Atoms, Solids, Liquids

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Atoms, Solids, Liquids

• Physics 1010
• Dr. Don Franceschetti
• March 16, 2005

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Atomic Hypothesis

• Philosophical Speculation, to 1800
• c. 1800 John Dalton revives atomic hypothesis
• 1827 Robert Brown discovers Brownian Motion
• 1905 Einstein explains Brownian motion as
  resulting from molecular collisions

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The Elements

• About 88 naturally occurring elements
• About 120 total known.
• Atoms are described by an atomic number (Z)
• which is the number of protons in their nucleus.
• 90 of the atoms in the universe are hydrogen
  (Z1).
• Sun fuses hydrogen to form helium (Z2)
• Older stars create heavier elements up to iron
  (Z26) by fusion and up to bismuth (Z83) by
  neutron capture. Only supernovas can create
  heavier elements.
• We are made of stardust. (CHON)

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Atoms are

• Incredibly small,
• Numerous, about 100,000,000,000,000,000,000,000
  atoms in a gram of water
• Ageless, nearly as old as the Universe.
• And they get around. Every breath you take
  contains atoms that have been in the lungs of
  every human that ever lived!

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Atoms cannot be seen because

• The wavelength of light is much longer than the
  diameter of an atom.
• But atoms can be seen using scanning tunneling
  microscopy
• And atomic force microscopy

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First Picture Thorium 1979
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Electricity

• Rubbing dissimilar objects results in their
  becoming charged, with a positive or negative
  charge.
• Like charges repel, opposite charges attract.
• Static electricity results from transfer of
  electrons.

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Electrons

• Ben Franklin, believed electrical phenomena were
  due to an excess or a deficiency of electric
  fluid. Electricity can flow through a gas.
• Crooks experimented with cathode rays in
  partially evacuated tubes.
• 1897 J. J. Thomson showed that cathode rays are
  particles electrons. Determines charge to mass
  ratio.
• 1909 Millikan determines electron charge in oil
  drop experiment.

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MIllikan Oil Drop experiment
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The Nucleus

• Discovered by Ernest Rutherford in alpha particle
  scattering experiment.
• Atoms are mostly empty space

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Particles in the Nucleus

• Protons carry single positive charges.
• Atomic number is number of protons in the
  nucleus.
• Atomic number determines chemical behavior
• Neutrons are neutral particles.
• Isotopes differ in number of neutrons. For
  example C-12, C-13

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Shell Model of Atom
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We use chemical symbols to represent atoms

• Usually these make sense
• H is Hydrogen
• O is Oxygen
• S is Sulfur
• C is Carbon
• H2O is a molecule of water
• H2SO4 is a molecule of sulfuric acid
• Atoms combine to form molecules by sharing
  electrons
• Crystals and polymers are in effect giant
  molecules.

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Some symbols are stranger

• Na (natrium) is sodium
• K (kalium) is potassium
• W (wolfram) is Tungsten
• Sb (stibnum) is Antimony
• NaCl represents one formula unit of sodium
  chloride, table salt
• Atoms can also bind by exchanging electrons to
  form charged ions.

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Chemical Reactions

• Atoms are not changed but can change partners
• CH4 2O2 ? CO2 2H2O

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Atoms Cannot Reproduce!

• Biological reproduction is based on copying
  molecular structures.
• Atoms in a newborn infant come from food eaten by
  mother (except a very fewfrom father) .
• Prenatal nutrition is very important. Inadequate
  Ca or Fe will be leached from mothers body.

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Nutritional Issues

• We must take in as food, enough of each type of
  atom needed (C,H,O,N,P,S,Fe,Ca ).
• Many molecules in the body are polymers of
  simpler molecules. Your body can make many of
  the smaller molecules it needs but not all. You
  need some vitamins and some amino acids.

