.Connecting the correct ___________ of
an electrical device in an electrical circuit is
more important than which theory of
current flow is used.
► 13.A(n)
__________ circuit provides only
one path for electron flow.
► 14.A(n) __________ circuit provides more
than one path for electron flow.
If Electricity isn’t Static
it is Dynamic.
► Once
a pathway connects a place of
electrical excess to an electrical place of
lacking. Entropy kicks in and the electronics
start to flow (potential energy has changed
to kinetic). This movement of electrons is
most commonly referred to as CURRENT
flow. Due to normal thermal agitation
(heat), some electrons in the outer orbit of
an atom will break away and become free.
► They
remain free for only a fraction of a
second, yet at any given time there are
millions, indeed trillions of FREE
ELECTRONS12 in a particular substance. If
we can get these free electrons to move in
one direction we have what is known as
electronic CURRENT FLOW.
► Some
materials have many more free
electrons than others. Rubber, for example,
has almost none at any temperature, while
copper is just loaded with them. Rubber is
considered a good Insulator13. This means
that it will resist current flow because it has
so few free electrons that can move. Yet
copper is an excellent conductor because it
has many free electrons ready to move.
► To
get free electrons to move in one
direction (current flow), an outside force of
some type is needed. This outside force is
known as VOLTAGE. As it has been said, at
any given time there are trillions of free
electrons in a particular material such
copper. (Actually any metal will have
countless free electrons.)
► However,
these free electrons are "dancing
around" in all directions and do not
constitute current flow. A voltage force lines
up these free electrons and pushes them all
in the same direction.
► If
the voltage force where too strong it
would not only move free electrons but also
begin to force stable electrons from the
atoms they are attached to and break down
the materials integrity. This is what
happens when a wire is burned apart when
too current is sent through it. In order to
have the right amount of force applied to a
material to produce electron flow the force
has to be strong enough to overcome the
► The
opposition to current flow due to this
attraction is known as RESISTANCE. The
right amount of voltage must be enough to
overcome an electrons resistance yet not
enough to force too many electron to move
causing material deterioration. If the
resistance to electron flow is stronger than
the voltage pressure then current does not
flow but a build up of electrons continues;
this is known as STATIC electricity.
► The
term static mean stationary and is an
example of potential energy applied to
electricity. If the voltage continues to cause
pressure, it will increase in strength until it
produces enough force to overcome the
resistance barrier and then it will break
through causing now Kinetic energy or aka
current flow. This phenomenon is exhibited
in nature when lighting strikes the earth
during a storm.
summary
► If
all three of the characteristics discussed
above are present a closed electrical system
is created known as an electrical circuit.
The block diagram of a simple circuit shown
below in fig 1-1 shows a closed circuit that
has a source of voltage - the battery,
current flowing through the conductors, and
resistance in the wires and light bulb.
► It,
along with more complex circuits
contains the "big three" in electronics:
Current - Voltage - Resistance. If the circuit
is ever broken or any on of the “big three”
are missing the circuit is said to be open16.