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Electricity Basics
Getting Started
Electricity is all around us. Powering technology like our cell phones, computers,
lights, soldering irons, and air conditioners. It's tough to escape it in our modern world.
Electricity is defined as the flow of electric charge.
Where do the charges come from? How do we move them? Where do they move
to? How does an electric charge cause mechanical motion or make things light up? So
many questions! To begin to explain what electricity is we need to zoom way in,
beyond the matter and molecules, to the atoms that make up everything we interact with
in life.
Going Atomic
Atoms exist in over a hundred different forms as chemical elements like
hydrogen, carbon, oxygen, and copper. Atoms of many types can combine to make
molecules, which build the matter we can physically see and touch.
Atoms are tiny, stretching at a max to about 300 picometers long (that's 3x10-10
or 0.0000000003 meters). A copper penny (if it actually were made of 100% copper)
would have 3.2x1022 atoms (32,000,000,000,000,000,000,000 atoms) of copper inside
it.
Even the atom isn't small enough to explain the workings of electricity. We need
to dive down one more level and look in on the building blocks of atoms: protons,
neutrons, and electrons.
Building Blocks of Atoms
An atom is built with a combination of three distinct particles:
Electrons, Protons, and Neutrons.
Each atom has a center nucleus, where the protons and neutrons are densely
packed together. Surrounding the nucleus are a group of orbiting electrons.
Every atom must have at least one proton in it. The number of protons in an atom is
important, because it defines what chemical element the atom represents.
For example, an atom with just one proton is hydrogen, an atom with 29 protons is
copper, and an atom with 94 protons is plutonium. This count of protons is called the
atom's atomic number.
A very simple atom model. It's not to scale but helpful for understanding how an atom is built. A core
nucleus of protons and neutrons is surrounded by orbiting electrons.
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The proton's nucleus-partner, neutrons, serve an important purpose; they keep the
protons in the nucleus and determine the isotope of an atom.
Electrons are critical to the workings of electricity. In its most stable, balanced
state, an atom will have the same number of electrons as protons. As in the Bohr atom
model below, a nucleus with 29 protons (making it a copper atom) is surrounded by an
equal number of electrons.
The atom's electrons aren't all forever bound to the atom. The electrons on the
outer orbit of the atom are called valence electrons. With enough outside force, a
valence electron can escape orbit of the atom and become free. Free electrons
allow us to move charge, which is what electricity is all about. Speaking of charge...
Flowing Charges
Electricity is defined as the flow of electric charge. Charge is a property of
matter--just like mass, volume, or density. It is measurable. Just as you can quantify
how much mass something has, you can measure how much charge it has. The key
concept with charge is that it can come in two types: positive (+) or negative (-).
In order to move charge we need charge carriers, and that's where our knowledge of
atomic particles--specifically electrons and protons--comes in handy. Electrons always
carry a negative charge, while protons are always positively charged. Neutrons
(true to their name) are neutral, they have no charge. Both electrons and protons carry
the same amount of charge, just a different type.
A lithium atom (3 protons) model with the charges labeled.
The charge of electrons and protons is important, because it provides us the means to
exert a force on them. Electrostatic force!
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Electrostatic Force
Electrostatic force (also called Coulomb's law) is a force that operates between
charges. It states that charges of the same type repel each other, while charges of
opposite types are attracted together. Opposites attract, and likes repel.
The amount of force acting on two charges depends on how far they are from each
other. The closer two charges get, the greater the force (either pushing together, or
pulling away) becomes.
Thanks to electrostatic force, electrons will push away other electrons and be attracted
to protons. This force is part of the "glue" that holds atoms together, but it's also the tool
we need to make electrons (and charges) flow!
Making Charges Flow
Electrons in atoms can act as our charge carrier, because every electron carries a
negative charge. If we can free an electron from an atom and force it to move, we can
create electricity.
Consider the atomic model of a copper atom, one of the preferred elemental
sources for charge flow. In its balanced state, copper has 29 protons in its nucleus and
an equal number of electrons orbiting around it. Electrons orbit at varying distances
from the nucleus of the atom. Electrons closer to the nucleus feel a much stronger
attraction to the center than those in distant orbits. The outermost electrons of an atom
are called the valence electrons; these require the least amount of force to be freed from
an atom.
This is a copper atom diagram: 29 protons in the nucleus, surrounded by bands of circling electrons.
Electrons closer to the nucleus are hard to remove while the valence (outer ring) electron requires
relatively little energy to be ejected from the atom.
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Using enough electrostatic force on the valence electron--either pushing it with another
negative charge or attracting it with a positive charge--we can eject the electron from
orbit around the atom creating a free electron.
