Transcript Electricity

Electricity
You Light Up My Life!
What is Electricity?
 Electricity is one of the two long-range fundamental forces of nature;
the other one being gravity.
 Gravitational force between two bodies is always attractive and
depends on mass (in kg). Electric force can be both attractive and
repulsive and depends on charge (in Coulombs). In both cases the force
falls with the square of the distance apart.
 There are two kinds of electric charge; positive and negative. Like
charges repel and unlike charges attract.
 Gravity is a very weak force; electric forces are trillions of times stronger;
but most materials have the same number of positive and negative
charges, which cancel out, and so do not have any electric activity.
Atomic Theory
All matter in the universe is made up of around 90 different elements; with
Hydrogen (H) being the lightest (and most common) and Uranium (U) the
heaviest (there are artificial elements, mostly above U in the periodic
table).
Atomic Theory
 If you keep subdividing an element down, you reach the smallest
particle that has the chemical properties of the element. This particle is
called an atom (greek átomos; meaning indivisible).
 Atoms are incredibly small. For instance a you could fit around 70
million carbon atoms across one of your hairs (0.1mm). One atom weighs
0.000 000 000 000 000 000 000 02gm (or around 20 trillion trillion would
weigh a gram)!
 Atoms are smaller than the wavelength of
light and so cannot be seen even with the
most powerful optical microscope. However,
they can be visualised by bombarding with
electrons. The picture to the right shows an
array of carbon atoms taken with a scanning
tunnelling electron microscope.
Atomic Structure
 The big question of the late Victorian era was could an atom be made
up of even smaller components?
 In 1874, the Irish physicist Johnston Stoney at a British Association
conference meeting in Belfast predicted that there was a basic particle of
electric charge as a constituent of the atom. He called these electrons.
 In 1897 JJ Thompson applied a high voltage across electrodes (the
positive called the anode and the negative the cathode) in a vacuum tube
generated cathode rays, which seemed consist of negatively charged
corpuscles. These had the predicted unit of charge.
JJ Thompson and one of his cathode ray tubes
Atomic Structure
 Further experiments by Earnest Rutherford at the University of
Manchester showed that the atom comprised of a number of electrons
together with the same number of positively charged protons. Each
particle carried one of Stoney’s fundamental charge measured as 1.6 
10-19 Coulombs. Rutherford predicted that there would also be neutral
particles in the atom, and neutrons were discovered in 1932 by James
Chadwick at Cambridge.
 A proton weighs in at around 1.6  10-24 gm against the lightweight
electron which is around 9  10-38 gm, or 1/1836 of a proton. A neutron is
only slightly heavier than a proton.
Rutherford left and Chadwick on the right
Atomic Structure
 JJ Thompson thought that the atom consisted of a mixture
of electrons and protons all mixed together; the plum pudding
model (the positive and negative charges holding everything
together). Electrons moved in rings inside this blob.
 In 1909 Rutherford and Geiger shot alpha particles
(negative Helium nuclei) from radium (a radioactive element)
at very thin gold foil. Most went right through but a very few
bounced back. From this he deduced that the atom was
mostly empty space.
 If all the space was removed from the human population of
6 billion, then the solid remainder would be the size of an
apple!
Atomic Structure: Bohr model
 By 1913 Neils Bohr, a Danish physicist, developed a
model of the atom, where the electrons rotated in rings at
a great distance from the positive nucleus, giving an
overall neutral atom
 Only certain orbits were allowed (like harmonics in a
vibrating violin string) and only a maximum number of
electrons could populate each orbit (inner 2, next out 8
etc). These electrons were stable, that is they wouldn’t
spiral into the positive nucleus.
 Electrons absorbing energy can make a quantum leap
to a higher orbit, and conversely moving down causes
radiation of energy as discrete frequencies of electromagnetic waves (light, X-rays etc).
Atomic Structure: Bohr model
 It is electrons in the outer orbit that interact with
other elements, and thus give chemical properties.
Thus elements in the same column in the periodic
table have similar (not identical) properties; e.g.
Carbon, Germanium, Silicon all have four
electrons in their outer orbit. This orbit can hold a
maximum of eight, so tend to steal electrons from
other atoms; e.g. a molecule of Carbon Dioxide
CO2 shares two electrons with two oxygen atoms
back and forth.
