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HEAT_ WORK AND INTERNAL ENERGY

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					 HEAT, WORK AND
INTERNAL ENERGY

         GLOBAL
        WARMING?
  THERMODYNAMICS: the science of energy,
specifically heat and work, and how the transfer of
    energy effects the properties of materials.
Thermodynamics:

"thermo": Greek therme heat
"dynamics": Greek dynamikos
powerful

Physics that deals with the mechanical
action or relations between heat and
work
Example 1: Heat to work

 Heat Q from flame provides energy
 to do work
---------------------------------------------------------
 Example 2: Work to heat.
 Work done by person is converted
  to heat energy via friction.
A “system” is the “collection of objects on which
  attention is being focused”
The “surroundings” are everything else in the
  environment
The system and surroundings must be separated by
  walls which can either insulate or allow heat flow
OPEN SYSTEM: Mass and energy freely moves in
  and out between the system and the surrounding
ISOLATED SYSTEM: No interaction between the
  system and the surrounding
CLOSED SYSTEM: fixed mass
Heat, Q, energy caused by
 temperature difference
Heat
 ... is the amount of internal energy
entering or leaving a system
... occurs by conduction, convection,
or radiation.
... causes a substance's temperature
to change
... is not the same as the internal
energy of a substance
... is positive if thermal energy flows
into the substance
... is negative if thermal energy flows
out of the substance
... is measured in joules
             Thermal Equilibrium
Systems (or objects) are said to be in thermal
   equilibrium if there is no net flow of thermal energy
   from one to the other. A thermometer is in thermal
   equilibrium with the medium whose temperature it
   measures, for example.
If two objects are in thermal equilibrium, they are at
   the same temperature.
Work, W, energy caused by physical
             motion
                           WORK
W is positive if work is   W is negative if work is done on the
done by system.            system.




                           Environment (man) does work on
                           system: W < 0
                           (Alternative: system does negative
                           work because force by air pressure
Air does work on the       on thumb is opposite to the direction
environment: W > 0.        of motion of the thumb.)
RELATIONSHIP BETWEEN HEAT AND WORK
Why does the volume of gas expands when it is heated?

 W=Fxd
 Pressure (P) = (Force) F   or   F=PA
                (Area) A
 Volume (V) = L x W x H or A x d
 d=V
     A
   W=PAV =PV
          A
Internal Energy (U or E)
: (measured in joules)               Vibrational kinetic
 - Sum of random                     energy in solids.
translational, rotational,           The hotter the
and vibrational kinetic              object, the larger
energies                             the vibrational
DU: change in U                      kinetic energy
DU > 0 is a gain of
internal energy
DU < 0 is a loss of
internal energy
----------------------------------
Thermal Energy:
 same as internal                                          Motions of a
energy                                                     diatomic
                                                            molecule in a
                                                           fluid
      INTERNAL ENERGY (U or E)
is the total of the kinetic energy due to the motion of
   molecules (translational, rotational, vibrational) and the
   potential energy associated with the vibrational and
   electric energy of atoms within molecules or crystals.
The First Law of Thermodynamics states that :
    The internal energy of a system changes from
 an initial value Ui to a final value Uf due to heat
 added (Q) and work done by the system (W)


                 DU = Uf – Ui = Q – W

   Q is positive when the system gains heat, and
    negative when the system loses heat.
   W is positive when it is done BY the system, and
    negative when it is done ON the system
Example: 1000 J of thermal energy
flows into a system (Q = 1000
J). At the same time, 400 J of work
is done by the system (W = 400 J).
What is the change in the system's
internal energy U?
----------------------------------------------------------

Solution:
DU = Q - W
   = 1000 J - 400 J
   = 600 J
Example: 800 J of work is done on a
system (W = -800 J) as 500 J of
thermal energy is removed from the
system (Q = -500 J).
What is the change in the system's
internal energy U?
-----------------------------------------------------
Solution:

DU = Q - W
    = -500 J - (-800 J)
    = -500 J + 800 J
    = 300 J
  Work Done by an Expanding Gas