Consider a copper wire: matter filled with countless copper atoms. As our free
electron is floating in a space between atoms, it's pulled and prodded by surrounding
charges in that space. In this chaos the free electron eventually finds a new atom to
latch on to; in doing so, the negative charge of that electron ejects another valence
electron from the atom. Now a new electron is drifting through free space looking to do
the same thing. This chain effect can continue on and on to create a flow of electrons
called electric current.
Conductivity
Some elemental types of atoms are better than others at releasing their electrons. To get
the best possible electron flow we want to use atoms which don't hold very tightly to
their valence electrons. An element's conductivity measures how tightly bound an
electron is to an atom.
Elements with high conductivity, which have very mobile electrons, are called
conductors. These are the types of materials we want to use to make wires and other
components which aid in electron flow. Metals like copper, silver, and gold are usually
our top choices for good conductors.
Elements with low conductivity are called insulators. Insulators serve a very
important purpose: they prevent the flow of electrons. Popular insulators include glass,
rubber, plastic, and air.
Static or Current Electricity
Two forms electricity can take: static or current. In working with electronics, current
electricity will be much more common, but static electricity is important to understand
as well.
Static Electricity
Static electricity exists when there is a build-up of opposite charges on objects
separated by an insulator. Static (as in "at rest") electricity exists until the two groups of
opposite charges can find a path between each other to balance the system out.
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When the charges do find a means of equalizing, a static discharge occurs. The
attraction of the charges becomes so great that they can flow through even the best of
insulators (air, glass, plastic, rubber, etc.). Static discharges can be harmful depending
on what medium the charges travel through and to what surfaces the charges are
transferring. Charges equalizing through an air gap can result in a visible shock as the
traveling electrons collide with electrons in the air, which become excited and release
energy in the form of light.
One of the most dramatic examples of static discharge is lightning. When a cloud
system gathers enough charge relative to either another group of clouds or the earth's
ground, the charges will try to equalize. As the cloud discharges, massive quantities of
positive (or sometimes negative) charges run through the air from ground to cloud
causing the visible effect we're all familiar with.
Static electricity also familiarly exists when we rub balloons on our head to make
our hair stand up, or when we shuffle on the floor with fuzzy slippers and shock the
family cat (accidentally, of course). In each case, friction from rubbing different types
of materials transfers electrons. The object losing electrons becomes positively charged,
while the object gaining electrons becomes negatively charged. The two objects become
attracted to each other until they can find a way to equalize.
Working with electronics, we generally don't have to deal with static electricity.
When we do, we're usually trying to protect our sensitive electronic components from
being subjected to a static discharge. Preventative measures against static electricity
include wearing ESD (electrostatic discharge) wrist straps, or adding special
components in circuits to protect against very high spikes of charge.
Current Electricity
Current electricity is the form of electricity which makes all of our electronic
gizmos possible. This form of electricity exists when charges are able to constantly
flow. As opposed to static electricity where charges gather and remain at rest, current
electricity is dynamic; charges are always on the move. We'll be focusing on this form
of electricity.
Circuits
In order to flow, current electricity requires a circuit: a closed, never-ending
loop of conductive material. A circuit could be as simple as a conductive wire
connected end-to-end, but useful circuits usually contain a mix of wire and other
components which control the flow of electricity. The only rule when it comes to
making circuits is they can't have any insulating gaps in them. If you have a wire full of
copper atoms and want to induce a flow of electrons through it, all free electrons need
somewhere to flow in the same general direction. Copper is a great conductor, perfect
for making charges flow. If a circuit of copper wire is broken, the charges can't flow
through the air, which will also prevent any of the charges toward the middle from
going anywhere. On the other hand, if the wire were connected end-to-end, the
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electrons all have a neighboring atom and can all flow in the same general direction.
We now understand how electrons can flow, but how do we get them flowing in the
first place? Then, once the electrons are flowing, how do they produce the energy
required to illuminate light bulbs or spin motors? For that, we need to understand
electric fields.
Electric Fields
We have a handle on how electrons flow through matter to create electricity.
That's all there is to electricity. Well, almost all. Now we need a source to induce the
flow of electrons. Most often that source of electron flow will come from an electric
field.
What's a Field?
A field is a tool we use to model physical interactions which don't involve any
observable contact. Fields can't be seen as they don't have a physical appearance, but
the effect they have is very real. We’re all subconsciously familiar with one field in
particular: Earth's gravitational field, the effect of a massive body attracting other
bodies. Earth's gravitational field can be modeled with a set of vectors all pointing into
the center of the planet; regardless of where you are on the surface, you'll feel the force
pushing you towards it.
The strength or intensity of fields isn't uniform at all points in the field. The further you
are from the source of the field the less effect the field has. The magnitude of Earth's
gravitational field decreases as you get further away from the center of the planet.
As we go on to explore electric fields in particular remember how Earth's gravitational
field works, both fields share many similarities. Gravitational fields exert a force on
objects of mass, and electric fields exert a force on objects of charge.
