Thermodynamics

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Thermodynamics Chapter 12
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Temperature and Heat

• The temperature of a hot cup of coffee left
  sitting on the table will fall until it reaches
  thermal equilibrium with the air temperature in
  the room.
• When a soda can is taken out of the refrigerator
  and left on the kitchen table, its temperature
  will rise rapidly at first but then more slowly
  until the temperature of the soda equals that
  of the air in the room. At this point, the soda
  and the air temperature in the room are in
  thermal equilibrium.
• The change in temperature is due to the transfer
  of energy between the object and the environment.

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Temperature and Heat

• Thermal energy the total potential and kinetic
  energy associated with the random motion and
  arrangement of the particles of a material.
• Heat, Q, is thermal energy that is absorbed,
  given up, or transferred from one body to
  another.
• Heat is thermal energy in motion.
• Heat is used when the transfer of thermal energy
  from one body to another body at a different
  temperature is involved.

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Temperature

• Temperature is a measure of the average kinetic
  energy of all the particles within an object.

• The temperature of a substance will increase if
  the average kinetic energy of its particles is
  increased.
• If the average kinetic energy of particles
  decreases, so does the temperature of the
  substance.

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Temperature

• Central concept of thermodynamics is temperature.
• Our temperature sense is often unreliable.
• On a cold winter day, an iron railing seems much
  colder to the touch than a wooden fence post,
  even though both are at the same temperature.
• This error in perception results because the iron
  removes energy from our fingers more quickly than
  the wood does.

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• Specific heat Every substance gains or loses
  heat based on its identity. This physical
  property of the substance is called the specific
  heat capacity of the object.
• The specific heat capacity, C, of a solid or
  liquid is defined as the heat required to raise a
  unit mass of the substance by one degree of
  temperature.

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Define Specific Heat

• Amount of energy required to raise the
  temperature of 1kg by 1oC

Heat Energy (mass)x specific x change
heat in temp OR Q m c DT
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Heat Change

• To determine the amount of thermal energy gained
  or lost by a mass
• Heat energy is gained if Q is positive.
• Heat energy is lost if Q is negative.

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• Ex. A 4.0 kg sample of glass heated from 1o C to
  41o C, and was found to have absorbed 32 J of
  energy. What is the specific heat of the glass?
• Q C x m x ?T
• ?T 41oC 1oC 40oC
• 32 J C (4.0 kg)(40oC)
• 32 J C (160 kg oC)
• C 0.2 J/kgoC

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Law of Heat Exchange

• For a closed system in which heat energy cannot
  enter or leave, the heat lost by objects at a
  higher temperature is equal to the heat gained by
  objects at lower temperature until thermal
  equilibrium is reached (at which point the final
  temperature of both objects is the same).
• The final temperature will be somewhere between
  the initial low temperature and the initial high
  temperature.

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Law of Heat Exchange

• Conservation of Energy
• Q lost Q gained
• To avoid problems with signs, for
• Q lost Q gained problems,
• it is best to make ?T Thi Tlo

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Heats of Transformation

• When energy is absorbed as heat by a solid or
  liquid, the temperature of the object does not
  necessarily rise.
• The thermal energy may cause the mass to change
  from one phase, or state, to another.
• The amount of energy per unit mass that must be
  transferred as heat when a mass undergoes a phase
  change is called the heat of transformation, L.

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Phase Changes
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Thermal Expansion of Solids

• Solids expand when heated and contract when
  cooled (with a few exceptions).
• Heated solids increase or decrease in all
  dimensions (length, width, and thickness).
• When a solid is heated, the increase in thermal
  energy increases the average distance between the
  atoms and molecules of the solid and it expands.

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Thermal Expansion of Solids

• Thermal expansion can be explained on a molecular
  basis.
• Picture the interatomic forces in a solid as
  springs, as shown in the picture on the right.
• Each atom vibrates about its equilibrium
  position. When the temperature increases, the
  amplitude and associated energy of the vibration
  also increase.

