c10 – thermal energy c11 – energy in bondsrfinn/generalphysics/schedule_files/c10and11.pdf ·...
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General Physics 1
• C10 – Thermal Energy• C11 – Energy in Bonds
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General Physics 2
Disappearing Energy
• Contact friction interaction doesn’tchange potential energy because theseparation between the two objectsdoesn’t change
• Kinetic energy of object decreases, butwhere does the energy go?
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General Physics 3
• All objects are constructed of a hugenumber of tiny molecules
• These molecules are in ceaselessrandom motion
• An object’s temperature is a measure ofthe intensity of motion
Thermal Energy
TkK Bavg2
3=
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General Physics 4
Friction and Thermal Energy
• When a surface moves past another other, theirsurface molecules often become entangled.
• The entangled molecules are stretched away fromtheir normal positions
• A small amount of kinetic energy is transformed topotential energy in the bonds between the molecules.
• When the molecules snap back, they oscilate• The energy is transmitted to the rest of the
molecules in the objects
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General Physics 5
• Heat and work are defined to describe energytransfer across a system boundary
• Heat is any energy that crosses the boundarybetween two objects because of the temperaturedifference between them
• Work is any other kind of energy flowing across anobject’s boundary
• The change in internal energy of an object is theheat added to and the work flowing into the system
Heat and Work
WQU +=!
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General Physics 6
Specific Heat
McdTdUth=
• A small change in an object’s thermal energyis proportional to the mass, the specific heat,and the temperature change
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General Physics 7
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General Physics 8
Example C10.3
• Imagine that we place a 50-gblock of aluminum with aninitial temperature of 100°Cinto an insulating coveredStyrofoam cup containing250 g of water at 22°C.Heat will flow from thealuminum to the water untilboth are at the sametemperature. What is thattemperature?
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General Physics 9
Potential Energy Diagrams
• Scan p 194
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General Physics 10
Bonds
• Typical potential energycurve for a bond.
• When bonds form,energy is released
• When bonds break,energy is absorbed
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General Physics 11
Latent Heat
mLUmLU
mLUmLU
lath
lath
+=!"=!
"=!+=!
L is the latent heat, m is the mass
For vaporization or melting
For condensation or freezing
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General Physics 12
Group Problems
• C10B.6• C10S.2• C11B.5• C11S.5