Download Physics 102, Class 11 “The Atomic Nature of Matter” Physics 102

Physics 102, Class 11
““The
The Atomic Nature of Matter
”
Matter”
10/3/2005
Announcements
• Good News
– No Class this Wednesday!
– Homework due on Monday
• Bad News
– Reading Assignment: Chapters 12, 13, 14
(Solids, Liquids, Gases) by next Monday
Empty Space
• There’s a whole lot of nothing going on!
– In the solar system, from 109 m to 1016 m
– In atoms, from 10-14 m to 10-10 m
Early Thinking About Atoms
• Parmenides of Elea (~500
BCE): How can change ever
occur?
– Means that something in a
state of “not being” has to
convert to “being”--How
could that make sense?
– we have to distrust our senses
and rely solely on our intellect
www.livius.org/gi-gr/greeks/philosophers.html
Early Thinking About Atoms
• Democritus of Abdera (~400500 BCE)
– Change occurs through
movement and reorganization:
everything is made of tiny little
particles called “atoms”, which
are constantly moving and
forming various temporary
clusters
– you can use your senses, but
you should be careful!
www.livius.org/gi-gr/greeks/philosophers.html
Aristotle Accidentally Kills Off Science for
2000 Years
—Oops!
Years—Oops!
• Aristotle of Stagira (384-322 BCE)
– 4 elements, no atoms
Air
Fire
Earth
Water
www.livius.org/gi-gr/greeks/philosophers.html
CPS Question
• Atoms were proposed by Democritus to
explain
– A: how fine dust could get
– B: chemical reactions
– C: how change could occur
CPS Question
• Atoms were proposed by Democritus to
explain
– A: how fine dust could get
– B: chemical reactions
– C: how change could occur
see the preceding
slides
Modern Revival of Atomic Hypothesis
• Galileo, Boyle, Descartes, Newton…then
• John Dalton
– “law of simple and multiple proportions”: elements
combine by mass in ratios of small whole numbers
– simplest explanation: atoms! (but not conclusive)
John Dalton (1766 - 1844), English
chemist and schoolteacher
http://www.bbc.co.uk/radio4/science/media/Dalton.portrait1.jpg
Modern Revival of Atomic Hypothesis
• “Brownian Motion” (1828)
– when mixed with water and viewed under a
microscope, tiny particles jiggle continuously
– he initially thought they were alive, but then
also saw it with dust and soot particles
• Puzzled scientists for decades
Robert Brown (1773-1858)
Scottish botanist
“Jupiter Botanicus”: Brown was the botanist on
Matthew Flinders's voyage of discovery to Australia
Modern Revival of Atomic Hypothesis
• Einstein’s PhD thesis, 1905: theory for
Brownian motion
– smaller particles, too small to see, (atoms or
molecules) are randomly impacting the larger,
visible particles
• http://groups.physics.umn.edu/demo/thermo/4D1020.html
• http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/brownian/brownian.html
• allowed calculation of the dimensions of the
particles (the water molecules)
Einstein in 1905, 26
years old
Modern Revival of Atomic Theory
• Kinetic Theory of Gases
– similar to the picture used for Brownian motion
• If You Try to Calculate
– diffusion
– viscosity
– heat conduction
• You Can Determine
– size of particles
– velocity of particles
– distance between particles
James Clerk Maxwell (1831-1879), Scottish
mathematical physicist. Image from
http://www.jcmax.com/maxwell2.html
• Additional Evidence for Atomic Theory
Where all the Variety Comes From
• Okay, So Everything Is Made
of Atoms
Where all the Variety Comes From
• Okay, So Everything Is Made
of Atoms
• So is there a Beer atom?
Where all the Variety Comes From
• Okay, So Everything Is Made
of Atoms
• So is there a Beer atom?
• Are the bubbles in beer caused
by fission of beer atoms?