 The Bohr model is far too simplistic, and by the
1920s quantum mechanics painted a much more
complex and mystical picture of sub-atomic
physics, but the Bohr model still explains most of
the phenomena useful in engineering
The Discovery of Electricity
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The ancient Greek mathematician Thales wrote in around 600bce that
rubbing amber (fossilised tree resin) with fur etc could cause attraction
between the two or even cause a spark. The Greek for amber is electron.
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Study of magnetism goes back to the observation that certain naturally
occurring stones attract iron.
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There is some evidence that electroplating was used in Mesopotamia
around 300bce (the Bagdad battery).
Attracting feathers with amber
Two Thousand Years Later
 Around 1600, William Gilbert, a physician who lived in London at the
time of Queen Elizabeth I and Shakespeare, studied magnetic
phenomena and demonstrated that the Earth itself was a huge magnet.
(Magnetism is really due to moving charges.)
 He also studied the attraction produced when materials were rubbed,
and named it the "electric" attraction. This is static electricity, usually
caused when some electrons are rubbed off a material into another. In the
picture below the little girl’s hair has been charged up and the hairs repel.
Benjamin Franklin
 In 1752, Franklin proved that lightning and
the spark from amber were one and the same
thing. This story is a familiar one, in which
Franklin fastened an iron spike to a silken kite,
which he flew during a thunderstorm, while
holding the end of the kite string by an iron key.
When lightening flashed, a tiny spark jumped
from the key to his wrist. The experiment
proved Franklin's theory, but was extremely
dangerous - he could easily have been killed.
 Franklin coined the terms positive and
negative charge, battery and conductor; still
used today.
Galvani and Volta
In 1786, Luigi Galvani, an Italian professor of
medicine, found that when the leg of a dead frog
was touched by a metal knife, the leg twitched
violently. Galvani thought that the muscles of the
frog must contain electricity.
By 1792, another Italian scientist, Alessandro Volta,
disagreed: he realized that the main factors in
Galvani's discovery were the two different metals the steel knife and the tin plate - upon which the frog
was lying. Volta showed that when moisture comes
between two different metals, electricity is created.
This led him to invent the first electric battery, the
voltaic pile, which he made from thin sheets of
copper and zinc separated by moist pasteboard.
Volta…continued
In this way, a new kind of electricity was discovered, electricity
that flowed steadily like a current of water instead of discharging
itself in a single spark or shock. Volta showed that electricity
could be made to travel from one place to another by wire,
thereby making an important contribution to the science of
electricity. The unit of electrical potential, the Volt, is named after
him.
Alessandro Volta and one of his
piles (batteries)
Andre Marie Ampere
Andre Marie Ampére, 1775 – 1836, a
French mathematician who devoted
himself to the study of electricity and
magnetism, was the first to explain the
electro-dynamic theory. A permanent
memorial to Ampere is the use of his
name for the unit of electric current.
http://www.corrosiondoctors.org/Biographies/AmperBio.htm
Ohm
Georg Simon Ohm, a German
mathematician and physicist, was a
college teacher in Cologne when in 1827
he published, "The Galvanic Circuit
Investigated Mathematically". His theories
were coldly received by German
scientists, but his research was
recognized in Britain and he was awarded
the Copley Medal in 1841. His name has
been given to the unit of electrical
resistance.
http://www.corrosiondoctors.org/Biographies/OhmBio.htm
Voltage = Current x Resistance
V = IR
Michael Faraday
The credit for generating electric
current on a practical scale goes
to the famous English scientist,
Michael Faraday (the unofficial
patron saint of Electrical
engineering). Faraday was
greatly interested in the invention
of the electromagnet, but his
brilliant mind took earlier
experiments still further. If
electricity could produce
magnetism, why couldn't
magnetism produce electricity?
Faraday….continued
In 1831, Faraday found the solution. Electricity could be produced
through magnetism by motion. He discovered that when a magnet
was moved inside a coil of copper wire, a tiny electric current flows
through the wire. Of course, by today's standards, Faraday's electric
generator was crude (and provided only a small electric current), but
he had discovered the first method of generating electricity by
means of motion in a magnetic field.
Faraday …. continued
Faraday also realized that magnetic and electric forces acting at a
distance can be conceptualized as a force field; hence electric and
magnetic fields.
Left: Magnetic field from a
bar magnet visualized
using iron filings
(miniature magnets lining
up in the force field).
