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This Week
 Fission and fusion
What made our world
 Atomic and Hydrogen bombs
Big bangs in small packages
 Nuclear power plants
The solution to the energy problem?
 Enrichment
What does it mean and why is it necessary?
 Toxic Disposal
 Radiation in everyday life
Limit your exposure (same for the stock market)
 Fusion: The Holy Grail
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Physics 214 Fall 2012
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Fission and fusion
If we make an atom from it’s
constituents energy is released. This is
called the binding energy.
Iron is the most tightly bound so if we
can make elements up to Iron by the
fusion of light elements energy is
released.
Similarly if we can break a very heavy
atom into lighter elements we also
have energy release.
E = mc2
Mass is another form of energy a
nucleus is lighter than the sum of the
constituent particle masses
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Physics 214 Fall 2012
Fusion processes are
how the sun produces
energy and fission
processes are how
nuclear reactors
produce energy
2
Chain reaction
If a U235 atom is broken into two lighter
elements energy is released.
This is accomplished using a neutron. The
process produces 3 neutrons which can
then cause 3 more atoms to undergo fission.
Without control this chain reaction produces
an enormous amount of energy in a fraction
U235
of a second. It is an atomic bomb.
This process was first observed by Enrico
Fermi in a laboratory under the Chicago
University football stadium
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Manhattan project and the bomb
In the early forties all the top scientists
were taken to Los Alamos to design
the atomic bomb.
In order for a bomb to work there had
to be sufficient Uranium but it could
not be in one piece because it fission
would spontaneously occur.
The main challenges were to obtain
enough U235 and to design a system to
bring the uranium together with a
neutron source when the bomb was
dropped.
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The hydrogen (fusion) bomb
Fusion gives a much bigger
energy release than fission
Hydrogen bombs are designed to
use an atomic bomb to provide
the temperature and pressure so
that fusion takes place.
There is a central core of light
elements surrounded by
plutonium or uranium.
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Enrichment
Naturally occurring uranium is nearly all U238
U235 is the fissionable material.
Complicated processes have been developed
to increase the fraction of U235, this is
enrichment.
For a reactor the final fraction of U235 is about
3.5%. For the original bomb several
kilograms were required.
Plutonium is produced
and can be obtained
from the spent fuel rods
In a reactor some of the U238 is turned into
plutonium which is also a fissionable
material that is used in atomic bombs.
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Nuclear reactors
In order to produce energy we need to have
controlled fission.
Fuel rods are inserted into a moderator,
which is used to slow the neutrons down
and not let them escape.
Control rods are used to absorb neutrons.
The control rods can be moved in and out to
set the level of energy production including
shutting the reactor off completely.
The energy is taken out with a circulating
liquid and steam is generated to drive a
turbine.
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Toxic disposal
Most of the elements produced in the fission process are highly
radioactive. All materials inside the containment vessel also
become highly radioactive.
Disposal of this toxic nuclear waste is a very big problem since it
must not enter the environment. There is controversy over a
proposed waste disposal site at Yucca mountain.
In 1986 operator error at the Chernobyl nuclear plant caused an
uncontrolled chain reaction. The resulting release of radiation
had effects over almost all the northern hemisphere and thyroid
cancer rates have increased 10 fold in the Ukraine
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Radiation in everyday life
We are exposed to radiation at low levels from a number of
sources
Cosmic rays
Radon and other natural radioactivity
Medical imaging
Consumer products fire alarms
Radiation is also used
to treat cancer
PET scans
All kinds of medical tracers e.g. radioactive iodine
Used in industry to control thickness of rolled steel and paper
Sterilization of food
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Fusion as an energy source
Every second the sun turns 600
million tons of hydrogen into 596
million tons of helium (with 4 million
tons transformed into luminous
energy via E=mc2).
To emulate the sun we need
extremely high temperatures and
pressures to squeeze light elements
together.
Livermore National lab has tried to
do this using high power laser
beams to compress pellets of
deuterium and tritium.
There is a new international effort
based in France called ITER to try
and produce controlled fusion.