1988
Where all the Variety Comes From
Physics
electrons,
protons,
neutrons
combine
into
92
different
kinds of
naturallyoccurring
atoms
(elements)
Chemistry
combine
into
countless
combinations
of atoms
(molecules)
Materials
Science
combine
into
countless compounds and mixtures and
solutions and structures of molecules and
atoms (all matter, including beer)
Facts About Atoms
• They’re really, really little
– ~10-10 m diameter
– an atom is to an apple as an apple is to the Earth
• The non-radioactive ones last forever
– or practically forever, anyway
– you are made of recycled atoms (ick)
• They’re always moving, or at least jiggling
around
Facts About Atoms
• There are more of them than you can think
about
• in the water:
– about 1023 molecules of water per cm3
– about 1.37x109 km3=1.37x1018 m3=1.37x1024
cm3 of water in all the oceans in the world
• (http://home.comcast.net/~igpl/Oceans.html )
Facts About Atoms
• There are more of them than you can think
about
• in the water:
– about 1023 molecules of water per cm3
– about 1.37x109 km3=1.37x1018 m3=1.37x1024
cm3 of water in all the oceans in the world
• (http://home.comcast.net/~igpl/Oceans.html )
• in the air:
– about 1022 molecules per liter (breath) of air
– about 1022 liters of air in the atmosphere
– 15 breaths per minute is normal, 500 million
breaths in a 60-year life
Facts About Atoms
• There are more of them than you can think about
• in the water:
– about 1023 molecules of water per cm3
– about 1.37x109 km3=1.37x1018 m3=1.37x1024 cm3 of
water in all the oceans in the world
• (http://home.comcast.net/~igpl/Oceans.html )
• in the air:
– about 1022 molecules per liter (breath) of air
– about 1022 liters of air in the atmosphere
– 15 breaths per minute is normal, 500 million breaths in
a 60-year life
– so use mouthwash!
Where do Atoms Come From?
• Except for hydrogen and some helium, All
the atoms in the Earth (and in You!) are
From Exploded Stars
During
Before
www.ast.cam.ac.uk/AAO/images/captions/aat050.html
After
In 1987 supernova “SN1987A” was
observed in a satellite galaxy of the Milky
Way called the Large Magellanic Cloud.
Elements heavier than iron can only be
produced in supernovas.
http://csep10.phys.utk.edu/guidry/violence/sn87a.html
CPS Question
• By studying rocks from the Earth, the
Moon, and from Meteorites, we know that
the Solar System is 4.54 billion years old.
Other than hydrogen and helium, how old
are the atoms in your body?
–
–
–
–
A: less than 4.54 billion years old
B: 4.54 billion years old
C: at least 4.54 billion years old
D: Hey, I was born in 1986, what’s all this
billions of years stuff?
for more on this dating, see http://pubs.usgs.gov/gip/geotime/age.html
CPS Question
This is sort of like asking
• By studying rocks from the
Earth,
the
how old the lumber in your
Moon, and from Meteorites,
we
house
is. know
If your that
house was
the Solar System is 4.54 billion
yearsago,
old.
built 10 years
then the
wood musthow
have been
Other than hydrogen and helium,
old cut
more than 10 years ago, so
are the atoms in your body?
the lumber in your house
– A: less than 4.54 billion years
mustold
be more than 10 years
– B: 4.54 billion years old old.
Similarly,
– C: at least 4.54 billion years
old if the solar system
condensed from a cloud 4.54
– D: Hey, I was born in 1986,
what’s all this
billion years ago, then the
billions of years stuff?
atoms in the cloud must have
been made before then.
for more on this dating, see http://pubs.usgs.gov/gip/geotime/age.html
Facts About Atoms: You Can
’t See Them
Can’t
• They’re there, but they’re smaller than the
lightwaves you use to see!
• What do they “look like”?
– they don’t “look like” anything
– but you can build instruments to measure where
they are, etc.
• Need something “smaller than light” to see
atoms: electrons
– Scanning electron microscope
– Scanning tunneling microscope
Figuring Out Where the Atoms Are
• Scanning Tunneling Microscope
http://physics.nist.gov/Divisions/Div841/Gp3/Projects/STM/t
unnel_prog.html
Double Row of Cs atoms on a
GaAs Plane
http://www.nano.geo.unimuenchen.de/external/research/instruments/STM/STMPrinciple_detail.jpg
Since We
’re on the Topic of Electrons
…
We’re
Electrons…
http://www.orau.org/ptp/collection/xraytubes/maltes2.jpg
• Rays cast a shadow
– travel in nearly straight lines
from the negative electrode
1 2
• Rays fall due to gravity d = gt
2
• Rays act like particles!
