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• “And through the window in the wall, comes streaming in on sunlight wings, a million white ambassadors of morning.”

• Pink Floyd

• Quoted by Jim Baggott in “Farewell to Reality” as “...a particularly pleasant metaphor for photons, the elementary particles that constitute all electromagnetic radiation, including light.”

• (J.B also points out: “However on a bright morning, there are rather more than a million of them streaming through the window.”)

• Today’s topic: Discovery of the laws of electromagnetic radiation.

• Particularly the roles of Michael Faraday 1791-1867,

• James Clerk Maxwell 1831 - 1879).

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Michael Faraday - some statisticsBorn in London 1792, father a poorly blacksmith.

Michael’s schooling almost non-existent.

Death of father meant Michael and his elder brother became bread winners

At 14 began work as a dogsbody to a bookbinder

Rapid progression to apprentice bookbinder.

Here his education really began – all those lovely books!

Owner encouraged Michael to study his favourite subject -

Natural Philosophy, or as we now say – SCIENCE .

Joined several informal Philosophical societies and then secured a

temporary appointment as assistant at to Humphry Davy - brilliant Prof. at

the Royal Institution

• Later, he succeeded Sir H. as the most brilliant experimentalist and communicator of science of his generation.

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Faraday’s electrical and magnetic experiments• 1820, H.-C. Oersted discovered that a magnet near a

vertical wire carrying an electric current is deflected.

• But the compass needle moves so that it lies at right angles to the current (flow of electric charge).

Magnetic force on compass needle

Electric current• 1821 Faraday (age 29) repeated Oersted’s experiments & found that a current-carrying wire rotates around a bar magnet.

• Built toy apparatus to demonstrate.

• First electric motor!

• For all other forces known in 1820 - gravity, static electricity, pushing a pram, firing a gun/arrow,etc., the force and its result lie in the same direction.

Faraday’s demonstration electric motor

Stationary magnet

Rotating wire

Rotating magnet

Stationary wire

Jars are filled with liquid mercury

Electric current can enter either jar and out through the central pillar.

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Problems• 1821 Faraday published right away, unfortunately

before he was able to check with Davy (his boss) whose work he mentioned.

• Result was that Faraday - dogsbody remember -was considered unfavourably by the English scientific establishment.

• Could have prevented his election to the Royal Society (1824) - Davy voted against.

• Faraday decide to concentrate on other problems.

• 1825 Davy retired and Faraday appointed to his place.

• Started Christmas lectures.

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Electromagnetic Induction

• Partly as a result over the unhappiness over his 1821 experiment, Faraday only returned to the subject 10 years later.

• He was influenced by the work of Augustin Fresnel who advocated a wave theory of light unlike Newton’s “corpuscles”

• Fresnel’s work was very mathematical but a popular review made his work accessible to Faraday.

• Faraday experimented on sound waves transmitted through air and cause “induction” (resonance); for example: sopranos and wine glasses.

• Also fascinated by patterns of waves in sand sprinkled on a sheet.

• Here were mechanical forces produced by waves.

Magnetic (Force) Fields - Faraday’s montage of the patterns formed by iron filings on paper placed over magnets

When the switch connects coil A to the battery, the meter “flickers”.Switching off gives motion in the opposite direction

Inserting a magnet into the coil makes the meter needle rotate.Remove the magnet and it rotates in the opposite direction.

First Dynamo

Prime Minister Robert Peel visiting R.I.: What is the name of that strange machine?

Faraday: ”I know not but I wager that one day your government will tax it.”

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What happened next?• Even though Michael Faraday now had an established

reputation as a brilliant experimentalist,

• Through his lectures was an immensely influential populariser of science,

• He was unable to take his ideas forward through his lack of mathematics.

• Moreover he had no wish to gain any financial reward, so others exploited his results.

• This owed much to his strong religious convictions: he was a member of a fundamentalist Christian group “known, if known at all” as Sandemanians.

• Pursuit of worldly privileges was not encouraged.

• But a remarkable mathematician had been impressed by Faraday’s experiments: James Clerk-Maxwell.

James Clerk Maxwell – Early years

• Born 1831. Home – Maxwell estate in Galloway.

• Only child. James played with local kids& developed thick Galloway accent (which he never lost).

• Very inquisitive child, loved animals especially frogs.

• Father John – a pioneer; designed own (practical) clothes & shoes for himself and James.

• Mother Frances responsible for early education.

• But she died at age 48 when James was 8.

• Shattering blow for both father & son.

• Education now placed in hands of local teenager.

• Total disaster.

• Enrolled at Edinburgh Academy in 1840 at age 9, living with aunt.

Maxwell’s Magnum Opus – Maxwell’s Equations

• Faraday’s “lines of force” proved difficult to understand: Astronomer Royal (Airy) dismissed them.

• Thomson – more receptive but tried to squeeze them into his own notions.

• Maxwell had corresponded with Faraday, whom he admired, but was determined to digest Faraday’s work before trying to extend it.

• Realised that Faraday might have a different view of the right solution.

• In fact Faraday’s method “was also a mathematical one, though not exhibited in the conventional form of mathematical symbols.”

• Embodies the gist of Faraday’s results in a more mechanistic “thought machine”

Axis of magnetic line of force

“Idle wheel to prevent friction

“molecular vortex”

Hexagonal shapes are purely for “artistic” reasons

A B

If speed of rotation increases from A to B idle wheels (electrical current ) will flow from A to B

Molecular vortices

From Vortices to waves• Several steps later with this magical machine, Maxwell

realised that it acted like an elastic sheet.

• And elastic sheets could be made to vibrate

• So electro-magnetic vibrations should exist.

• He knew how to calculate the speed and found it to be close to the velocity of light

• Maxwell, on holiday in Galloway needed the latest data.

