Transcript SCE 18 – Part 2

Atoms- What do we know?
• “Nothing exists except atoms and empty space: everything
else is opinion.” Democritus: 400 BC.
• Atoms in 1900:
– Can be grouped into types: for example the halogens:
Fluorine, Chlorine, Iodine, Bromine
– Different atoms have different atomic weights: usually
multiples of the atomic weight of Hydrogen
– They are very small. If we model atoms as 1mm grains of
sand. Then the model of a REAL sand grain would be a
cube, edge length 1-10 km.
– Up to half the distance Cambridge to Ely
Atoms – Views of scientists in 1890-1900
• Actual existence of atoms controversial.
• Problems were philosophical and scientific
• Theory of gases (Maxwell and Boltzmann): “billiard ball”
collisions of molecules based on Newton’s laws of
motion.
• Apparent inconsistency with Laws of Thermodynamics:
• First law: Energy cannot be created or destroyed
• Second Law: Despite this, actual physical processes do
lead to “degradation” of energy so that some is
effectively lost.
• Therefore, assumption of existence of molecules (and
therefore atoms) must be incorrect.
Electrical currents through gases at low pressure
• Several people, including Faraday, investigated currents
through gases at low pressure.
• Geissler invented an improved mercury pump
• Lower pressures obtained by raising &
lowering mercury column.
•
This lead to coloured discharges
depending on the gas – Geissler tubes:
• Further improvement in pumps lead
to enormous advances in physics. “
The biggest revolution in the history
of science…”
Cathode
High voltage generator
Anode
William Crookes
With much lower gas pressures, “Crookes
Tubes” were produced in which something
(particles or rays ?) travelled from the negative
cathode) to positive (anode) in straight lines.
When they hit the tube they caused a glow.
Cathode Rays - PARTICLES OR WAVES??
• 1871 Cromwell Varley suggested particles
• 1876 Golstein & Helmhotz : “cathode rays”.
• Showed that rays were deflected by magnets but thought
they were electromagnetic rays like light.
• 1879 Crookes thought the “rays” were particles and
other British scientists took this view.
• Hertz showed that electric field had no effect so it
seemed that they were really rays. However his vacuum
was not very good.
• Several workers noticed nearby wrapped photographic
plates became fogged.
• Crookes complained to the manufacturers!
• So what are cathode rays?
1894 Philipp Lenard showed
cathode rays could penetrate
very thin foils of metals.
Evidence for “rays”?
Röntgen used tubes with lower
pressure in Nov1895. Tried to
check whether rays would pass
through glass.
Covered tube with cardboard
and hoped to detect rays with a
fluorescent screen.
But out of corner of his eye saw
a glow from another screen.......
X-rays!!
Wilhelm Röntgen
Born 1845 in Lennep Germany
X-ray of Röntgen’s wife’s hand
22 12 1895
X-ray taken during Röntgen’s
public lecture 23 1 1896
Radioactivity
Henri Bequerel – one of a dynasty.
Father and grandfather were
interested in minerals that glow in
the dark
Text
Radioactivity Winter 1895 Paris
• Henri Bequerel continued investigating minerals that glow in
the dark after Röntgen’s discovery.
• Wondered whether any connections to X-rays.
• Exposed crystal to light then
• placed them under wrapped
photographic plates.
• Plates darkened.
• Even in dull weather with
little light.
• Plates still darkened - so light
• was not necessary.
• “Radioactivity” – a term coined
• by Marie Curie, who devoted her career to this.
Beta particles - electrons
• J. J. Thomson
Born 1856 Cheetham Hill Manchester.
• Mother textile worker, Father antiquarian
bookseller.
• Father died when J.J. was16. Changed
course from Engineering to Maths.
• Then Cambridge (Trinity again).
• Started research some of which predated
Einstein’s result that mass of charged
• Later became Master of Trinity - first
particle increases with speed.
Master not a clergyman.
• 4 years later appointed Cavendish
Professor.
• William Thomson first choice but again
refused.
• Also first Cambridge Prof of Physics
to originate from lower middle classes
of North of England
Several workers showed that
cathode rays can be deflected by
magnets.
