Second Exam Review

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Second Exam Review

• Physics 1010
• November 1, 2007

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Quiz 7 answers

• 1. The molecules at in a room temperature glass
  of water jostle around at at great variety of
  speeds.
• This is generally true in liquids and gases and
  explains why evaporation is a cooling process and
  why only some molecules escape from the Earths
  atmosphere into space.

3
Quiz 7 answers

• To say that evaporation is a cooling process is
  to say that the more energetic particles escape.
• See comment on last question.

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Quiz 7 answers

• 3. We feel uncomfortably warm on a muggy day
  because water molecules are
• B. Condensing on our bodies. They deposit energy
  which helps raise our skin temperature.

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Quiz 7 answers

• When a gas is changed to a liquid state, the gas
• Releases energy (the latent heat). Think about
  steam burns.
• The lowest temperature possible in nature is
• -273 degrees Celsius. Actually -273.15 degreed
  if you want to be picky. (Dont)

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Quiz 7 answers

• To wholly convert a given amount of heat energy
  into mechanical energy is
• Impossible. If you could, you would reduce the
  entropy of the universe and this would violate
  the second law of thermodynamics. Also this
  would solve the problem of global warming

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Quiz 7 answers

• Systems that are left alone, tend to move to a
  state of
• More entropy. When they reach their maximum
  entropy they stop moving and are in thermal
  equilibrium.

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Quiz 7 answers

• Blow on your hand with your open mouth and you
  feel the warmth of your breath, Pucker your lips
  so your breath expands as you blow and your
  breath is
• Cooler. The expansion is adiabatic (no time for
  heat to come in, and the gas cools.

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Quiz 7 answers

• When a system does no work and heat is added to
  the system, its temperature
• Rises. Think of heating something on a stove.
• When a volume of air is conpressed and no heat
  leaves, the air temperature will
• Increase.

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Second Test will cover

• Gravity
• Projectile and Satellite Motion
• Atoms
• Solids
• Liquids
• Gases
• Temperature
• Heat Transfer
• Change of Phase
• Thermodynamics
• Thats a lot of stuff, so I can only test on the
  important ideas.

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Newton discovered

• that gravity is universal.
• F G m1m2 / d2
• and so there is nothing special about the center
  of the Earth.
• Understand the effect of changing the mass or
  distance.
• For spherical bodies d is the distance between
  centers.

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By the way,

• The gravity force is always attractive.

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With Newtons universal gravitation, we can

• Determine the mass of the earth, the moon, the
  sun, the other planets, etc.
• Predict the location of undiscovered planets.
• Understand how to aim spacecraft.
• Predict the acceleration of gravity on other
  planets. g GM/d2

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Weightlessness

• Is really apparent weightlessness or free fall.
• What we measure as weight is really the support
  force.
• Satellites fall around the earth.

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In each second, the distance a freely falling
object will fall is

• A) about 5 m.
• B) about 10 m.
• C) the same, but not 5 m or 7 m.
• D) increasing
• E) none of these

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When a rock thrown upward gets to the exact top
of its path, its

• A) velocity is zero and its acceleration is zero.
• B) velocity is zero and its acceleration is about
  10 meters per second per second.
• C) velocity is about 10 m/s and its acceleration
  is zero.
• D) velocity is about 10 m/s and its acceleration
  is about 10 meters per second per second.
• E). None of these.
• -again, you must distinguish between acceleration
  and velocity.

17
A bullet is fired straight down from the top of a
high cliff. Neglecting air resistance, the
acceleration of the bullet in meters per second
per second is

• A) less than 9.8.
• B) is 9.8
• C) is more than 9.8
• D) depends on the height of the cliff.

18
Compared to a 1-kg. block of solid iron a 2-kg
block of solid iron has twice as much

• A) inertia
• B) mass
• C) volume
• D) all of these
• E) none of these

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Compared to the mass of a certain object on
Earth, the mass of the same object on the moon is

• A) one sixth as much.
• B) the same.
• C) six times as much
• D) zero
• Mass is the amount of stuff and does not depend
  on location.

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The newton is a unit of

• A) force
• B) mass
• C) density
• D) inertia

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An objects weight may properly be expressed in
units of

• A) meters
• B) kilograms
• C) newtons
• D) cubic centimeters
• It is a force, after all.

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Einsteins theory of gravitation

• Gravitational field equivalent to constant
  acceleration.
• Mass causes space to curve.
• Black holes are objects so massive that not even
  light can escape.
• Apparently the fate of the most massive stars.
• Known by their effect on matter as it falls in.
• Probably one at center of each galaxy including
  ours!

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Projectile and Satellite Motion

• Projectiles and satellites are objects in free
  fall that have a sideways component of motion.
• Projectiles eventually impact earth.
• Satellites fall around earth.