Electric Fields
Electric fields (e-fields) are an important tool in understanding how electricity
begins and continues to flow. Electric fields describe the pulling or pushing force in a
space between charges. Compared to Earth's gravitational field, electric fields have one
major difference: while Earth's field generally only attracts other objects of mass (since
everything is so significantly less massive), electric fields push charges away just as
often as they attract them.
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The direction of electric fields is always defined as the direction a positive test
charge would move if it was dropped in the field. The test charge has to be infinitely
small, to keep its charge from influencing the field.
We can begin by constructing electric fields for solitary positive and negative
charges. If you dropped a positive test charge near a negative charge, the test charge
would be attracted towards the negative charge. So, for a single, negative charge we
draw our electric field arrows pointing inward at all directions. That same test charge
dropped near another positive charge would result in an outward repulsion, which
means we draw arrows going out of the positive charge.
The electric fields of single charges. A negative charge has an inward electric field because it attracts
positive charges. The positive charge has an outward electric field, pushing away like charges.
Electric fields provide us with the pushing force we need to induce current flow.
An electric field in a circuit is like an electron pump: a large source of negative charges
that can propel electrons, which will flow through the circuit towards the positive lump
of charges.
Electricity Basics
When beginning to explore the world of electricity and electronics, it is vital to
start by understanding the basics of voltage, current, and resistance. These are the three
basic building blocks required to manipulate and utilize electricity. At first, these
concepts can be difficult to understand because we cannot "see" them. One cannot see
with the naked eye the energy flowing through a wire or the voltage of a battery sitting
on a table. Even the lightning in the sky, while visible, is not truly the energy exchange
happening from the clouds to the earth, but a reaction in the air to the energy passing
through it. In order to detect this energy transfer, we must use measurement tools such
as multimeters, spectrum analyzers, and oscilloscopes to visualize what is happening
with the charge in a system.
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Electrical Charge
Electricity is the movement of electrons. Electrons create charge, which we can
harness to do work. Your light bulb, your stereo, your phone, etc., are all harnessing the
movement of the electrons in order to do work. They all operate using the same basic
power source: the movement of electrons.
The three basic principles for this tutorial can be explained using electrons, or more
specifically, the charge they create:
Voltage is the difference in charge between two points.
Current is the rate at which charge is flowing.
Resistance is a material's tendency to resist the flow of charge.
So, when we talk about these values, we're really describing the movement of
charge, and thus, the behavior of electrons. A circuit is a closed loop that allows charge
to move from one place to another. Components in the circuit allow us to control this
charge and use it to do work.
Georg Ohm was a Bavarian scientist who studied electricity. Ohm starts by
describing a unit of resistance that is defined by current and voltage. So, let's start with
voltage and go from there.
Voltage
Voltage as the amount of potential energy between two points on a circuit.
One point has more charge than another. This difference in charge between the two
points is called voltage. It is measured in volts, which, technically, is the potential
energy difference between two points that will impart one joule of energy per coulomb
of charge that passes through it. The unit "volt" is named after the Italian physicist
Alessandro Volta who invented what is considered the first chemical battery. Voltage is
represented in equations and schematics by the letter "V".
When describing voltage, current, and resistance, a common analogy is a water
tank. In this analogy, charge is represented by the water amount, voltage is represented
by the water pressure, and current is represented by the water flow. So for this analogy,
remember:
Water = Charge
Pressure = Voltage
Flow = Current
Consider a water tank at a certain height above the ground. At the bottom of this tank
there is a hose.
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The pressure at the end of the hose can represent voltage. The water in the tank
represents charge. The more water in the tank, the higher the charge, the more pressure
is measured at the end of the hose.
We can think of this tank as a battery, a place where we store a certain amount of
energy and then release it. If we drain our tank a certain amount, the pressure created at
the end of the hose goes down. We can think of this as decreasing voltage, like when a
flashlight gets dimmer as the batteries run down. There is also a decrease in the amount
of water that will flow through the hose. Less pressure means less water is flowing,
which brings us to current.
Current
We can think of the amount of water flowing through the hose from the tank as
current. The higher the pressure, the higher the flow, and vice-versa. With water, we
would measure the volume of the water flowing through the hose over a certain period
of time. With electricity, we measure the amount of charge flowing through the circuit
over a period of time.
Current is measured in Amperes (usually just referred to as "Amps"). An
ampere is defined as 6.241*10^18 electrons (1 Coulomb) per second passing
through a point in a circuit. Amps are represented in equations by the letter "I".
Let's say now that we have two tanks, each with a hose coming from the bottom.
Each tank has the exact same amount of water, but the hose on one tank is narrower
than the hose on the other.
