Thermodynamics

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Thermodynamics

• The study of thermal properties of matter. The
  science of Heat.

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Temperature
The quantity indicating how warm or cold an
object is relative to some standard is
TEMPERATURE (T). T does not depend on quantity of
a substance. A cup and a thimble of boiling
water both have same T. When two objects have the
same temperature, they are in thermal equilibrium.
Heat is the flow of energy due to a temperature
difference. Heat always flows from objects at
high temperature to objects at low temperature.
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Temperature Scales
() Values given at 1 atmosphere of pressure.
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Temperature Scales
() Values given at 1 atmosphere of pressure.
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Temperature Scales
() Values given at 1 atmosphere of pressure.
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The temperature scales are related by
Fahrenheit/Celsius
Absolute/Celsius
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Example (a) At what temperature (if any) does
the numerical value of Celsius degrees equal the
numerical value of Fahrenheit degrees?
(b) At what temperature (if any) does the
numerical value of kelvins equal the numerical
value of Fahrenheit degrees?
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Question

• Which is smaller, a change of 1oF or 1oC?
• A) 1oF
• B) 1oC
• C) they are the same

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Absolute Temperature

• There is a coldest possible temperature Absolute
  zero.
• All objects will transfer heat to an object at
  absolute 0.
• Experiment show (e.g. V?0 ) that the coldest
  possible T is -273.15oC.
• Kelvin scale measures T from Absolute 0 in units
  of 1oC
• TKTc273o

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• Thermal Energy is the total kinetic energy of the
  moving molecules in a gas

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Temperature, for a gas, is related to average KE

• k is a constant of physics, the Boltzman constant

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The Ideal Gas Temperature and Heat
kB 1.4 x 10-23 J/K T is absolute
temperature measured in K
Slide 11-18
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Thermal Energy
Heating changes thermal energy The molecules
have both internal KE and PE
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Question

• If the temperature of a quantity of an ideal gas
  is raised from 20o C to 40o C, its thermal energy
  is________
• A) doubled
• B) tripled
• C) unchanged
• D) none of the above

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Heat
Touch the back of your chair. Energy flows out
of your hand into the chair. If you touched a hot
stove energy would flow into your hand. Thermal
Energy always flows from a warmer object into a
(touching) cooler one. Heat is thermal Energy in
transit T tells us how hot. (measured in
degrees) Heat is energy (measured in Joules or
calories)
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Mechanical equivalent of heat We can increase
the temperature of an object Either by applying
heat or Doing work on the system
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Joule showed the quantity of work done on a
system or the same quantity of heat flowing into
a system causes the same increase in the systems
thermal energy.
Heat is measured in joules or calories. 1 cal
4.186 J (the mechanical equivalent of heat) 1
Calorie (used on food packaging) 1 kcal.
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• Two containers of the same gas (ideal) have
  these masses and temperatures
• Which gas has atoms with the largest average
  
  thermal energy?
• Which container of gas has the largest thermal
  energy?
• P, Q
• P, P
• Q, P
• Q, Q

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Answer

• Two containers of the same gas (ideal) have
  these masses and temperatures
• Which gas has atoms with the largest average
  
  thermal energy?
• Which container of gas has the largest thermal
  energy?
• P, Q
• P, P
• Q, P
• Q, Q

Slide 11-22
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Question

• Which has more thermal energy, the water in a
  large partially frozen lake or boiling water in a
  coffee mug?
• The lake
• The mug
• Depends on the fraction of the lake that is
  frozen.

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Question

• Which is hotter, the water in a large partially
  frozen lake or boiling water in a coffee mug?
• The lake
• The mug
• Depends on the fraction of the lake that is
  frozen.

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Question

• If you throw the mug into the lake heat will flow
  from
• The lake into the mug
• The mug into the lake
• There is no heat flow .

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Conservation of Energy

• K U Echem Eth E is constant
• We can change the Energy of an isolated system by
  doing work on (or by) it or causing Heat flow
• ?K ? U ? Echem ? Eth ? E W Q
• W,Q can be or
• Many times we are only concerned with Eth

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The First Law of Thermodynamics
The first law of thermodynamics says the change
in thermal energy of a system is equal to the
heat flow into the system plus the work done on
the system (conservation of energy).
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Internal energy U is what we call thermal energy
Eth
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• When the temperature of an ideal gas is
  increased, which of the following also increases?
  (1) The thermal energy of the gas (2) the
  average kinetic energy of the gas (3) the
  average potential energy of the gas (4) the mass
  of the gas atoms (5) the number of gas atoms.
• 1, 2, and 3
• 1 and 2
• 4 and 5
• 2 and 3
• All of 15

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Answer

• When the temperature of an ideal gas is
  increased, which of the following also increases?
  (1) The thermal energy of the gas (2) the
  average kinetic energy of the gas (3) the
  average potential energy of the gas (4) the mass
  of the gas atoms (5) the number of gas atoms.
• 1, 2, and 3
• 1 and 2
• 4 and 5
• 2 and 3
• All of 15

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Using a fan to move air in a room will make you
feel cooler, but it will actually warm up the
room air. A small desk fan uses 50 W of
electricity all of this energy ends up as
thermal energy in the air of the room in which it
operates. The air in a typical bedroom consists
of about 8.0 x 1026 atoms. Suppose a small fan is
running, using 50 W. And suppose that there is no
other transfer of energy, as work or heat, into
or out of, the air in the room. By how much does
the temperature of the room increase during 10
minutes of running the fan?
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Operation of a Heat Engine
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Practical Heat Engines

• Gasoline engine (car) Heat generated in
  cylinder(high T) moves car, exhaust (hot)
• Electrical Turbine Heat steam-gt turns
  turbine-gtgenerates Electricity, exhaust (hot)
• Steam engine etc.

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Can we convert all Eth to W?
N
NO!
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QC can never be 0
QC 0
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T is Absolute Temperature (K)
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• An engine operates at maximum theoretical
  efficiency of 60. The hot reservoir has
  T427oC. TC ?

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Strange Things happening

• We must always waste some Heat, QC never 0
• Efficiency, e is always less than 1
• Heat never spontaneously flows from cold to hot
• Glass never un-breaks
• None of these violate the First Law (conservation
  of energy)
• Something else is going on

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Entropy
Higher entropy states are more likely. Systems
naturally evolve to states of higher entropy.
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Entropy
Equilibrium state is the most likely
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Second Law of Thermodynamics
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