What is Quantum Mechanics ?

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What is Quantum Mechanics ?
Quantum Mechanics describes the laws that govern
the time evolution and the behaviour of any
physical system. They are the laws of motion
and incorporate the principles of relativity.
It explains the stability and identity of atoms
why are all hydrogen atoms wherever we find them
the same and why are they stable. It
quantitatively explains the interaction of
radiation and matter.
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The Bohr Complementarity Principle
To describe the motion of an object, at each
instant of time one must have complete knowledge
of its position and its velocity two
complementary quantities. The wave and particle
properties of object can be regarded as
complementary aspects of a single reality, like
two sides of a coin. According to quantum
mechanics it is impossible to measure both
complementary quantities simultaneously to
complete precision. If you know position exactly
you cannot know its velocity. This is the
essence of the Uncertainty Principle
of Heisenberg. Just as a tossed coin may fall
either heads up or tails up, not both at
once. Therefore it is impossible to know
everything about a physical system. This shows
up spectacularly when dealing with microscopic
objects electron or photon.
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Copenhagen Interpretation
As a physical system evolves in time it can lead
to many different outcomes. Quantum mechanics
only predicts the probabilities for all possible
outcomes. Reality consists of actual individual
outcomes. Bohr proposed that the act of
observation turns the many possibilities into a
single actuality a proposition that is
called the Copenhagen interpretation. By
repeatedly doing the experiment one can
determine the quantum mechanical probabilities.
According to Bohr this is the only meaningful
interpretation.
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The Nucleus
Matter is made up of atoms.(19th century) Whole
atoms are electrically neutral. Atoms have
structure shown by Rutherford(1911) Most of
the mass of the atom is concentrated in a small
volume called the nucleus. It is
positively charged. 99.97 of the mass of the
atom is in the Nucleus. Atom is 10,000 bigger
than the nucleus . Negative electrons move in
this large volume and make the atom as a whole
electrically neutral.
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Atom
Mostly empty space !
10 kilometers
Electrons
Nucleus 1 meter in size Mass of the atom
Contains protons and neutrons
Electric Force
True size 0.00000000005 m
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Nucleus of the atom
Element Lithium (used in batteries)
Four neutrons and three protons
True size 0.000000000000001 m
Neutron
Protons have positive electric charge
Proton
Neutrons are electrically uncharged
Atomic mass 7 Atomic charge 3
Neutrons and protons move around in the nucleus
Held together by Strong nuclear force
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Isotopes
Different varieties of the same element for
which the nucleus has the same number of protons
but different number of neutrons so their
masses are different.
Example Hydrogen comes in three varieties, each
having only one proton. Hydrogen
1 proton Deuterium 1 proton and 1
neutron Tritium -- 1 proton and 2 neutrons .
Tritium is radioactive with a half life
of 12 years ! Tritium is three times more
massive than hydrogen.
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H
1
2
H

D
D
1
1
3
H
T
1
1
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Isotopes Continued
The common isotope of Uranium has 92 protons and
146 neutrons for a total atomic mass of
238 A rare isotope of Uranium, important in
making a bomb or reactors has a mass of 235
consisting of 92 protons and 143 neutrons.
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Why does the nucleus not fly apart ?
Like charges repel each other, hence the protons
try to fly away from each other in the nucleus.
What keeps the nucleus together is the attractive
nuclear force a force different from the
electric force and called the Strong force. To
break a nucleus one has to supply energy to
overcome the energy which binds it and keeps it
together. Similarly the atom itself is bound
together by electrical force between the protons
and the surrounding electrons. Energy usually
has to be supplied to break up atoms.
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Fission
The process by which a nucleus with a given
number of protons(Z) and neutrons(N) can split
up into two other nuclei with different nuclei
with different numbers of protons and neutrons.
A1 N1Z1
N1
Z1
N, Z
neutrons
A NZ
A2N2Z2
N2
Z2
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As protons and neutrons cannot be destroyed in
fission
Number of Protons of the fissioning nucleus
must equal the number of protons of the fission
product nuclei together. Z Z1Z2.
However the number of neutrons of the
fissioning nucles equals the number of neutrons
in product nuclei plus the number of neutrons
released in a fission. N N1 N2 number of
neutrons emitted
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There are two kinds of fission processes 1.
Spontaneous fission where the nucleus by
itself fissions, without external excitation.
2. Induced fission, caused by the absorption
of a neutron by a nucleus which then
undergoes fission. Uranium 235 can easily
capture a slow neutron and undergo fission
and in turn releases 2 or more neutrons
making it possible to sustain a chain reaction.
Hence it is important for generating power
or making an explosion.
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Neutron Induced fission of Uranium 235
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Chain Reaction with Uranium 235
Fission products Krypton and Barium radioactive
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Critical Mass
If you can pack 24 pounds of Uranium235 in a
sphere of radius 3.4 inches the fission process
can run away and in a millionth of a second and
release enormous amount of energy an atom bomb
will result. This is the critical mass of Uranium
235
Controls rods are devices made up of a substance
which absorbs Slow neutrons and remove them from
the chain reaction.
If control rods are not operative in a reactor,
the energy release can be exponentially large and
the reactor can have a meltdown !
In Copenhagen, Bohr asks Heisenberg about use of
control rods in a reactor a point Heisenberg
did not seem to be aware of.
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Radioactivity is the spontaneous emission of
different kinds of radiation from the nucleus of
an atom. Three kinds of radiation can be
emitted 1.Electrons or positrons called
beta-rays 2. Alpha particles which are nuclei of
helium atoms 3. Gamma radiation which is more
energetic than X-rays. It is gamma radiation
which is most lethal radiation in bomb explosions.
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Cyclotron
Cyclotron is a particle accelerator which
produces very energetic protons or similar
nuclei. (Invented by Ernest O. Lawrence of
Berkeley in 1930s) In Copenhagen, at Bohr's
institute there was such a cyclotron, while
there were no such machines in Germany at the
time of World War II. German physicists wanted
to use this machine in studying processes which
might help in making an atom bomb or a reactor.