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Summary of Chapters 18 and 19
n
1
2
3
# electrons
(2L+1) x 2 = 2 L = 0
(2L+1) x 2 = 8 L = 0 or 1
(2L+1) x 2 = 18 L = 0 or 1 or 2
The accelerating voltage is
typically 30,000 volts and
X rays are produced as the
beam hits the screen
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The
nucleus
Decay
Periodic table
E = mc2
Chain reaction
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Questions Chapter 18
Q11 Assuming that cathode rays are a beam of charged particles,
how could you demonstrate that these particles are negatively
charged?
By deflection in a magnetic field
Q13 Would you expect X rays to be produced by a television
picture tube?
Yes the accelerated electrons hitting the screen do produce X rays
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Q14 If the electron beam in a television tube is striking just one point
on the screen at a time, how can we get a full picture?
Because the beam sweeps over the screen 30 times/second and
we do not see this motion because of the speed and persistence
of vision.
Q20 What role did Rutherford’s scattering experiment play in our
developing understanding of atomic structure?
Rutherford discovered the nucleus made up of protons and
neutrons surrounded at a large distance by electrons. The atom
was not of uniform density like a marshmallow
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Q23 Does the spectrum of hydrogen consist of randomly spaced
wavelengths or is there a pattern to the spacing?
There is unique pattern corresponding to the allowed energy
levels. This allows the observation of hydrogen in deep space
and the exansion of space by the shift to the red in the pattern
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Ch 18 E 6
How many electrons are required to produce 1
microcoulomb of negative charge?
e = 1.6 x 10-19C
1 microcoulomb = 10-6C = ne
n = 10-6/(1.6 x 10-19) = 6.25 x 1012 electrons
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Ch 18 E 10
Suppose a photon has wavelength λ = 520 nm
a) What is frequency of photon?
b) What is photon’s energy in Joules?
a) c = fλ , f = c/λ = (3 x 108)/(520 x 10-19) = 5.769 x 1014Hz
b) E = hf = (6.626 x 10-34)(5.769 x 1014) = 3.823 x 10-19J
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Ch 18 E 12
An electron in the hydrogen atom jumps from an orbit in
which the energy is 1.89 eV higher than the energy of
the final lower-energy orbit.
a) What is the frequency of photon emitted in this
transition? (h = 4.14 x 10-15eVs)
b) What is the wavelength of emitted photon?
}
a)
∆E = 1.89 eV
∆E = hf , f = ∆E/h = 1.89/(4.14 x 10-15) = 4.565 x 1014 Hz
b) c = λf , λ = c/f = (3 x 108)/(4.565 x 1014) = 6.571nm
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Ch 18 CP 4
An electron (m = 9.1 x 10-31kg) moves with velocity
v = 1500m/s
a) What is the electron’s momentum?
b) What is the electron’s deBroglie wavelength?
c) How does this wavelength compare to visible light?
a) P = mv = (9.1 x 10-31)(1500) = 1.365 x 10-27 kg.m/s
b) λ = h/P = (6.626 x 10-34)/(1.365 x 10-27) = 485.4 nm
c) Blue light has λ = 440-490 nm. Electron has the same
wavelength as blue light.
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Questions Chapter 19
Q1 In 1919, Rutherford bombarded a sample of nitrogen gas with
a beam of alpha particles.
A. In addition to alpha particles, what other particle emerged from
the nitrogen gas in this experiment?
B. What conclusion did Rutherford draw from this experiment?
Explain.
Q3 Is it possible for two atoms of the same chemical element to
have different masses?
Yes by adding or subtracting neutrons e.g. C12 and C14
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Q4 Is it possible for atoms of the same chemical element to have
different chemical properties?
No the chemical properties are determined by the electrons
Q5 Which number, the mass number or the atomic number,
determines the chemical properties of an element?
The atomic number gives the number of electrons
Q7 In a nuclear reaction, can the total mass of the products of
the reaction be less than the total mass of the reactants?
Yes. E = mc2 and mass can be turned into energy
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Q13 In a time equal to two half-lives of a radioactive isotope, would
you expect all of that isotope to have decayed?