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We measure the same amount of pressure at the end of either hose, but when the water
begins to flow, the flow rate of the water in the tank with the narrower hose will be less
than the flow rate of the water in the tank with the wider hose. In electrical terms, the
current through the narrower hose is less than the current through the wider hose. If we
want the flow to be the same through both hoses, we have to increase the amount of
water (charge) in the tank with the narrower hose.
This increases the pressure (voltage) at the end of the narrower hose, pushing
more water through the tank. This is analogous to an increase in voltage that causes an
increase in current.
Now we're starting to see the relationship between voltage and current. But there
is a third factor to be considered here: the width of the hose. In this analogy, the width
of the hose is the resistance. This means we need to add another term to our model:
Water = Charge (measured in Coulombs)
Pressure = Voltage (measured in Volts)
Flow = Current (measured in Amperes, or "Amps" for short)
Hose Width = Resistance
Resistance
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In electrical terms, this is represented by two circuits with equal voltages and different
resistances. The circuit with the higher resistance will allow less charge to flow;
meaning the circuit with higher resistance has less current flowing through it.
This brings us back to Georg Ohm. Ohm defines the unit of resistance of "1
Ohm" as the resistance between two points in a conductor where the application of 1
volt will push 1 ampere, or 6.241×10^18 electrons. This value is usually represented in
schematics with the Greek letter "Ω", which is called omega, and pronounced
"ohm".
Ohm's Law
Combining the elements of voltage, current, and resistance, Ohm developed the
formula:
Where
• V = Voltage in volts
• I = Current in amps
• R = Resistance in ohms
This is called Ohm's law. Let's say, for example, that we have a circuit with the
potential of 1 volt, a current of 1 amp, and resistance of 1 ohm. Using Ohm's Law we
can say:
All materials are made up from atoms, and all atoms consist of protons, neutrons
and electrons. Protons have a positive electrical charge. Neutrons have no electrical
charge (that is they are Neutral), while Electrons have a negative electrical charge.
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Atoms are bound together by powerful forces of attraction existing between the atoms
nucleus and the electrons in its outer shell.
When these protons, neutrons and electrons are together within the atom
they are happy and stable. But if we separate them from each other they want to
reform and start to exert a potential of attraction called a potential difference.
Now if we create a closed circuit these loose electrons will start to move and drift
back to the protons due to their attraction creating a flow of electrons. This flow of
electrons is called an electrical current. The electrons do not flow freely through the
circuit as the material they move through creates a restriction to the electron flow. This
restriction is called resistance.
Then all basic electrical or electronic circuits consist of three separate but very
much related electrical quantities called:
Voltage, ( v ), Current, ( i ) and Resistance, ( Ω ).
Electrical Voltage
Voltage, ( V ) is the potential energy of an electrical supply stored in the form of
an electrical charge. Voltage can be thought of as the force that pushes electrons
through a conductor and the greater the voltage the greater is its ability to “push” the
electrons through a given circuit. As energy has the ability to do work this potential
energy can be described as the work required in joules to move electrons in the form of
an electrical current around a circuit from one point or node to another.
Then the difference in voltage between any two points, connections or junctions
(called nodes) in a circuit is known as the Potential Difference, ( p.d. ) commonly called
the Voltage Drop.
The Potential difference between two points is measured in Volts with the circuit
symbol V, or lowercase “v“, although Energy, E lowercase “e” is sometimes used to
indicate a generated emf (electromotive force). Then the greater the voltage, the greater
is the pressure (or pushing force) and the greater is the capacity to do work.
A constant voltage source is called a DC Voltage with a voltage that varies
periodically with time is called an AC voltage.
Voltage is measured in volts, with one volt being defined as the electrical
pressure required to force an electrical current of one ampere through a resistance
of one Ohm.
Voltages are generally expressed in Volts with prefixes used to denote sub-
multiples of the voltage such as microvolts ( μV = 10-6 V ), millivolts ( mV = 10-3 V )
or kilovolts ( kV = 103 V ). Voltage can be either positive or negative.
Batteries or power supplies are mostly used to produce a steady D.C. (direct
current) voltage source such as 5v, 12v, 24v etc in electronic circuits and systems.
While A.C. (alternating current) voltage sources are available for domestic
house and industrial power and lighting as well as power transmission.
The mains voltage supply in the United Kingdom is currently 230 volts a.c. and
110 volts a.c. in the USA.
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General electronic circuits operate on low voltage DC battery supplies of
between 1.5V and 24V dc The circuit symbol for a constant voltage source usually
given as a battery symbol with a positive, + and negative, – sign indicating the direction
of the polarity. The circuit symbol for an alternating voltage source is a circle with a
sine wave inside.
Electrical Current
Electrical Current, ( I ) is the movement or flow of electrical charge and is
measured in Amperes, symbol i, for intensity). It is the continuous and uniform flow
(called a drift) of electrons (the negative particles of an atom) around a circuit that are
being “pushed” by the voltage source. In reality, electrons flow from the negative (–ve)
terminal to the positive (+ve) terminal of the supply and for ease of circuit
understanding conventional current flow assumes that the current flows from the
positive to the negative terminal.
