lecture 15 - umd physicslecture 15 • today: • next lecture: chapter 1: work, heat and 1st law of...
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Lecture 15
• today:
• next lecture:
Chapter 1: Work, Heat and 1st Law of Thermodynamics
work, heat as energy transfers between system and environment
how state of system changes in response to work, heat
Ca lor imet r y ( spec i fic hea t and hea t o f transformation)
(1st law of Thermodynamics: energy conservation)
adiabatic process
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Work-Energy Theorem
• earlier: how K and U is transformed into each other
• now: energy is transferred between system and environment (work and heat)
• associated with T: isothermal process (except phase changes)
Thermal energy
!K = Wc + Wdiss + Wextconservative forces:
dissipative:
external
!U = !Wc !Eth = !Wdiss
E m ec h = K + U (mot ion of system as a whole: macroscopic)vs. Eth (microscopic energy of atoms and bonds)
!Esys = !Eth + !Emech = Wext
isolated system: We x t = 0 ! !Es y s = 0 (energy conservation)
Eth = Kmicro + Umicro
Eint = Eth + Echem + Enucl Esys + Emech + Eint: here, Eint = Eth
moving atoms spring-like bonds
!T = 0
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Work• energy transferred due to net force over a
distance (mechanical interaction): sign tells us which way transfer
• not a state variable
• energy transferred in thermal interaction e.g. water on stove
Energy transfer
Heat
W e x t = 0; ! E m ec h = 0, bu t ! E t h = 0
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1st Law of Thermodynamics
• basic energy model (work included, but not heat) vice versa in thermodynamic... ener
• 1st law does not tell us , only
• change in other state variables (use other laws such as ideal-gas)
W + Q = !Es y s = !Em ec h + !Et h : here, !Em ec h = 0
E t h
! E th
! E th
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Work in Ideal-gas Processes (I)
W = s f
s iF s ds (force parallel
to or opposi te to displacement )
F e x t = F g a s = ! p A(piston in equilibrium)
dW = F e x t dx = ! p A dx = ! pdV< ( > )0 (expansion / compression)
quasi-st at ic process:W = !
V f
V ipdV
(cannot t ake p ou t of in general)
= negative of area under pVcurve between V i and V f
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Work in Ideal-gas Processes (II)• isochoric (constant V)
• isobaric (constant p)
• isothermal (constant T)
multi-step process(1 2 3):calculate W for each step and add not equal to W in 1 step (1 3)
Work done depends on path and not just initial and final states: forces not conservative; path is of thermodynamic states, not particle
W = ! V f
V ipdV = !
V f
V i
n R TV dV = ! n R T ln V f
V i
= ! p i V i ln V fV i
= ! p f V f ln V fV i
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Heat
• more elusive than work: “something” flows from hot to cold object (heat fluid or caloric)
• Joule...heat is energy transferred: raise temperature by heating or doing work (different ways of transferring energy to/from system)
• thermal interaction (temperature difference): faster moleclues collider with slower (no macro.motion)
• heat is not state variable: energy transferred; units joule or calorie: 1 cal = heat to change 1 g water by 1 C 1 food calorie = 1 Cal = 1000 cal
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Thermal energy vs. heat vs. temperature• Thermal energy (state variable): energy due to motion of atoms
(even for isolated system)
• Heat: energy transferred between system and environment; heat is not a form of energy or state variable; heat can cause thermal energy to change
• Temperature: state variable, related to thermal energy per molecule; temperature difference required for thermal interaction (observed temperature difference can be due to work done on system or heating
• fix volume by locking pin
• change pressure (unlock piston)
• heat transferred thru’ bottom (piston/sides insulated)
A Model...
pg a s = pa t m + M gA