NO. In one half life ½ decay so in two half lives ¼ would be left
Q15 Chemical reactions and nuclear reactions can both release
energy. On the average, would you expect the energy released per
unit of mass in a chemical reaction to be greater than, equal to, or
less than what is released in nuclear reaction?
Chemical reactions involve changes in the electron energy
levels and these are very low energy compared to nuclear
reactions
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Q17 Suppose that you light a match to a mixture of oxygen and
hydrogen, which then reacts explosively to form water. Is this a
chemical reaction or a nuclear reaction?
It is a chemical reaction in which two hydrogen atoms and one oxygen
atom join.
Q18 The most common isotope of uranium is uranium-238. Is this
the isotope that is most likely to undergo fission?
NO U235
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Q19 What property of the fission reaction leads to the possibility of
a chain reaction?
The emittance of more than one neutron.
Q21 Do the control rods in a nuclear reactor absorb or emit
neutrons?
They absorb neutrons
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Q22 If you wanted to slow down the chain reaction in a nuclear
reactor, would you remove or insert the control rods?
Insert
Q28 How does nuclear fusion differ from nuclear fission?
Fission is breaking a very heavy element into two lighter
elements
Fusion is joining two very light elements into on heavier element
Both produce energy. Fusion produces the most and is cleaner
but much more difficult
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Q29 Is nuclear fission the main process involved in the energy
generated in the sun?
No, the process is fusion.
Q31 Which can produce larger yields of energy, a fission weapon
or a fusion weapon?
A fusion weapon. The original bomb was a fission bomb. The
hydrogen bomb uses a fission bomb to trigger a fusion bomb.
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Ch 19 E 2
94Pu239 is an isotope of Plutonium produced in
nuclear reactors.
a) How many protons in the nucleus of the isotope?
b) How many neutrons in the nucleus of the isotope?
a) Atomic number = # of protons = 94
b) Mass number = # of protons + # of neutrons = 239
# of neutrons = mass number - # of protons = 239-94 = 145
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Ch 19 E 6
The fission fragment of iodine – 131 undergoes
negative Beta Decay. Complete the reaction equation
and identity the daughter nucleus.
unknown
131 → Xm + e° + --°
53I
a
-1
o
Conserve mass number: 131 = m + 0 + 0, m=131
Conserve atomic number: 53 = 9 + -1 + 0, a = 54
131
54X
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= 54Xe131
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Ch 19 E 10
How many half lives must go by for radioactivity of a
give isotope to drop to:
a) one-sixteenth its original value?
b) one-sixty fourth its original value?
a) In one half life, radioactivity drops by ½.
1/16 = 1/2·1/2·1/2·1/2 → four half lives
b) 1/64 = 1/2·1/2·1/2·1/2·1/2·1/2 → six half lives
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Ch 19 E 12
Suppose two deuterium nuclei (1H2 fuse in a reaction
that emits a neutron. Complete the reaction equation
and identify the resulting nucleus.
1H
2
+ 1H2 → aXm + on1
Conserve mass number 2 + 2 = m + 1, m = 3
Conserve atomic number 1 + 1 = a + 0, a = 2
3
2X
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= 2He3
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Ch 19 CP 4
Nuclear power has been a source of controversy over
the last few decades. Although the use of nuclear
power has grown during this time, we still get more
than half our power by burning fossil fuels. Discuss
the different environmental & economic impacts of
these power sources:
a) Burning fossil fuels produces CO2 as a natural byproduct, a gas that
contributes to the greenhouse effect and to global warming. Is this a
problem with nuclear power?
b) What environmental problems are associated with nuclear power that
are not present in the burning of fossil fuels?
c) What environmental problems are associated with burning fossil
fuels that are not present with nuclear power?
d) If alternatives were not available, which power source would you
choose to develop further?
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CH 19 CP 4 cont.
a) No. CO2 is not created in the nuclear reaction associated with the
fission of Uranium.
b) Disposal of nuclear wastes. Reactor safety. Depleted uranium fuel
rods must be buried as they are still radioactive. Complex processes
associated with nuclear reaction and reactor stability lend nuclear
reactors to events like that at Chernobyl!
c) Availability of fossil fuels. Green house gasses and other pollutants.
d) Nuclear power.
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