Atomic Particles Atoms are made of protons, neutrons and electrons 99.999999999999% of the atom is empty space Electrons have locations described by probability functions Nuclei have protons and neutrons nucleus m p = 1836 m e
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Atomic Particles
Atoms are made of protons, neutrons and electrons
99.999999999999% of the atom is empty space Electrons have locations
described by probability functions
Nuclei have protons and neutrons
nucleus
mp = 1836 me
Atomic sizes
Atoms are about 10-10 m Nuclei are about 10-14 m Protons are about 10-15 m The size of electrons and
quarks has not been measured, but they are at least 1000 times smaller than a proton
What is Light?
Properties of lightReflection, Refraction
A property of both particles and waves Interference and Diffraction
Young’s double slitsA Property of Waves Only
PolarisationA Property of Waves Only
Classical Physics
Light is a waveYoung’s Double Slit Experiment Faraday’s experimentsMaxwell’s equations
JtEB
tBE
B
E
000
0
1
0
Line-Emission Spectrum
ground state
excited state
ENERGY IN PHOTON OUT
Bohr Model
e- exist only in orbits with specific amounts of energy called energy levels
Therefore…
e- can only gain or lose certain amounts of energy
only certain photons are produced
Bohr Model
12
3456 Energy of photon
depends on the difference in energy levels
Bohr’s calculated energies matched the IR, visible, and UV lines for the H atom
Other ElementsEach element has a unique bright-line
emission spectrum. “Atomic Fingerprint”
Helium
Bohr’s calculations only worked for hydrogen!
The Birth of the QuantumMax Planck
The energy contained in radiation is related to the frequency of the radiation by the relationship
n is a positive integer called the quantum number f is the frequency of the oscillation
A discreet packet of energy, later to become known as “a photon”
nhfE
Implications of Planck’s Law
The energy levels of the molecules must be discreet
Only transitions by an amount E=hf are allowed
The implication is that light is discreet or quantised
These quantum levels are now known as number states
43210
energy4hf3hf2hf1hf0
energy n
Photoelectric effect
When light strikes the When light strikes the cathodecathode, electrons , electrons are emittedare emittedElectrons moving between the two plates Electrons moving between the two plates constitute a currentconstitute a current
Photoelectric Effect Explanation
Einstein: the quanta of energy are in fact localised “particle like” energy packets
Each having an energy given by hf Emitted electrons will have an energy given by
Where is known as the “work function” of the material hft where ft is the threshold frequency for the metal.
hfKmax
Louis de Broglie1892 - 1987
Wave Properties of Matter
In 1923 Louis de Broglie postulated that perhaps matter exhibits the same “duality” that light exhibits
Perhaps all matter has both characteristics as well For photons,
h
chf
cEp
mvh
ph
Which says that the wavelength of light is related to its Which says that the wavelength of light is related to its momentummomentum
Making the same comparison for matter we find…Making the same comparison for matter we find…
Quantum Theory
Particles act like waves?!The best we can do is predict the
probability that something will happen.
Heisenberg Dirac Schrodinger
SchrödingerErwin
(1887 – 1961)
“The task is not so much to see what no-one has yet seen, but to think what nobody has yet thought, about that
which everybody sees.”
Schrodinger’s cat After consultation with Einstein, Schrodinger proposed a
thought experiment in which he highlighted the apparent inconsistencies between the so-called Copenhagen interpretation of Quantum Mechanics and the reality of macroscopic measurements.
He proposed that a cat be placed in a sealed box. The release of a poison is then subject to the probabilistic decay of a radioactive isotope. If the isotope decays, the poison is released. If no decay occurs, the poison is not released.
The result is that the cat is in a superposition of states between being dead, and being alive. This is very unintuitive.
Quantum mechanics
Wave-particle dualityWaves and particles have interchangeable
propertiesThis is an example of a system with
complementary properties The mechanics for dealing with systems
when these properties become important is called “Quantum Mechanics”
Measurement disturbes the system
The Uncertainty Principle
The Uncertainty Principle Classical physics
Measurement uncertainty is due to limitations of the measurement apparatus
There is no limit in principle to how accurate a measurement can be made
Quantum Mechanics There is a fundamental limit to the accuracy of a
measurement determined by the Heisenberg uncertainty principle
If a measurement of position is made with precision x and a simultaneous measurement of linear momentum is made with precision p, then the product of the two uncertainties can never be less than h/2
xx p
The Uncertainty Principle
Virtual particles: created due to the UP
E t
In Search of the Higgs Boson
Higgs boson is “cosmic molasses” – the Holy Grail of particle physics
Interactions with the Higgs Field are theorized to give all the particles their masses
LHC detectors should be able to confirm or disprove initial hints for Higgs at E=115 GeV
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