Right: Electric field
showing direction of force
(on a +ve charge) near a
negative charge –q.
Edison and Swan
Nearly 40 years went by before a really
practical DC (Direct Current) generator was built
by inventor Thomas Edison.
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In 1878 Joseph Swan, a British
chemist/electrician, invented the incandescent
filament lamp and within twelve months Edison
made a similar discovery in America.
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“The aggregate capital now actually invested in
electrical industries, principally electric lighting,
(electric) railway and power distribution, is
estimated by the same authority, as not less than
$275,000,000”. Quote from the National Electric
Light Association in 1889!
www.edisonian.com/p004b002.htm
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Edison and Swan…continued
Swan and Edison later set up a joint company to produce the first
practical filament lamp. Prior to this, electric lighting had been very
powerful (too powerful for households) but crude arc lamps.
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Edison used his DC generator to provide electricity to light his
laboratory and later to illuminate the first New York street to be lit
by electric lamps, in September 1882. Edison's successes were not
without controversy, however - although he was convinced of the
merits of DC for generating electricity, other scientists in Europe
and America recognized that DC brought major disadvantages.
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Left: A lamp used at the historic 1879 New Year’s
Eve demonstration of the Edison Lighting System
in Menlo Park, New Jersey.
Nichola Tesla
Power is the product of voltage and current (V  I). High
voltages in the home are dangerous! Thus Edison had to
generate and distribute his dc power at lowish voltages
(110V), but the cables had to carry large currents. Losses in
the cables are proportional to current squared (I2R), but the
problem with dc is that it is very difficult to change the
voltage. With ac it is easy; just use a transformer. However,
motors at the time would only run on dc.
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Nichola Tesla, a Croatian engineer working for Edison,
conceived the idea of 2- and 3-phase generation (in a dream)
and on this basis patented a motor running alternating
current. This removed the chief objection to ac, but Edison
fought this tooth and nail. With Westinghouse, Tesla was
instrumental in the design and implementation of the
Niagara Falls hydroelectric scheme, which supplied New
York, over 20 miles away, with electricity. This effectively
won the battle of the currents.
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Nichola Tesla continued…
Left: Tesla
monument at
Niagra Falls
(Canadian
side), Queen
Victoria Park,
unveiled on
July 9,
2006. Tesla is
standing atop
an AC motor.
Right: Tesla
took out over
700 patents!
http://www.tesla
society.com/
The Information Revolution
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The use of electricity is critically important in
lighting, heating, and in mechanical
actuators/motors.
Equally important is the use of electrons to
generate, transmit, store and reproduce
information.
Information is a measure of change and
predictability. Consider the two statements:
1.
Tomorrow the sun will rise and darkness will
be banished.
2.
Tomorrow an extinct volcano will erupt in
Belfast.
Which one carries the most information?
Because electrons are so light, changes (called
signals) can be sent along a conductor or
propagated in space using radio or light waves at
speeds approaching that of light.
The Information Revolution
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Up to the early 1800s the fastest you could send
information was on horse by land or sailing ship
by sea. A horseman carrying a message had to
transport around 500kg of animal over rocks,
muddy ruts and fallen trees with plenty of food
for the two mammals.
With a reliable source of electricity, around 1830
many experiments were made in sending
currents along wires to deflect a needle at the far
end (magnetic field).
Wires were strung on poles along railway lines to
signal oncoming trains and synchronise time
(railway time). In UK by 1838 there was 20km (12
miles) of line, by 1852 there were 6,000km (4,000
miles).
The British system (Wheatstone & Cook) used
multiple wires and five needles to point to each
letter in turn!
The Information Revolution
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Reducing the number of wires and reliability of the telegraph was a
priority, and the number of needles was steadily reduced and various
codes were used to encode alphanumerics.
Samuel Morse (portrait Painter) with Alfred Vail came up with a code,
which relied on each letter being coded by a series of dots and
dashes. The more common letters had a shorter code:
. .-.. . -.-. - .-. .. -.-. .. - -.-E l e c t r i c i t y
These current pulses could be used to close a relay switch and thus
regenerate the signal along the link, and at the receiver mark a paper
tape or actuate a buzzer.
In 1844 first government-funded demonstration between Baltimore
and Washington (37 miles). Message sent “What has God wrought?”