Generally in circuit diagrams the flow of current through the circuit usually has
an arrow associated with the symbol, I, or lowercase i to indicate the actual direction of
the current flow. However, this arrow usually indicates the direction of conventional
current flow and not necessarily the direction of the actual flow.
Resistance
Resistance, ( R ) is the capacity of a material to resist or prevent the flow of
current or, more specifically, the flow of electric charge within a circuit. The circuit
element which does this perfectly is called the “Resistor”.
Resistance is a circuit element measured in Ohms, Greek symbol ( Ω, Omega )
with prefixes used to denote Kilo-ohms ( kΩ = 103Ω ) and Mega-ohms ( MΩ = 106Ω ).
Note that resistance cannot be negative in value only positive.
A basic summary of the three units is given below.
➢ Voltage or potential difference is the measure of potential energy between
two points in a circuit and is commonly referred to as its” volt drop “.
➢ When a voltage source is connected to a closed loop circuit the voltage will
produce a current flowing around the circuit.
➢ In DC voltage sources the symbols +ve (positive) and −ve (negative) are used to
denote the polarity of the voltage supply.
➢ Voltage is measured in Volts and has the symbol V for voltage or E for electrical
energy.
➢ Current flow is a combination of electron flow and hole flow through a circuit.
➢ Current is the continuous and uniform flow of charge around the circuit and is
measured in Amperes or Amps and has the symbol I.
➢ Current is Directly Proportional to Voltage ( I ∝ V )
➢ The effective (rms) value of an alternating current has the same average power
loss equivalent to a direct current flowing through a resistive element.
➢ Resistance is the opposition to current flowing around a circuit.
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➢ A low value of resistance implies a conductor and high values of resistance
implies an insulator.
➢ Current is Inversely Proportional to Resistance ( I 1/∝ R )
➢ Resistance is measured in Ohms and has the Greek symbol Ω or the letter R.
Electrical Power in Circuits
Electrical Power, ( P ) in a circuit is the rate at which energy is absorbed or
produced within a circuit. A source of energy such as a voltage will produce or
deliver power while the connected load absorbs it. Light bulbs and heaters for
example, absorb electrical power and convert it into either heat, or light, or both.
The higher their value or rating in watts the more electrical power they are likely
to consume.
The quantity symbol for power is P and is the product of voltage
multiplied by the current with the unit of measurement being the Watt ( W ).
Prefixes are used to denote the various multiples or sub-multiples of a watt, such
as: milliwatts (mW = 10-3W) or kilowatts (kW = 103W).
Then by using Ohm’s law and substituting for the values of V, I and R the
formula for electrical power can be found as:
To find the Power (P)
[ P = V x I ] P (watts) = V (volts) x I (amps)
Also:
[ P = V2 ÷ R ] P (watts) = V2 (volts) ÷ R (Ω)
Also:
[ P = I2 x R ] P (watts) = I2 (amps) x R (Ω)
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Difference between AC and DC
Electric current flows in two ways: either in an alternating current
(AC) or in a direct current (DC).
In Alternating current, current keeps switching directions periodically
– forward and backward.
While in the direct current it flows in a single direction steadily.
The main difference between AC and DC lies in the direction in which the
electrons flow. In DC, the electrons flow steadily in a single direction while
electrons keep switching directions, going forward and then backwards in AC.
What is an Alternating Current (AC)?
In alternating current, the electric charges flow changes its direction
periodically. AC is the most commonly used and most preferred electric
power for household equipment, office, and buildings, etc. It was first tested,
based on the principles of Michael Faraday in 1832 using a Dynamo Electric
Generator.
Alternating current can be identified in waveform called a sine wave,
in other words, it can be said as the curved line. These curved lines represent
electric cycles and are measured per second. The measurement is read as Hertz
or Hz. AC is used to powerhouses and buildings etc because generating and
transporting AC across long distances is relatively easy. AC is capable of
powering electric motors which are used on refrigerators, washing machine etc
What is a Direct Current (DC)?
Unlike alternating current, the flow of current in direct current does not
change periodically.
The current electricity flows in a single direction in a steady voltage. The
major use of DC is to supply power for electrical devices and also to charge
batteries. For example, mobile phone batteries, flashlights, flat-screen television,
hybrid and electric vehicles. DC has the combination of plus and minus sign, a
dotted line or a straight line.
Everything that runs on a battery and uses an AC adapter while plugging
into a wall or uses a USB cable for power relies on DC. Examples would be Cell
phones, Electric vehicles, flashlights, Flat-screen TVs(AC goes into the TV and
is converted into DC).
The major differences between Alternating Current and Direct Current are given in the table below:
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Alternating Current Direct Current
AC can carry and safe to transfer longer distance
even between two cities, and maintain the electric
power.