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Examples of Uses of Thermal Expansion

• Dental materials used for fillings must be
  matched in their thermal expansion properties to
  those of tooth enamel, otherwise consuming hot
  drinks or cold ice cream would be painful.
• In aircraft manufacturing, rivets and other
  fasteners are often cooled using dry ice before
  insertion and then allowed to expand to a tight
  fit.

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• You can loosen a tight metal jar lid by holding
  it under a stream of hot water. Both the metal
  of the lid and the glass of the jar expand as the
  hot water adds energy to their atoms. With the
  added energy, the atoms can move a bit farther
  from each other than usual, against the
  interatomic forces that hold every solid
  together. However, because the atoms in the
  metal move farther apart than those in the glass,
  the lid expands more than the jar and is
  loosened.
• Expansions slots are often placed in bridges to
  accommodate roadway expansion on hot days. This
  prevents buckling of the roadway. Driveways and
  sidewalks have expansion slots for the same
  reason.

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Methods of Heat Energy Transfer

• Conduction is the transfer of heat energy by
• Between particles of objects in direct contact
• Convection is the transfer of heat energy by
• the movement of fluids(gas or liquid)
• convection currents due to hot fluid rising and
  cold fluid sinking
• Radiation is the transfer of heat energy by
• electromagnetic waves
• does not involve the movement of matter

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A Conduction transfers energy as heat along the
wire and into the hand
B. Embers swirl upward in the convection currents
that are created by the warmed air above the fire
which rises
C. Electromagnetic waves emitted by the hot
campfire transfer energy by radiation
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Heat Transfer by Conduction

• Conduction is the transfer of thermal energy
  without any net movement of the material itself.
• When a metal poker is put in a hot fire, the
  exposed end of the poker soon becomes hot as
  well, even though it is not directly in contact
  with the source of heat. We say that heat has
  been conducted from the hot end to the cold end.

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Heat conduction in many materials can be
visualized as the result of molecular collisions.
As one end of the object is heated, the
molecules there move faster and faster. As they
collide with their slower-moving neighbors, they
transfer some of their energy to these molecules
whose speeds thus increase. These in turn
transfer some of their energy by collision with
molecules farther along the

• object. Thus the energy of
• thermal motion is transferred by
• molecular collision along the object.
• Good thermal conductors such as
• silver, copper, aluminum, and gold
• are also good electrical conductors.

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Heat Transfer by Convection

• Convection is the process of heat transfer
  through the mass motion or flow of some fluid,
  such as air or water.
• When a pot of water is heated, convection
  currents are set up as the heated water at the
  bottom of the pot rises because of its reduced
  density and is replaced by cooler water from
  above.

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Heat Transfer by Convection

• Although liquids and gases are generally not very
  good conductors of heat, they can transfer heat
  quite rapidly by convection. Convection is the
  process whereby heat is transferred by the mass
  movement of molecules from one place to another.
  Whereas conduction involves molecules (and/or
  electrons) moving only over small distances and
  colliding, convection involves the movement of
  molecules over large distances.

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Perhaps the first thing that most people say is
"heat rises". While not wrong, what you should
say is "hot air rises" or "hot water rises".
Anything fluid - that is gases or liquids - will
tend to change density with changes in
temperature. For example, if heated, air
decreases in density. The surrounding air is
cooler and denser. This makes it heavier, so it
falls beneath the hot air, forcing it upwards.
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Heat Transfer by Radiation

• Radiation is a more rapid transfer of
• thermal energy in the form of electromagnetic
• radiation accomplished by a process that
• requires neither contact nor mass flow.
• A hot object also loses heat energy by
• radiation. This radiation is similar to light
• and can pass through empty space. The
• warmth you fell when you warm yourself by
• a fire is due to this radiation. If the
• object is hot enough, some of the radiation
• is visible and can indeed be seen.

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What type of heat transfer is shown in the
following pictures?
B.
Radiation
A.
Convection
C.
Conduction
D.
Radiation