The Information Revolution
“It is difficult to imagine how strange the telegraph must
have seemed to our great, great grandparents. People
had only the vaguest idea about the technology involved.
One railway passenger who left her umbrella on the train
asked at the station if it could be found. The
stationmaster said he'd try to use the telegraph to
arrange for its return and wired to the end of the line to
see if it had been found on the train. Soon, he received a
message back that it had and would be sent back 'down
the line'. When he told the anxious passenger this good
news, she expressed amazement that items such as
umbrellas could be returned using the telegraph!
Rather than disappoint her, the station staff hooked the
returned umbrella over the telegraph wire - as if it had
literally come back 'down the line'.”
http://www.connected-earth.com/Galleries/index.htm
The Information Revolution
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Key to building an international communications
web was undersea cables; first across rivers and
then seas.
Needs great strength and good insulation;
invention of gutta-percha (rubber) led in 1850 to
first international submarine telegraph between
Dover and Cap Gris Nez (France). Four private
investors each put up £500. Failed after a few
messages!
The wonder of the Victorian age (equivalent to
putting a man on the moon) was the transatlantic
link. Can you think of any problems laying 1,852
miles (2,980 km) of cable?
In 1857 and 1858 the HMS Agamemnon and USS
Niagara met in mid-Atlantic, spliced the cable and
sailed back towards their respective continents.
Queen Victoria sent President Buchanan a 98-word
message. Took 17 hours!
Authenticated left-over pieces of transatlantic cable sold
The Information Revolution
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In an attempt to increase the signalling rate some
genius decided to use 2,000 volts and ……………
It would take 12 years (and an American civil war)
and seven attempts before a working link was
established, with an investment of the equivalent
of billions of pounds.
The final cable (all 5,000 tonnes) was laid by
Brunel’s giant Great Eastern ship from Valentia
(Dingle Bay) to Heart’s Content in Newfoundland.
Lord Kelvin had invented the mirror galvanometer
(very sensitive) and this allows a transmission rate
of up to 20 words per minute with low voltages!.
In 1871 a cable was laid to Australia via Singapore.
By 1902 with the completion of a line from British
Columbia to New Zealand, telegraph cables now
circumnavigate the globe.
The first Telephone (speech) transatlantic cable
was not laid until 1956!
The Information Revolution
The Information Revolution
1924
The Information Revolution
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The electric telegraph was a digital communications
network; people speak in tones.
To send sounds down a wire, you need to:
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Convert sounds to electric current vibrations (that
is an analogue to the original air pressure
variations).
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Transmit these currents to the desired receiver.
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Turn electrical current variations back to pressure
waves (sounds)
Many people working at transmitting tones down a
telegraph wire around 1870s, in order to try and send
more than one morse-code message at a time ―
multiplexing.
Also experiments in teaching deaf people to recognise
sounds with vibrating membranes.
Telephone-like instruments 1862  1872, developed by
Philipp Reis; German physics instructor.
http://atcaonline.com/phone/
The Information Revolution
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The invention of the first practical telephone is normally attributed to
Alexander Graham Bell, a Scottish scientist (with a deaf wife) who was
working in Canada. Patented in 1876. Also Edison’s carbon microphone.
Lord Kelvin exhibited Bell's telephone to the British Association for the
Advancement of Science at Glasgow in September. He described it as "the
greatest by far of all the marvels of the electric telegraph". 1877
Bell demo’ed to Queen Vic in 1878, with a long-distance call to Southampton.
What do you consider to be the major problem with distance connections?
1879 first public telephone exchange: Eight subscribers.
1880 first London telephone directory in January covered three exchanges
and 250 subscribers. By April, 7 London exchanges, 16 provincial exchanges
and 350 subscribers …..
The first operators were boys, who turned out to be impatient and rude when
dealing with phone customers. Their rudeness made them extinct within only
a few years, replaced by females who were, "calm and gracious”
The Information Revolution
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Long-distance links require amplification:
active electronic devices.
In 1904 Ambrose Fleming invents the thermonic
diode.
Followed by Lee DeForest’s triode amplification
valve (tube) in 1906. A small voltage on a grid
could control a large current flowing between a
hot cathode and anode.
This led to the electronic revolution, with radio
(wireless), telephone repeaters, audio
amplifiers and television etc.
Telephone exchanges were automated during
the 20th century (In Donegal not until late 1980s)
and the switching technology formed the
technological basis for the comeback of digital
networks, such as computers.