DC cannot travel for a very long
distance. It does, it loses electric
power.
The rotating magnets cause the change in direction
of electric flow.
The steady magnetism makes DC
flow in a single direction.
The frequency of AC is depended upon the
country. But, generally, the frequency is 50Hz or
60Hz.
DC has no frequency of zero
frequency.
In AC the flow of current changes its direction
backwards periodically.
It flows in a single direction
steadily.
Electrons in AC keep changing its directions –
backward and forward
Electrons only move in one
direction – that is forward.
Why AC can’t be Stored in Batteries like DC?
We cannot store AC in batteries because AC changes their polarity up to
50 (When frequency = 50 Hz) or 60 (When frequency = 60 Hz) times in a
second. Therefore the battery terminals keep changing Positive (+ve) becomes
Negative (-Ve) and vice versa, but the battery cannot change their terminals with
the same speed so that’s why we can’t store AC in Batteries.
In addition, when we connect a battery with AC Supply, then It will charge
during positive half cycle and discharge during negative half cycle, because the
Positive (+ve) half cycle cancel the Negative (-Ve) half cycle, so the average
voltage or current in a complete cycle is Zero. So there is no chance to store AC
in the Batteries.
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What is Electrical Earthing or Grounding?
To connect the metallic (conductive) Parts of an Electric appliance or
installations to the earth (ground) is called Earthing or Grounding.
To connect the metallic parts of electric machinery and devices to the
earth plate or earth electrode (which is buried in the moisture earth)
through a thick conductor wire (which has very low resistance) for safety
purpose is known as Earthing or grounding.
Earthing and Grounding are the same terms used for earthing. Grounding
is the commonly word used for earthing in the North American standards like
IEEE, NEC, ANSI and UL etc while, Earthing is used in European, Common
wealth countries and Britain standards like IS and IEC etc.
Why Earthing is Important?
The primary purpose of earthing is to avoid or minimize the danger of
electrocution, fire due to earth leakage of current through undesired path and to
ensure that the potential of a current carrying conductor does not rise with respect
to the earth than its designed insulation.
When the metallic part of electrical appliances (parts that can conduct or
allow passage of electric current) comes in contact with a live wire, maybe due to
failure of installations or failure in cable insulation, the metal become charged and
static charge accumulates on it. If a person touches such a charged metal, the
result is a severe shock.
To avoid such instances, the power supply systems and parts of appliances have
to be earthed so as to transfer the charge directly to the earth. This is why we need
Electrical Earthing or Grounding in electrical installation systems.
Below are the basic needs of Earthing.
o To protect human lives as well as provide safety to electrical devices and
appliances from leakage current.
o To keep voltage as constant in the healthy phase (If fault occurs on any one
phase).
o To Protect Electric system and buildings form lighting.
o To serve as a return conductor in electric traction system and
communication.
o To avoid the risk of fire in electrical installation systems.
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Key Differences between Single Phase and Three Phase Supplies
The key differences between single phase and three phases include the following.
➢ The definition of the single-phase power supply is, the power supplies
through a single conductor
➢ The definition of the three-phase power supply is, the power flows through
three conductors.
➢ The single-phase power supply has one distinct wave cycle whereas; three
phase has three distinct wave cycles.
Single Phase Three Phase
➢ Single phase requires the single wire to connect the circuit whereas; 3-phase
needs 3-wires.
➢ The voltage of the single phase is 230V, whereas three phase voltage is 415V.
➢ The phase name of the single phase is split phase, whereas three phase has no
other name.
➢ The capacity of power transfer in the single phase is minimum, whereas three
phase has the maximum.
➢ The connection of single phase is simple whereas in 3-phase is complicated.
➢ The power failure happens in a single phase, but not occurs in three phase.
➢ The loss in single phase is maximum whereas in three phase is minimum.
➢ The single-phase efficiency is less whereas in three phase is high.
➢ The single-phase is inexpensive whereas the 3-phase is expensive.
➢ The single-phase AC power supply is utilized for home appliances and three
phase power supply is used in huge industries to run heavy loads.
➢ RYB simply stands for Red, yellow and Blue respectively in a three phase
electrical system.
Note:
Phase is a time gap between the voltages and mentioned in degrees.
One cycle in AC is 360°. This is same as 0°. AC is the voltage which changes with
time. When another AC voltage has little time difference from the first one, the gap is
phase difference.
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Capacitor:
Capacitors are simple passive device that can store an electrical charge on their
plates when connected to a voltage source.
The capacitor is a component which has the ability or “capacity” to store energy
in the form of an electrical charge producing a potential difference (Static Voltage)
across its plates, much like a small rechargeable battery.