Hertz and Philipp Lenard found that
the rays were not deflected by an
electrical field.
But JJT showed that cathode rays
can be deflected electrically.
He had tubes with a lower
pressure.
Schematic of J.J.T’s apparatus
(original in Cavendish Museum)
•By using equations derived from Faraday, JJT calculated
the ratio of the charge on each particle, e to its mass, m;
(that is e/m).
•He found e/m was about 2000 times larger than
expected.
•The mass of the electron is about 2000 times smaller
than the mass of the smallest atom.
•When he announced this at the RI listeners thought he
was pulling their legs
Pieter Zeeman
• Born in a small village in Zeeland
• Assistant in University of Amsterdam
• Decided to repeat Faraday’s attempts to
measure changes in spectrum of a flame.
• Also wished to contradict Maxwell’s view that
energy of light could not be altered by magnetic
forces.
• Studied sodium light from a Bunsen flame in a
magnetic field.
Spectrum of sodium light
• Zeeman saw a very small broadening of two closely
spaced yellow lines.
• Hendrik Lorentz applied his theory and obtained a value
for e/m - also very high.
• Published in 1896, so this value was known to J.J.
• The two values for e/m for an electron outside the atom
& one inside the atom corresponded.
What does the electron tell us about atomic
physics?
• The electron is clearly the unit of electric charge and may
be involved in electrical conduction in metals and
electrolysis.
• The electron is involved in oscillations within the atom
which give rise its optical spectrum.
• Outside the atom, electrons can exist alone as cathode
rays.
• Nobel prizes
1901 (first)
Wilhelm Rontgen
1902
Henri Becquerel, Marie and Pierre Curie
1904
Philipp von Lenard
1905
Pieter Zeeman and Hendrik Lorentz
1906
J.J. Thomson
Centenary celebrations in Germany, France and
UK
1900, Following J.J., what is the atom?
• Clearly atoms are no longer inviolable.
• They can be split apart to yield a particle only about
1/2000th the mass of the lightest atom, Hydrogen.
• Motion of electrons along a wire can provide a model for
electric currents.
• Thus removing old definitions of electricity :
• “Imponderable fluid” and “Electric virtue”
• (William Gilbert court physician to Q. Elizabeth I)
• Note. J.J. didn’t call them electrons – he called them
corpuscles.
• “Electron” from Greek word for amber
Ernest Rutherford
Cambridge 1895-1898
• Rutherford: Cambridge University’s first Research
student.
• J.J. & Mrs J.J. welcomed him warmly.
• Not so other laboratory Physicists.
• Gained Street Cred. via his wireless devices.
• News of X-rays reached Cavendish. J.J. and R. start
work on electrification of gases by X-rays.
• R. moved to Radioactivity after Becquerel’s work.
• Rutherford identified two types of radiation:
• α rays: easily absorbed e.g. by paper, air.
• β-rays: long range – greater penetration.
• Rutherford also aware of a third type of radiation,
subsequently named γ-radiation by Paul Villard in
1900.
Manchester - Geiger and Marsden :scattering of α particles by nucleus
Glass tube filled with Radium “emanation”
Foil of various metals
Lead
Gold
Platinum
Tin
Silver
Copper
Iron
Aluminium
Lead screen
Zinc Sulphide screen
to show scintillations
Microscope
Rutherford
• Discovery of the (electrically negative) electron within atoms
raises a problem.
• Since the atom is electrically neutral, where is the positive
charge?
•
J.J. favoured a ‘plum pudding’ with charges spread out.
•
Rutherford (in Manchester) set up an
experiment with α-particles (Helium
nuclei) from a radioactive source.
•
Most particles went straight
through the thin gold foil.....
but some bounced back!
•
•
•
Rutherford said it was ‘like firing
a 15 inch shell at a piece of tissue
paper and it came back and hit you.’
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Rutherford’s diagram showing how alpha particles are
deflected when they pass close to a heavy nucleus
Dimensions of nucleus
• Rutherford (eventually) calculated the size of the
nucleus: 10-15 m.