J. J. Thomson (1856-1940), English Physicist
http://nobelprize.org/physics/laureates/1906/thomson-lecture.pdf
Corpuscular Negative Electricity
• Negative electricity is carried by “corpuscles”
• “corpuscles” have 1/1700 the mass of a hydrogen atom
• “corpuscle” character (e/m) is independent of the
elements used for the electrodes or the gas!
• are the “corpuscles” a universal component of all atoms?
• “The case is entirely different with positive electricity.”
Thomson
’s Picture of the Atom
Thomson’s
• The “Plum Pudding” Model:
Positive Charge is uniformly
distributed (no “corpuscular”
character)
Negative Charge is in
“corpuscles” (electrons), like
plums in a plum pudding
Objection from Hertz
• Hertz’s objection to “corpuscles”:
– If we place a thin metal foil
between the cathode and the
glass, the glass still glows, albeit
at a reduced luminosity—How
can this be true for particles?
(Hertz thought the cathode rays
must be a wave phenomenon, not
particles)
Heinrich-Rudolf HERTZ (1857-1894,
only 37 years). German Physicist.
An Answer to Hertz
’s Question
Hertz’s
• electron interacting with “plum pudding” atom
– a high-velocity electron is not likely to be completely
stopped passing through a few atoms
• Thomson found they were going 5,000 to 60,000 mph!
– many or most electrons will get through a very thin
foil
http://www.omniinstruments.com/classical.html
CPS Question
• J. J. Thomson:
– A: found that negative electricity was
transported by tiny particles
– B: found the first particles smaller than the
hydrogen atom
– C: found that negative electricity particles were
always exactly the same, regardless of the
materials used to make the electrodes or the gas
in the tube
– D: All of these
– E: None of these
CPS Question
• J. J. Thomson:
– A: found that negative electricity was
transported by tiny particles
– B: found the first particles smaller than the
hydrogen atom
– C: found that negative electricity particles were
always exactly the same, regardless of the
materials used to make the electrodes or the gas
in the tube
review the previous slides, and
– D: All of these
read Chapter 11 in Hewitt.
– E: None of these
The Gold Foil Experiment
• Alpha particles:
– ~7000 times more massive than electrons
– positively charged
• Pass them through gold foil (leaf)
– should go right through, but a small number were
scattered backward!
– What did they hit? The “atomic nucleus”
Ernest Rutherford (1871-1937), New
Zealand physicist, worked in England.
http://nobelprize.org/chemistry/laureates/1908/rutherford-lecture.html
Everything is Made of Atoms,
But Atoms are Mostly Made of Nothing!
electrons are distributed out to
diameter ~1x10-10 m (size of atom)
tiny nucleus has 99.95%
of the mass of the whole
atom, contained in
~1/10,000 of its
diameter
Atoms Repel Each Other at Close Range
-
+
-
-
-
-
+
-
-
-
-
-
-
-
-
-
-
-
• Far Apart, Electrostatic Interactions nearly cancel
-
-
-
-
+
-
-
-
-
-
-
-
+
-
-
-
Note: we are
ignoring all
chemistry and
quantum
mechanics for
now
• Close Together, Repulsive Electrostatic
Interaction between electron clouds dominates
The Nucleus is Very Dense
• Neutron Stars are composed entirely of
nuclear matter
– Mass of the Earth Would be < 220 m radius
• Huge pressures or very high temperatures
are required to push nuclei together so they
can fuse. This process only happens
naturally in stars.
– fission reactors
– particle accelerators
– fusion reactors
www.noaanews.noaa.gov/stories2005/s2372.htm
CPS Question
• Professor Boyd is mostly empty space.
What force keeps him from sinking into the
floor, which is also mostly empty space?