Difference between a Battery and a Capacitor
The main difference between a battery and a capacitor is that Battery stores
charge in the form of chemical energy and convert to the electrical energy whereas,
capacitor stores charge in the form of electrostatic field.
Electric Motor
An electric motor is an electrical machine that converts electrical energy into
mechanical energy.
Types of Motors
Induction Motor
Induction motor works on the principle of induction.
When the power supply is given to the stator, it produces rotating magnetic
field which gets induced in the rotor of induction motor causing rotor to rotate.
As they run at Asynchronous speed they are called Asynchronous motor.
(Induction motor works on the principle of induction where electro-magnetic
field is induced into the rotor when rotating magnetic field of stator cuts the stationary
rotor.)
Synchronous motor is a machine whose rotor speed and the speed of the
stator magnetic field is equal. Synchronous motor does not have slip.
Asynchronous motor is a machine whose rotor rotates at the speed less than
the synchronous speed. ... AC Induction Motor is known as the Asynchronous Motor.
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Single phase induction motor
The single-phase induction motor is not self-starting. When the motor is
connected to a single-phase power supply, the main winding carries an alternating
current.
There are different types based on their way of starting since these are of not self
starting. Those are split phase, shaded pole and capacitor motors. Again capacitor
motors are capacitor start, capacitor run and permanent capacitor motors. Permanent
capacitor motor is shown below.
Applications of Single Phase Induction Motor
These are used in low power applications and widely used in domestic
applications as well as industrial. And some of those are mentioned below
Pumps, Compressors, Small fans, Mixers, Toys, High speed vacuum cleaners,
Electric shavers, Drilling machines.
Three-Phase Induction Motor:
Three Phase Induction motors are self-starting and use no capacitor, start
winding, centrifugal switch or other starting device. Three-phase AC induction motors
are widely used in industrial and commercial applications. These are of two types,
squirrel cage and slip ring motors. Squirrel cage motors are widely used due to their
rugged construction and simple design. Slip ring motors require external resistors to
have high starting torque.
Induction motors are used in industry and domestic appliances because these are
rugged in construction requiring hardly any maintenance, that they are comparatively
cheap, and require supply only to the stator.
Applications of Three Phase Induction Motor
Lifts, Cranes, Hoists, Large capacity exhaust fans, Driving lathe machines,
Crushers, Oil extracting mills, Textile and etc.
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CHARACTERISTICS
SINGLE PHASE
INDUCTION MOTOR
THREE PHASE INDUCTION MOTOR
Power Source
Single Phase (1-phase)
Only
Three Phase (3-phase) Only
Starting Mechanism
They are NOT self-
starting.
They are self-starting.
Efficiency
Low as only one winding
has to carry all the
current
High because three winding are available to
carry the current
Types
Shaded pole
Split Phase
Capacitor Start Inductor
Run
Capacitor Start Capacitor
Run
Squirrel cage type
Slip ring type or wound induction motor
Cost Cheaper Quite Expensive
Slip (s)
There are two slips:
Forward slip (s)
Backward slip (2-s)
It has only forward slip
Size (for same power
rating)
Larger in size Smaller in size
Power Factor Low High
Repair & Maintenance Easier to repair Difficult to repair and maintain
Structure
Simple and easy to
manufacture
More complicated to construct because of extra
components involvement
Starting Torque Low High
Operational Reliability More Reliable Less Reliable
Motor Rotation
There is no mechanism
to change the rotation.
Can be changed easily by changing the phase
sequence in stator.
Uses
Frequently used for
lighter loads. such as:
Blowers
Vacuum cleaner fans
Centrifugal pump
Washing machine
Grinder
Compressor
Extensively employed in industrial and
commercial drives since they are more rugged
and economical in terms of operational
efficiency.
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Slip
Slip(S) is defined as difference between Synchronous Speed (Ns) (Speed of
rotation of magnetic field) and Rotor Speed Nr (Speed of rotatory part of induction
motor).
Slip Speed = Ns - Nr
% Slip =
𝑁 𝑠−𝑁 𝑟
𝑁 𝑠
*100
The speed of rotor cannot be equal to synchronous speed i.e. Nr < Ns, the
value of slip s is always less than one. For induction motor, 0<s<1.
Value of Slip (s) Significance
s = 0 (Zero Slip)
Zero slip means that rotor speed is equal to
synchronously rotating magnetic flux. Under this
condition, there will not be any relative motion
between the rotor coils and rotating magnetic
flux. Therefore, there will not be any flux cutting
action of rotor coils. Hence, no emf will be
generated in rotor coils to produce rotor current.
This means no electromagnetic torque will be
produced. Induction motor will not work.