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3 phases (states) of matter

• Gas fills any containerlots of space between
  molecules
• Liquid takes shape of container but doesnt
  expand to fillmolecules bumping into each other
• Solids retain shape--atoms bound to each other

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Crystal structure

• Solids are either crystalline or amorphous
• Crystals have a three-dimensional orderly
  arrangement of atoms
• X-ray diffraction permits determination of
  crystal structure
• Many solids are polycrystalline, example copper
  wire
• Amorphous solids have no long range order

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Bonding in Solids

• Solids can be
• Ionic, electrons transferred between atoms which
  then attract each other e.g. NaCl
• Metallic, electrons shared by all atoms, e.g. Au
• Covalent, chemical bonds between neighboring
  atoms, e. g. diamond
• Hydrogen bonded, e.g. ice
• van der Waals bonded, e.g. dry ice

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Density

• Density mass/volume and is characteristic of
  the substance.
• Gold 19,320 kg/m3
• Mercury 13,600 kg/m3
• Water 1,000 kg/m3
• Gases, roughly 1 kg/m3 so 99.9 of a gas is empty
  space.

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Another Metric Moment

• The liter is a secondary unit of volume.
• Originally exactly 1 dm3
• Now the volume occupied by 1 kg of pure water.

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Elasticity

• Hookes Law Force proportional to stretch Fkx
• Longitudinal Stress
• Tension (tensile strength)
• Compression
• Shear Stress

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Solids

• Archescompression strengthens structure
• Pre-Stressed Concreteditto
• Scaling
• Mass proportional to length cubed
• Surface proportional to length squared

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Liquids

• Liquids take the shape of whatever container they
  are poured into.
• Pressure concept becomes very useful
• PressureForce/Area
• In liquid pressure weight density x depth
• Pressure is perpendicular to any surface

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Buoyancy

• Pressure on bottom of submerged object greater
  than on top
• Archimedes Principle An immersed body is buoyed
  up by a force equal to the weight of the fluid it
  displaces.
• Principle of Flotation A floating object
  displaces a weight of fluid equal to its own
  weight.

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Will it Sink?

• If it is less dense than the fluid it will
  displace its weight and some will be left over to
  float on the surface.
• If it is more dense than the fluid it will sink
  but appear to weigh less.

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Specific Gravity

• S.G. density/density of water
• Human body S. G. between 0.9 and 1.05
• Iceberg S.G. about .9
• Submarines take in/pump out water.
• Land masses float on Earths liquid mantle.Plate
  tectonics

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Pascals Principle

• A change in pressure at any point in an enclosed
  fluid at rest is transmitted undiminished to all
  points in the fluid
• Basis of hydraulic lift

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Surface tension

• Molecules at surface bond more strongly to each
  other than in bulk
• Extra force is needed to lift a ring through a
  liquid surface
• Insects can walk on water.
• Pull brush from liquid. Bristles pulled together
  by surface tension

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Capillarity

• Force of adhesion to glass may be stronger than
  cohesion

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Review

• Q 12.1 What is an ion? How can an atom become
  an ion?
• Q 12.2 How does the arrangement of atoms in a
  crystalline substance differ from that in a
  non-crystalline substance?
• Q 12.3 What evidence can you cite for the
  microscopic crystal nature of certain solids? For
  macroscopic crystal nature?

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Review

• Q12.7 Which has the greater density, a heavy bar
  of pure gold or a pure gold ring?
• Tell Archimedes story
• Q12.10 What is Hookes law?
• Stress is proportional to strain
• Ut tensio sic vis
• ceiiinosssttuv

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Review

• Q12.13 If the weight of a 1-kg body stretches a
  spring by 2 cm, how much will the spring be
  stretched when it supports a 3-kg load?
• Q12.24 If the linear dimensions of an object are
  doubled, by how much does the surface area
  increase? By how much does the volume increase?

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Review

• Q13.2 Distinguish between force and pressure.
• Q13.3 What is the relationship between liquid
  pressure and the depth of a liquid. Between
  liquid pressure and density?
• Q13.5 How does water pressure one meter below the
  surface of a small pond compare to water pressure
  one meter below the surface of a huge lake?

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Review

• Q13.7 Why does buoyant force act upward on an
  object submerged in water.
• Q13.12 What is the mass of 1 L of water? What is
  its weight in newtons?
• Q13.18 How is the density of a fish controlled?
  How is the density of a submarine controlled?

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For Extra Credit

• Seeing Stars
• Dr. Keith Strong
• Lockheed Martin
• Palo Alto, CA
• Thursday March 18, 400pm MN 201
• Cookies, coffee, etc, 330 MN 222