• So volume of nucleus is 10-45 cubic metres
• Volume of atom is 10-30 cubic metres.
• Nucleus is 1015 times smaller than an atom.
• Rutherford pictured this as “A gnat in the Albert Hall”.
• But the gnat’s density is enormous: a gnat-sized gold
particle weighs a million tonnes.
• Roughly the mass of the Albert Hall!
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Bohr- Rutherford anatomy of an atom
• In the 1910s Ernest Rutherford
and Niels Bohr devised a model
for the atom.
• Electrons – electrically negative
– surround a positive nucleus.
•The nucleus contains protons (+)
and also (neutral) neutrons
•(James Chadwick 1932).
• Bohr- Rutherford model of electron absorption and
emission
Incoming radiation
causes ejection of
an electron to a
higher quantum
orbit.
Electron falling
to a lower quantum
orbit releases
radiation
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Problems with the “Astronomical”model
of the atom.
• Classical theory requires that a circulating
charged particle emits radiation.
• So the circulating electron would lose energy
continuously.
• And quickly spiral down into the nucleus.
• Solution to the problem demands that energy
should not be emitted continuously.
• Another opportunity for Quantum science!
• If one young man (Einstein) could revolutionise
science, why not another?
• Enter Niels Bohr.
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• Niels Bohr (again) made the quantum hypothesis the centre
of his new theory (1913) of the structure of the atom.
Bohr bolted together Newtonian
dynamics and Quantum Theory.
Proposed that there are stable orbits –
which do not spiral into the nucleus as
classical physics demands.
Transitions between orbitals obey
Planck’s Quantum law. Results agree
with spectrum for Hydrogen.
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Beyond the Bohr atom
• Bohr’s hypothesis agreed with the spectrum of
Hydrogen.
• H is the lightest atom - 1 proton in the nucleus
and 1 orbiting electron,
• Bohr’s method failed for larger elements
containing many electrons.
• But the key ingredient - “Quantisation of stable
electron states and
• changes in energy was established.
43
Louis de Broglie
After detailed reflection on Einstein’s work,
“I suddenly had the idea, during the year 1923, that the
discovery made by Einstein in 1905 should be generalised
by extending it to all material particles and notably to
electrons.”
E = mc2 (Einstein)
E = hν (Planck);
So simply coupling the two
equations we get:
E = hν = mc2
So electron “mass” has an
associated wave motion.
Particles become “Wavicles”
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DeBroglie’s Harmonics
De Broglie had in mind wave patterns
for a stretched string.
• Rather than electrons occupying
“orbits”- as Bohr had suggested, he
thought that the lowest level electron
state would be the fundamental mode
of “vibration”.
• Higher orbitals could be
represented by harmonics.
De Broglie’s thesis supervisor and examiners had doubts
about the validity of the work and consulted Einstein who
was supportive: “I believe it is a first feeble ray of light on this
worst of our physics enigmas.”
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Contemporary reaction to De Broglie
• In France... The thesis was published in full in a special
edition of “Annales de Physique” in 1925.
• Outside France, wave-particle duality met a sniffy
reaction: “ la Comédie Française”.
• (Dirac was not impressed.)
• Einstein reacted positively: drew De Broglie’s work to the
attention of Erwin Schrödinger, a 38-year old Austrian
(who had not read De Broglie’s papers).
• Schrödinger realised this paper covered similar ground to
some of his earlier work and he was also attracted to the
idea of harmonics.
• He talked about his ideas in an informal seminar but was
told that he should try to construct a wave equation to
replace “this rather childish way of talking”.
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Some reactions to De Broglie’s proposal: hν =E = mc2
• Roger Penrose in “The Emperor’s New Mind”:
• “Thus, according to De Broglie’s proposal, the
dichotomy between particles and fields that had been
a feature of classical theory is not respected by nature.
• Somehow, Nature contrives to build a consistent world
in which particles and field-oscillations are the same
thing.”
• Or, rather, her world consists of some more subtle
ingredient, the words ‘particle’ and ‘wave’ conveying
but partially appropriate pictures”.
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