Therefore, it very important for induction motor
to have a positive value of slip. This is the
reason; slip is never zero in an induction motor.
s = 1 (Slip equal to 1) Slip = 1, means that rotor is stationary.
s = Negative (Negative Slip)
Negative value of slip in induction motor can be
achieved when the rotor speed is more than the
synchronously rotating magnetic flux. This is
only possible, when the rotor is rotated in the
direction of rotating magnetic flux by some
prime mover. Under this condition, the
machine operates as an Induction Generator.
s > 1 (Slip greater than 1)
Slip more than 1 implies that, rotor is rotating
in a direction opposite to the direction of
rotation of magnetic flux. This means if
magnetic flux is rotating in clockwise
direction, then rotor is rotating in
anticlockwise direction or vice versa.
Therefore, the relative speed between them will
be (Ns + Nr). In Plugging or Braking of
Induction motor, slip more than 1 is achieved to
quickly bring the rotor at rest.
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What is the difference between an AC motor and a DC motor?
While both A.C. and D.C. motors serve the same function of converting electrical
energy into mechanical energy, they are powered, constructed and controlled
differently.
The most basic difference is the power source. A.C. motors are powered from
alternating current (A.C.) while D.C. motors are powered from direct current (D.C.),
such as batteries, D.C. power supplies or an AC-to-DC power converter.
D.C wound field motors are constructed with brushes and a commutator, which
add to the maintenance, limit the speed and usually reduce the life expectancy of
brushed D.C. motors.
A.C. induction motors do not use brushes; they are very rugged and have long
life expectancies.
The final basic difference is speed control. The speed of a D.C. motor is
controlled by varying the armature winding’s current while the speed of an A.C. motor
is controlled by varying the frequency, which is commonly done with an adjustable
frequency drive control.
Electricity
It is a form of energy resulting from the existence of charged particles (such as
electrons or protons). Electrons (negative charges) and protons (positive charges) have a
force field associated with them. (Electricity is defined as the flow of electric charge.)
A fundamental form of energy created by the movement of electrons (negative
charges), protons, or positrons (positive charges) and generating current.
Electric Current
Electric current is the rate of flow of electrons in a conductor.
An electric current is a flow of particles (electrons) flowing through wires and
components. It is the rate of flow of charge. (Current is the rate at which electric
charge is flowing.)
Electric Power
Electric power is the rate, per unit time, at which electrical energy is transferred
by an electric circuit. The SI unit of power is the watt, one joule per second
Electric power is defined as the rate at which electrical energy is consumed
in an electrical circuit.
By Ohm's Law, V = IR, and so there are additional forms of the electric power formula for
resistors.
Power is measured in units of Watts (W), where a Watt is equal to a Joule per second (1 W = 1 J/s).
General form:
Electric power = voltage difference x current
P = VI
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Resistors:
P = electric power (W)
V = voltage difference (V = J/C)
I = electric current (A = C/s)
R = resistance (Ω = V/A)
Formula Wheel
Plug
Plug is a device for making an electrical connection between an appliance and the
mains, consisting of an insulated casing with metal pins that fit into holes in a socket.
2Pin Plug
An electrical plug that has two pins for inserting into a socket. Live and Neutral.
3Pin Plug
An electrical plug with three pins or metal projections to fit into a socket. Live,
Neutral and Earth.
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Live wire carries electric current to the appliances.
Neutral wire completes the circuit by forming a path for the current back to mains.
The earth wire is for our protection, in case the live wire makes a contact with metal
casing of an appliance. When this happens, the current will pass to the earth instead of
our body, thus saving us.
Why Earth Pin is Longer and Bigger in 3-Pin Plugs?
Why it is Longer?
The earth pin should be the first to connect and the last to disconnect with electric
supply. This is why earth pin is longer than the live and neutral pin on 3-pin plugins.
Below are the reason to do so,
When we insert a 3-pin plugin into a 3-pin socket, the earth pin is the first to make a
contact with the socket as compared to the Live and Neutral Pins.
The earth pin is the last to disconnect from the socket when removing the plug from the
socket. i.e. Line and Neutral disconnect first and then the earth pin.
This way, earthing is well maintained during the operation for proper safety and
protection.
Why it is Thicker?
First reason, to prevent the wrong way to operate a 3-pin plug and connected electrical
machine for the safety purpose. In other words, 3-Pin plug won’t be connect to the
socket upside down as the earth pin is thicker, so it won’t adjust and fit in another slot
made for Neutral or Live Pin in the socket.
In short, the earth pin is bigger and it cannot be inserted in the live or neutral slot
of the socket even by mistake.
Second reason, for thicker pin is that the law of resistance. The resistance is inversely
proportional to the area of the conductor. i.e. thicker of the conductor, lesser is the
resistance.
Third reason, modern 3 pin wall sockets have safety shutter on the Line and Neural
lines to prevent someone (especially children) to insert conducting materials in it which
cause electric shock. In this case, the longer earth pin help to open the shutters for Line
and Neutral pins i.e. without longer earth pin, The shutters for Line and Neutral
remains closed for better safety.
Thank You