heat_work_and_internal_energy.ppt............

Science 9

1. HEAT, WORK AND HEAT, WORK AND
INTERNAL ENERGYINTERNAL ENERGY
GLOBAL GLOBAL
WARMING?WARMING?

2. THERMODYNAMICSTHERMODYNAMICS : : the science of energy, the science of energy,
specifically heat and work, and how the transfer of specifically heat and work, and how the transfer of
energy effects the properties of materials.energy effects the properties of materials.

3.  
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.


                         


                     

4. A “A “systemsystem” is the “collection of objects on which ” is the “collection of objects on which
attention is being focused”attention is being focused”
The “The “surroundingssurroundings” are everything else in the ” are everything else in the
environmentenvironment
The system and surroundings must be separated by The system and surroundings must be separated by
walls which can either insulate or allow heat flowwalls which can either insulate or allow heat flow
OPEN SYSTEMOPEN SYSTEM:  Mass and energy freely moves in :  Mass and energy freely moves in
and out between the system and the surroundingand out between the system and the surrounding
ISOLATED SYSTEMISOLATED SYSTEM:  No interaction between the :  No interaction between the
system and the surroundingsystem and the surrounding
CLOSED SYSTEMCLOSED SYSTEM:  fixed mass:  fixed mass

5. HeatHeat, , QQ, energy caused by , energy caused by
temperature differencetemperature difference

6. 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


                    
      

7. Thermal EquilibriumThermal Equilibrium
Systems (or objects) are said to be in thermal Systems (or objects) are said to be in thermal
equilibrium if there is no equilibrium if there is no net net flow of thermal energy flow of thermal energy
from one to the other.  A thermometer is in thermal from one to the other.  A thermometer is in thermal
equilibrium with the medium whose temperature it equilibrium with the medium whose temperature it
measures, for example. measures, for example.
If two objects are in thermal equilibrium, they are at If two objects are in thermal equilibrium, they are at
the same temperature.the same temperature.

8. Work, W,Work, W, energy caused by physical energy caused by physical
motionmotion

9. WORKWORK
W is positive if work is
done by system.


                           
Air does work on the
environment:
  W > 0.
W is negative if work is done
on
the
system.

                              
 
            

 Environment (man) does work on
system:
 W < 0
 
(Alternative:  system does negative
work because force by air pressure
on thumb is opposite to the direction
of motion of the thumb.)

10. INTERNAL ENERGY (U or E)INTERNAL ENERGY (U or E)
is the total of the kinetic energy due to the motion of is the total of the kinetic energy due to the motion of
molecules (translational, rotational, vibrational) and the molecules (translational, rotational, vibrational) and the
potential energy associated with the vibrational and potential energy associated with the vibrational and
electric energy of atoms within molecules or crystals. electric energy of atoms within molecules or crystals.

12. The The First Law of ThermodynamicsFirst Law of Thermodynamics states that : states that :
The internal energy of a system changes from an The internal energy of a system changes from an
initial value initial value UU
ii to a final value to a final value UU
ff due to heat due to heat
added (added (Q)Q) and work done by the system ( and work done by the system (W) W)
UU = = UU
ff – – UU
ii = = QQ – – WW

QQ is positive is positive when the when the system gains heatsystem gains heat, and , and
negativenegative when the when the system loses heatsystem loses heat..

WW is positive is positive when it is when it is done BY the systemdone BY the system, and , and
negativenegative when it is when it is done ON the systemdone ON the system

13. 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:
U = Q - W
 
    =
1000 J - 400 J
 
    = 600 J


                        

14. 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:
U = Q - W
 
     =
-500 J - (-800 J)
 
     =
-500 J + 800 J
 
     =   300 J


                        

15. Work, Rubber Bands, and Internal EnergyWork, Rubber Bands, and Internal Energy

 
                                  
 
   
U = Q - W
Expand rubber band:
 
W < 0, Q = 0 U >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.
 

16. ISOTHERMALISOTHERMAL--Temperature remains constantTemperature remains constant

17. ISOBARIC - ISOBARIC - Pressure remains constantPressure remains constant

18. ISOMETRICISOMETRIC - - Volume remains constantVolume remains constant
(also (also ISOVOLUMETRICISOVOLUMETRIC or or ISOCHORICISOCHORIC))


                                  
 
       
 Since ΔV = 0, W = 0 then U = Q - W = Q
 

19. 
Refrigerators work by taking heat from the interior and Refrigerators work by taking heat from the interior and
depositing it on the exteriordepositing it on the exterior

The compressor raises the pressure and temperature of The compressor raises the pressure and temperature of
the refrigerant (freon or ammonia) while the coils the refrigerant (freon or ammonia) while the coils
OUTSIDE the refrigerator allow the now hot OUTSIDE the refrigerator allow the now hot
refrigerant to dissipate the heatrefrigerant to dissipate the heat

The warm refrigerant flows through an expansion valve The warm refrigerant flows through an expansion valve
from a high-pressure to a low-pressure zone, so it from a high-pressure to a low-pressure zone, so it
expands and evaporatesexpands and evaporates
•
The coils INSIDE the The coils INSIDE the
refrigerator allow the cold refrigerator allow the cold
refrigerant to absorb heat, refrigerant to absorb heat,
cooling the interiorcooling the interior
• The cool refrigerant flows The cool refrigerant flows
back to the compressor, and back to the compressor, and
the cycle repeatsthe cycle repeats

20. 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.


                        

21. ENTROPYENTROPY

22. Can you beat the Second Law?Can you beat the Second Law?

So, can you cool your kitchen by So, can you cool your kitchen by
leaving the refrigerator door openleaving the refrigerator door open

NO!NO!

The heat removed from the interior The heat removed from the interior
of the refrigerator is deposited back of the refrigerator is deposited back
into the kitchen by the coils on the into the kitchen by the coils on the
back!back!

And to make matters worse, the Second Law of And to make matters worse, the Second Law of
Thermodynamics says that work is needed to move the Thermodynamics says that work is needed to move the
heat from cold to hot, so the actual amount of heat heat from cold to hot, so the actual amount of heat
added to the kitchen is MORE than the amount added to the kitchen is MORE than the amount
removed from the refrigeratorremoved from the refrigerator

24. Hopefully, you understand today’s lesson. Hopefully, you understand today’s lesson.
Otherwise, you’ll end up like this cow.Otherwise, you’ll end up like this cow.