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Projectiles

• Motion is straight line with downward
  acceleration of 10 m. per sec. each sec.
• Path is a parabola.
• Max range for 45 degree launch.
• With horizontal motion at 8000 m/s orbit becomes
  circle around the earth.
• With faster horizontal motion orbit becomes
  ellipse.

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Kepler

• Used Tycho Brahes data on mars to deduce three
  laws of planetary motion.
• Newtons laws including gravity explain Keplers
  laws.
• The end of circular motion as special.

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Escape speed

• For a planet is determined from mass and radius.
• For Earth its 11.2 km/s.
• But it takes 42.2 km/s to escape from Sun on
  launch from Earth.
• Pioneer 10 launched in 1972, passes orbit of
  Pluto in 1984.

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Atoms

• Ordinary matter is composed of atoms, usually
  combined to form molecules.
• Einstein explained Brownian motion as resulting
  from the collisions of a microscopic particle
  with surrounding molecules. This was considered
  the final proof that atoms exist.

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Atoms are

• Very small--about 100 sextillion in a gram of
  water.
• Generally unchanging. We are constantly
  exchanging atoms with our environment.
• Made in starsWe are made of star stuff.
• Every breath we take contains atoms that have
  been in the lungs of every human that ever lived.

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We cant see atoms but

• We can form images of them by several techniques.
  
• We can talk about their mass. Mass of carbon 12
  atom defined to be 12 atomic mass units.

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Atomic structure

• Inside, a nucleus composed of protons (positive)
  and neutrons (negative) held together by strong
  nuclear force against proton-proton repulsion.
• Orbital electrons move around nucleus, arranged
  in shells or energy levels.
• Sharing or exchange of electrons results in
  chemical bonds.

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Elements and Isotopes

• All atoms of the same element have the same
  number of protons. (the atomic number)
• Atoms of the same element that have different
  numbers of neutrons are called isotopes of the
  element

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Compounds and mixtures

• A pure chemical substance that consists of more
  than one kind of atom is called a compound. It
  is described by a chemical formula.
• A mixture contains more than one compound.

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Solids

• Can be crystalline or amorphous. Crystal
  structures can be determined by the scattering of
  x-rays.
• Much of what we know about molecular structure
  comes from x-ray scattering by crystals made up
  of the molecules of interest.

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Density

• Density mass / volume.
• Density is characteristic of each substance (at
  fixed temperature and pressure).
• Archimedes story.

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Tension and Compression

• Stone and concrete are much stronger in tension
  than compression.
• Arch allows weight of structure to be supported
  by compressed materials. A revolution in
  architecture.

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Buoyancy

• A fluid exerts a force on a submerged object
  equal to the weight of the fluid displaced
  (because it occupies the same space).
• Object of density less than the fluid will float.
• So weight of air displaced by a hovering one ton
  blimp is one ton.

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Gases

• Obey Boyles Law
• PV const
• Molecules have a distribution of speeds
  determined by their mass and the temperature.

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Temperature and Heat

• Temperature is measured in degrees by a
  thermometer.
• Most temperature measurements are based on the
  expansion of a liquid as its temperature
  increases.

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Water

• Is an exception to the rule that substances
  expand as the temperature goes up.
• From 4 degrees Celsius up. Water expands with
  increasing temperature.
• From 4 degrees Celsius down water also expands.
• This means that a pond must be at 4 degrees
  Celsius before water at the surface can freeze.
  Since ice floats and is a good insulator, ponds
  freeze from the top down, protecting the life
  under the surface.

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Heat Transfer occurs by

• Conduction - transfer of kinetic energy from
  molecule to molecule
• Convection motion of fluids as density
  differences develop
• Radiation emitting electromagnetic radiation.
  Only mechanism that works in a vacuum.

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Phase changes

• Change from solid to liquid is melting. Requires
  heat input without temperature change.
• Change from liquid to solid is freezing, requires
  hear withdrawal.
• Change from liquid to gas is evaporation.
  Cooling occurs as faster moving molecules leave.
  
• Change from gas to liquid is condensation and
  releases heat. Steam burns.

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Heat Capacity and Thermal Conductivity

• Heat capacity tells you by how much the internal
  energy increases ad the temperature rises. Water
  has a high heat capacity. Sand and air have a
  lower heat capacity.
• Thermal conductivity tells you how fast heat
  energy travels through a substance. Air is a
  poor thermal conductor, so you can stick your
  hand in a hot oven without an immediate burn.
  Ice is a poor thermal conductor, so you loose
  heat slowly in an igloo or ice hotel.

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First Law of Thermodynamics

• The internal energy of a system in increased by
  heat flowing in or energy flowing out as the
  system does work.
• 1 calorie is 4.184 joules

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Second Law

• The entropy of the universe always increases.
• Because of this, you cant convert a given amount
  of heat energy entirely into work, and you cant
  transfer heat to a hotter body from a cooler body
  without putting in come work.
• Entropy is actually an amazing concept. We are
  just scratching its surface.