                  Area under pressure-volume
                  curve is the work done
W = PDV            -----------------------------------------
DV = Vf - Vi      Isobaric Process: "same
W = P (Vf - Vi)   pressure"
                   Greek: barys, heavy
Work and the Pressure-Volume Curve
           Work Done = Area Under PV
           curve
           -------------------------------------
           How much work is done by the
           system when the system is
           taken from:
           (a) A to B (900 J)
           (b) B to C (0 J)
           (c) C to A (-1500 J)
           -------------------------------------
           Each "rectangle" has an area of
           100 Pa-m3 = 100 (N/m2)-m3
                          = 100 N-m
                          = 100 Joules
                  Expanding Gas
                       10 grams of steam at 100oC at
                       constant pressure rises to
                       110oC:
                       P = 4 x 105 Pa      DT = 10oC
                       DV = 30.0 x 10-6 m3
                       c = 2.01 J/g oC

Example: If a gas      What is the change in internal
expands at a           energy?
constant pressure,     DU = Q - W
the work done by the
                       W = (4 x 105)(30.0 x 10-6) = 12 J
gas is:
                       Q = mcDT = (10)(2.01)(10) = 201 J
W = PDV                DU = Q - W = 201 J - 12 J = 189 J
Work, Rubber Bands, and Internal Energy
               DU = Q - W

               Expand rubber band:
               W < 0, Q = 0 DU >0
               temperature increases
               -------------------------------------------
               Press thick rubber band to
               forehead and expand
               it rapidly. The warming should
               be obvious.
               Now allow the band to
               contract quickly;
               cooling will also be evident.
ISOTHERMAL-Temperature remains constant
ISOBARIC - Pressure remains constant
 ISOMETRIC - Volume remains constant
(also ISOVOLUMETRIC or ISOCHORIC)




   Since ΔV = 0, W = 0 then DU = Q - W = Q
    Adiabatic Expansion of a Ideal Gas
No heat transfer therefore no temperature change (Q=0).
Generally obtained by surrounding the entire system
with a strongly insulating material or by carrying out the
process so quickly that there is no time for a significant
heat transfer to take place.
                          If Q = 0 then ΔU = - W
                          A system that expands under
                          adiabatic conditions does
                          positive work, so the internal
                          energy decreases.
                          A system that contracts
                          under adiabatic conditions
                          does negative work, so the
                          internal energy increases.
Adiabatic Expansion of a Ideal Gas
                 Both adiabatic expansion and
                 compression of gases occur in
                 only hundredths of a second in
                 the cylinders of a car’s engine.

             Blowing air through wide open mouth
             results to warm air. Blowing through
             small opening results to cooler air due
             to adiabatic expansion.



             Compresses air leaking out through
             a small opening also results in
             adiabatic cooling.
   PROCESS DIAGRAMS: visualize
processes using properties (T, P, V, etc.)



                         Area underneath the
                         slope represents the
                         amount of work
                         done (P x V).
CYCLE: a system undergoes processes -
    returning to its initial state

                           Area
                           underneath
                           the slope
                           represents
                           the amount
                           of work done
                           (P x V).
  Refrigerators work by taking heat from the interior and
   depositing it on the exterior
 The compressor raises the pressure and temperature of
   the refrigerant (freon or ammonia) while the coils
   OUTSIDE the refrigerator allow the now hot
   refrigerant to dissipate the heat
 The warm refrigerant flows through an expansion valve
   from a high-pressure to a low-pressure zone, so it
   expands and evaporates
• The coils INSIDE the
refrigerator allow the cold
refrigerant to absorb heat,
cooling the interior
• The cool refrigerant flows
back to the compressor, and
the cycle repeats
  Second Law of Thermodynamics
Heat flows naturally from a
region at high temperature to
a region at low
temperature. By itself, heat
will not flow from a cold to a
hot body.
When an isolated system
undergoes a change, passing
from one state to another, it
will do so in such a way that
its entropy (disorder) will
increase, or at best remain the
same.
ENTROPY
        Can you beat the Second Law?
   So, can you cool your kitchen by
    leaving the refrigerator door open
   NO!
   The heat removed from the interior
    of the refrigerator is deposited back
    into the kitchen by the coils on the
    back!
   And to make matters worse, the Second Law of
    Thermodynamics says that work is needed to move the
    heat from cold to hot, so the actual amount of heat
    added to the kitchen is MORE than the amount
    removed from the refrigerator
Hopefully, you understand today’s lesson.
 Otherwise, you’ll end up like this cow.

				
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posted:1/20/2013
language:English
pages:35