The Information Revolution
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Although all the theory was known by the end
of the 2nd World war it took the invention of the
transistor in 1948 by Bardeen, Brattain and
Shockley at Bell Laboratories to make it all a
practical reality. Transistors control electrons
travelling through a solid, such as silicon.
Such structures can be made down to a few
hundred atoms in size (which is where we came
in), no vacuum, no hot filament. Small size
means high speed and low energy required to
switch.
Hundreds of millions of these tiny switches can
be put on wafers of silicon to make up an
integrated circuit. Imagine a Pentium with 50
million hot, fragile and limited-life thermionic
tubes!
Electromagnetism
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James Clerk Maxwell (1831 - 1879)
developed the laws of
electromagnetism in the form we
know them today: Maxwell’s
Equations
Maxwell’s Equations are to
electromagnetism what Newton’s
Laws are to gravity
Note: It was Maxwell who realized the light is electromagnetic in nature
What is “Electricity”?
- "Electricity" means electric charge.
Examples: CHARGES OF ELECTRICITY. COULOMBS OF ELECTRICITY.
- "Electricity" refers to the flowing motion of electric charge.
Examples: CURRENT ELECTRICITY. AMPERES OF ELECTRICITY.
- "Electricity" means electrical energy.
Examples: PRICE OF ELECTRICITY. KILOWATT-HOURS OF ELECTRICITY.
- "Electricity" refers to the amount of imbalance between quantities of electrons and
protons.
Example: STATIC ELECTRICITY.
- "Electricity" is a class of phenomena involving electric charges.
Examples: BIOELECTRICITY, PIEZOELECTRICITY, TRIBOELECTRICITY,
THERMOELECTRICITY, ATMOSPHERIC ELECTRICITY ...ETC.
Electricity?
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Electricity is all about electrons, which are the
fundamental cause of electricity
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Static Electricity - involves electrons that are moved
from one place to another, usually by rubbing or
brushing
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Current Electricity - involves the flow of electrons in a
conductor
Electric Charge
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Two kinds: positive and negative (terms coined by
Benjamin Franklin)
When you rub a glass rod with silk, the charge that is
left on the glass was called positive. If you rub a hard
rubber rod with silk, the charge left on the rod was
called negative.
Like charges repel while unlike charges attract.
On the Move
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Electrons in the outer rings or shells of atoms are
bound more loosely to the nucleus
Such electrons tend to break free from the nucleus
and wander around amongst other nearby atoms
Such electrons are called free electrons
Current = Conduction
 Such movement of these free electrons creates an
electric current
 Materials with large numbers of free electrons are called
electrical conductors. They conduct electrical current.
 Movement of the electrons physically from one place to
another is slow. Transfer of the energy from one electron to
another happens fast.
Conductors and Insulators
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In conductors, electric charges are free to move
through the material. In insulators, they are not.
In conductors:
 The charge carriers are called free electrons
 Only negative charges are free to move
 When isolated atoms are combined to form a
metal, outer electrons of the atoms do not
remain attached to individual atoms but become
free to move throughout the volume of the
material
Other Types of Conductors
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Electrolytes
 Both negative and positive charges can move
Semiconductors
 In-between conductors and insulators in their
ability to conduct electricity
 Conductivity can be greatly enhanced by adding
small amounts of other elements
 Requires quantum physics to truly understand
how they work
Simple Circuits
Don’t let the name fool you
 Bottom line: For electric current to flow,
there has to be a complete pathway for
it…a complete circuit.
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Closed and Open Circuits
 Closed Circuit - an unbroken path of conductors
through which electric current flows
 Open Circuit - a circuit with a break in the conductive
path, so no current flows
Now, let’s play… “Know Your Electrical Symbols!”
Know Your Symbols
 Battery or Power Supply
 Resistor
 Capacitor
 Switch
 Conductive Wire
Series Circuits
 An electrical circuit with only one path for the
electrical current to follow
Parallel Circuits
 An electrical circuit that provides more than one
path for the electrical current to follow.
Static Electricity
Who hasn’t rubbed a balloon
on their hair and stuck it to
the wall?
 Buildup of charge (static, not moving)
in one place.
 Charge can be either positive or negative
Beware of Door Knobs That Bite
More apt to happen in dry weather…why?