Chapter 16.1 Announcements
The rest of the semester:
- Thursday, 4-22,
Nuclear Weapons & Reactors
- Tuesday, 4-27,
catch-up, review, some fun, last day of class
- All grades will continue to be posted at: http://www.wfu.edu/~gutholdm/Physics110/phy110.htm
- Listed by last four digits of student ID
Final:
Tuesday,
May 4,
2:00 pm – 5:00 pm
- Reading Assignment: Get ready for final. Bring questions.
- Go through old test and practice test.
- Go through homework and lecture notes.
Practice test will be put on web, Final is CUMULATIVE!!
Chapter 16.1
Nuclear reactions and nuclear bombs
Demos and Objects
- chain reaction
Concepts
- atoms
- atoms consist of a
nucleus and electrons
- chemical reaction
- nuclear reaction
- fission
- fusion
The building blocks of matter
• All matter consists of atoms (greek: atomos = not sliceable)
• All atoms consist of a nucleus surrounded by electrons
• Nuclei consist of protons and neutrons. The sum of neutrons and protons in the nucleus
of a particular element is called the atomic number of the element.
Atomic force microscope
image of gold surface
An atom: Nucleus
surrounded by electrons
Table salt
Solids and molecules:
An arrangement of atoms
Table salt (75x)
10,000x
10,000,000x
Size scales: Powers of ten
1m : (meter)
child
10-1m (decimeter, dm):
hand
10-2m (centimeter, cm): penny
10-3m (millimeter, mm): needle, width
10-4m (100 mm):
hair, width
10-5m (10 mm):
human cell
10-6m (1micrometer, mm): bacterial cell
10-7m (100 nm):
chromosome width (optics limit)
Size scales: Powers of ten, continued
10-8m (10 nm):
simple viruses
10-9m (1 nanometer, nm): DNA (radius)
10-10m (1 Ångstrom 1Å): atom
Limit of electron and other ultrahigh resolution microscopes
10-11m (10 picometer, pm):
inner electrons in atom
10-12m (1 picometer, 1 pm):
nothing
10-13m (100 femtometer):
nothing
10-14m (10 femtometer):
nucleus
10-15m (1 femtometer 1 fm):
proton and neutron
Atoms of a
gold surface
Atomic number = # of protons in nucleus
# of nucleons = # of protons + # of neutrons
THE PERIODIC TABLE
OF ELEMENTS
The number of neutrons can vary slightly for a given element (isotopes)
Atomic weight is equal to average number of nucleons in nucleus
Chemical reactions
CH4 + 2O2  CO2 + 2H2O + some energy
One molecule or element reacts with another one.
Get a rearrangement (different combination) of elements.
No new elements are created (C, H, O before and C, H, O after)
Nuclear reactions
• Very different from chemical reactions.
• In these reactions (fusion and fission) the chemical element
changes to a different element.
• Fusion: The nuclei of lighter elements (like hydrogen or helium)
combine to form a heavier element (e.g.: H + H  He). That’s
what’s going on in the sun.
• Fission: The nuclei of heavy elements split to form new, lighter
elements.
• These reactions emit an enormous amount of energy (~ 10 million
times more than chemical reaction, per kilogram)
• These reactions can be very difficult to start!!
Natural (radioactive) decay (fission)
Neutron-induced fission
• Many heavy elements (eg.
Uranium) decay (slowly) into
lighter elements (natural decay)
• However, this fission can also be
induced by an incoming neutron.
• Fission reaction release a lot of
energy.
• Fission often creates new
neutrons!!
Fission and
chain reaction
Fission releases neutrons …
… these neutrons cause new fission
reactions in surrounding Uranium …
… creating more neutrons …
… chain reaction …. huge explosion
http://lectureonline.cl.msu.edu/~mmp/applist/chain/chain.htm
Conditions for a chain reaction to occur (nuclear bomb):
1. Need a source of neutrons to trigger chain reaction.
2. Bomb material needs to be fissionable (splits when hit
by a neutron.
3. Each fission has to produce more neutrons.
4. Each fission needs to induce more than one subsequent
fission ((super-)critical mass = 60 kg; 7 inch sphere for 235U).
235U
(Uranium isotope with 92 protons and 143 neutrons) works!!
Releases 2.5 neutrons when fissioning.
238U
(Uranium isotope with 92 protons and 146 neutrons), which
naturally occurs more abundantly (99.28%) does not work.
For bomb: Need to enrich 235U (very difficult, fortunately).
Assembly of supercritical mass
(to initiate 235U bomb)
Hiroshima, Aug. 6, 1945
“Little boy”
Two-thirds of Hiroshima was
destroyed. Within three miles of
the explosion, 60,000 of the
90,000 buildings were
demolished. Clay roof tiles had
melted together. Shadows had
imprinted on buildings and other
hard surfaces. Metal and stone
had melted.
Hiroshima's population has been
estimated at 350,000;
approximately 70,000 died
immediately from the explosion
and another 70,000 died from
radiation within five years.
At the time this photo was made, smoke billowed
20,000 feet above Hiroshima while smoke from
the burst of the first atomic bomb had spread
over 10,000 feet on the target at the base of the
rising column.
239Pu
(Plutonium) bomb
Supercritical mass
(density) of
Plutonium core is
achieved when
explosion crushes
239Pu core.
Nagasaki, Aug. 9, 1945
“Fat man”
Approximately 40 percent of
Nagasaki was destroyed.
Though this atomic bomb
was considered much
stronger than the one
exploded over Hiroshima,
the terrain of Nagasaki
prevented the bomb to do as
much damage. Yet the
decimation was still
enormous. With a population
of 270,000, approximately
70,000 people died by the
end of the year.
A dense column of smoke rises more than 60,000 feet into
the air over the Japanese port of Nagasaki, the result of an
atomic bomb, the second ever used in warfare, dropped on
the industrial center August 8, 1945, from a U.S. B-29
Superfortress.
The fusion or Hydrogen bomb
• A fusion (e.g. H + H  He (two
hydrogen atoms make a Helium atom)) is
extremely difficult to start,because it
requires extremely high temperatures
(like inside sun).
• Fusion bombs release even more energy
than fission bombs
• Use a fission bomb to create 100 million
degrees
• This will trigger the fusion bomb
Civil use of nuclear reactions
• Controlled nuclear reactions (add material that slows
down reaction) are used in nuclear reactors to create
energy.
• All reactors thus far are fission reactors. (problem:
radioactive waste)
• Fusion reactions can not be controlled yet (just
bomb).
• Active research is underway to control fusion
reactions (How to initiate reaction, how to hold
material once reaction is going?)