• When he got back to London, he found the model DID predict a velocity near that of light.

• Using only the constants of nature appropriate to:

• Electricity, ε 0 , and Magnetism, μ0

• Velocity of light, c = 1/square root(ε0 μ0).

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Electricity and magnetism were now united

• And are now integral

parts of a new

Fundamental force - the

Electro-magnetic Force.

• With enormously wide

applications throughout

of Nature.

• For example - light and

other parts of the

electro-magnetic

spectrum.

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Electromagnetic spectrum

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Maxwell - Last Works

• Maxwell continued with the classical approach to light and other forms of electromagnetic radiation.

• Light from a star, say, would spread out across the universe as waves, and a part of those waves would be detected by our eyes.

• Maxwell believed that for waves to travel they must travel through some medium - the ether.

• This was thought of as an invisible, tenuous, transparent medium - the “luminiferous ether”.

• In other works he founded, with Austrian Ludwig Boltzmann, theories of gases and liquids based on probability - the statistical properties of collections of molecules.

• A departure from the deterministic “clockwork universe” models favoured by Newton in which properties of bodies could be determined exactly.

• Maxwell died in 1879 - leaving Boltzmann to fight on - as there were serious critics of his approach - and even in the existence of atoms.

• Boltzmann, succumbed to the critics and committed suicide in 1906.28

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Bonding - from atoms to molecules and then to solids and liquids

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Electrons make elements

For Principal Q.N. N=2: there are up to 2 electrons in 2s,

and 2 electrons in each of three polar (2p) orbitals,

pointing in 3 directions at right angles. Total = 8

Shells fill up in an orderly manner: 1s before 2s before 2p, before 3s etc. (Pauli Exclusion Principle).

“Ionic” Bonding of elements• Inert gases with filled shells represent the most stable electronic structures.

• Example: Ne, filled 1s & filled 2s & 2p shells (10 in all).

• H, Li & Na have 1 electron more than the filled shell configuration, which is easily removed so it is very reactive.

• F, Cl have one electron less than the filled shell and could gain one. So they too are very reactive.

• If, for example, Na donates its extra electron to Cl , then both achieve the stable filled shell “rare gas” configuration.

• The result is a positive Na+ ion and a negative Cl - ion.

• So the compound Na+ Cl - is formed with an electrostatic “ionic” bond between the two ions.

Bonding by electron transfer

Chlorine (a few more inner electrons) is chemically similar to Fluorine.

Bonding by electron sharing

Also called covalent bonding

Solids

From atoms to solids

Here we see the ionic bondtranslated into a three-dimensionalcrystal lattice.

FNa

Session 8 Electric currents• Alessandro Volta in 1790s found that 2 different

metals in his mouth tingled - and frogs’ legs twitched

• He developed a Voltaic pile (battery/ cell)

• This gives a Voltage which can drive an electric current.

• At first, electric currents were thought to involve flow of positive charges: convention now is to call them negatives.

• We now recognise that what moves are electrons.

• Distinction between two types of solids emerges:

• Metals, like copper, which “conduct” electric currents

• Insulators, like wax, plastics, which do not.

• What’s the difference?

The Lithium atom in the centre has 8 nearest neighbours and another 6(not shown on the diagram) at a slightly larger distance.

So the one easily moved electron is shared over 14 neighbouring Li atoms.

These bonds are very weak, so the outer electrons are very mobile.

In early models of metals, the outer electrons were thought to behave like molecules of a gas – which could therefore “flow” anywhere.

Metals – an example - Lithium

Metals and Insulators• Many metallic elements have one electron more

than the favoured “rare gas” configuration.

• Lithium 1s2 2s1 – one more than Helium (1s2)

• Sodium 1s2 2s2 2p6 3s1 one more than Neon.

• The extra electron can easily be lost to give the “ion”.

• Diamond is a good insulator.

• Shares outer electrons over

4 neighbours, giving strong

bonds, • with electrons that are

not easily moved.

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Diamond and Graphite• Both are pure Carbon.• Diamond is very hard - used in abrasives.• Graphite is very soft - used in pencil “leads”.• Graphite exhibits metallic sheen.• Diamond is transparent.• Graphite conducts electricity - is used in

batteries.• Diamond is an electrical insulator.• Graphite is very common - and is the most

stable form of elemental carbon. • Diamond a rare gem stone.

• What are the reasons??38

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In each of two adjacent C atoms, one outer shell orbitaloverlaps with its neighbour, to give bonds“floating” above and below the sheets.

The other 3 form bonds in the plane of the sheet.

Two-dimensional lattice structure of graphite

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Here are two slides which you might like to think about before next time when we can fill in any

gaps.

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How fast do electrons move?• When a voltage difference is applied across a

metal, electrons can move very quickly.

• In copper the instantaneous speed is around 1.6 million metres per second.

• But electrons meet resistance from

Grain boundaries

Thermal vibrations

• These cause the electron ”gas” to be scattered – like the scattering of molecules in a gas or liquid – “Brownian Motion”.

• Actual speed – “drift velocity” is 3/100th mm/sec.

• But the “resistance” does cause heating in a lamp filament.

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Electrical currents - a simple analogy• One simple analogy is an empty garden hose-pipe

connected to a tap.

• Turn on the tap and water exits at the other end.

• We could think of the flow of water modelling electrons moving down a metal wire.

• But it’s a poor model. Because electrons move so slowly, (drift velocity less than 3/100 mm per second) the electrical signal would take too long to cross the street, let alone the Atlantic.

• Better model: a hose-pipe already filled with water.

• Then, when the tap is turned on, a pulse of pressure moves rapidly along the pipe and water, already at the end comes out of the other end is forced out almost immediately.

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