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Electricity - PowerPoint PPT Presentation

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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.

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You Light Up My Life!

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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.

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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).

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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.

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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

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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

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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!

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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 electro-magnetic waves (light, X-rays etc).

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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

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The Discovery of Electricity

  • 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.

  • Study of magnetism goes back to the observation that certain naturally occurring stones attract iron.

  • There is some evidence that electroplating was used in Mesopotamia around 300bce (the Bagdad battery).

Attracting feathers with amber

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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.

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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.

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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.

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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)

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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.


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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.


Voltage = Current x ResistanceV = IR

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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?

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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.

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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.

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Edison and Swan

  • Nearly 40 years went by before a really practical DC (Direct Current) generator was built by inventor Thomas Edison.

  • In 1878 Joseph Swan, a British chemist/electrician, invented the incandescent filament lamp and within twelve months Edison made a similar discovery in America.

  • “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.

  • 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.

Left: A lamp used at the historic 1879 New Year’s Eve demonstration of the Edison Lighting System in Menlo Park, New Jersey.

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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.

  • 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!


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The Information Revolution

  • 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:

    • Tomorrow the sun will rise and darkness will be banished.

    • 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.

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The Information Revolution

  • 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!

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The Information Revolution

  • 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 yThese 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?”

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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'.”


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The Information Revolution

  • 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

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The Information Revolution

  • 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!

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The Information Revolution

  • The electric telegraph was a digital communications network; people speak in tones.

  • To send sounds down a wire, you need to:

    • Convert sounds to electric current vibrations (that is an analogue to the original air pressure variations).

    • Transmit these currents to the desired receiver.

    • 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.


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The Information Revolution

  • 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”

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The Information Revolution

  • 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.

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The Information Revolution

  • 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!

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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

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What is “Electricity”?

- "Electricity" means electric charge.


- "Electricity" refers to the flowing motion of electric charge.


- "Electricity" means electrical energy.


- "Electricity" refers to the amount of imbalance between quantities of electrons and protons.


- "Electricity" is a class of phenomena involving electric charges.


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  • Electricity is all about electrons, which are the fundamental cause of electricity

  • Static Electricity - involves electrons that are moved from one place to another, usually by rubbing or brushing

  • Current Electricity - involves the flow of electrons in a conductor

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Electric Charge

  • 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.

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On the Move

  • 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

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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.

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Conductors and Insulators

  • 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

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Other Types of Conductors

  • 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

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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!”

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Know Your Symbols

  • Battery or Power Supply

  • Resistor

  • Capacitor

  • Switch

  • Conductive Wire

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Series Circuits

  • An electrical circuit with only one path for the electrical current to follow

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Parallel Circuits

  • An electrical circuit that provides more than one path for the electrical current to follow.

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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

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Beware of Door Knobs That Bite

More apt to happen in dry weather…why?