– A: Gravity
– B: Magnetic Force
– C: Electrical Force
CPS Question
• Professor Boyd is mostly empty space.
What force keeps him from sinking into the
floor, which is also mostly empty space?
– A: Gravity
– B: Magnetic Force
– C: Electrical Force
See the figures on the previous
slides and review this topic in
Hewitt Chapter 11.
Schematic Structure of Atoms
-
-
+
+
+
Hydrogen
Helium
+
+ +
-
Lithium
• “Element” is determined by number of protons +
• Number of Electrons - =Number of Protons +
• Number of Neutrons
can vary
– usually appx. equal to or larger than the # of protons
– usually several stable versions or “isotopes”
Elements
• All of the typical properties we associate
with elements, molecules, and materials are
determined by their electronic structure,
which depends on the atoms’ proton count
– chemistry
– metals are ductile and conducting
– rock salt is brittle and insulating
• The Periodic Table Organizes the Elements
– here’s a good online one from Los Alamos Lab:
http://periodic.lanl.gov/
– atomic number
– atomic mass
CPS Question
• Iron has 26 protons. How many electrons
does it have?
– A: less than 26
– B: 26
– C: more than 26
CPS Question
• Iron has 26 protons. How many electrons
does it have?
– A: less than 26
– B: 26
– C: more than 26
Atoms are electrically neutral,
meaning that the number of
positively-charged protons must be
exactly balanced by the number of
negatively-charged electrons.
CPS Question
• Iron has 26 protons. Aluminum has 13
protons. An atom of iron has approximately
– A: half as much mass as an atom of aluminum
– B: the same mass as an atom of aluminum
– C: twice as much mass as an atom of aluminum
CPS Question
• Iron has 26 protons. Aluminum has 13
protons. An atom of iron has approximately
– A: half as much mass as an atom of aluminum
– B: the same mass as an atom of aluminum
– C: twice as much mass as an atom of aluminum
Remember that the mass of the atom is almost all in the protons
and neutrons. Each electron only weighs 1/1700 as much as a
proton, so the mass of the electrons can be neglected here.
Remember also that the number of neutrons is approximately
equal to or a bit larger than the number of protons. So if
aluminum has half as many protons, atoms of aluminum will
weigh about half as much as atoms of iron.
““Fundamental”
Fundamental” is Always a Moving Target
late 1800s
Atoms
early 1900s
Protons, Neutrons, Electrons
mid 1900s
Quarks
late 1900s
Strings, Nature of the Vacuum?
Antimatter
• Anti-protons plus anti-neutrons plus anti-electrons
could in principle make anti-atoms
• The ATRAP Experiment is making antihydrogen
and and measuring its properties
– http://hussle.harvard.edu/~atrap/
– http://www.mpq.mpg.de/~haensch/antihydrogen/introd
uction.html
– it’s not easy: total antimatter made to date would light a
100W bulb for 3 seconds
Antimatter
• The ultimate rocket fuel: 23 milligrams of
antimatter gives a kilo-ton of energy. 100 mg
would put the space shuttle into orbit!
– http://science.nasa.gov/newhome/headlines/prop12apr9
9_1.htm
– http://www.engr.psu.edu/antimatter/introduction.html
– http://livefromcern.web.cern.ch/livefromcern/antimatter
/everyday/AM-everyday05.html
• Does antimatter “fall up”? Theoretically, the
expectation is no. However, a number of
experiments have been performed, and more are
coming.
Dark Matter
• Need 10X as much mass as we can see to
explain galaxy rotation curves
• Strong Candidate: Weakly-Interacting
Massive Particles, or “WIMPs”
• The DRIFT experiment, here at UNM
– Dinesh Loomba, UNM PI
– unusual capability: can look for Dark Matter
“Wind” caused by the Earth’s motion through
the galaxy
Announcements
• Good News: No Class this Wednesday!
• Bad News: Reading Assignment: Chapters
12, 13, 14 (Solids, Liquids, Gases) by next
Monday
• More Bad News: Homework for Monday
will be issued on Wed. at the usual time.