Sistem koji cikli čki pretvara toplotu u mehaničku energiju
QA – TOPLOTA KOJU GORIVO PREDA VODI AT – RAD KOJI VRŠI TURBINA QK – TOPLOTA KOJU TREBA ODVESTI DA BI PARA PREŠLA U VODU AP – ENERGIJA POTREBA ZA RAD TURBINE
Q A − QK = AT − AP DRUGI ZAKON TERMODINAMIKE Problem konverzije TOPLOTE U RAD . Sav rad možemo prevesti u toplotu ali svu toplotu nemožemo prevesti u rad. Toplotni stroj je sistem koji c ikli čki radi i pretvara toplotu u mahani čku energiju (rad) .
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Termodinami čki proces je reverzibilan (povratan) ako dozvoljava povratak sistemna u po četno stanje bez promjena na okolinu ( neprigušene ocsilacije). Ireverzibilni (nepovratni) procesi su svi realni procesi ( trenje, difuzija, izmjena toplote itd.)
TOPLOTNA PUMPA (FRIŽIDER)
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Q1 = Q2 + W Q2 − negativna veličina Koeficijent korisnog djelovanja
K = −
Q2 W
=−
Carnotoov ciklus
Q2 Q1 + Q2
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Termički koeficijent korisnog djelovanja
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η = 1 +
Q2 Q1
=
T 1 − T 2 T 1
Najveći stepen korisnog djelovanja toplotnih strojeva ima Carnotoov stroj. Nijedan stroj koji radi izme đu dvije temperature nemože biti efikasniji od Carnootovog stroja. Svi Carnootovi strojevi koji rade izme đu dvije iste temperature imaju istu efikasnost bez obzira na prirodu radnog medija. II ZAKON TERMODINAMIKE NEMOGUĆE JE NAPRAVITI TOPLOTNI STROJ KOJI BI U PRIODI ČNOM CIKLUSU SVU DOVEDENU TOPLOTU PRETVORIO U RAD. To bi bio perpetuum mobile II vrste.
Kada ovo ne bi važilo!! Brod bi se mogao kretati po moru uzimaju ći toplotu iz mora ili iz okolnog zraka. NEMOGUĆ JE PROCES ČIJI BI JEDINI REZULTAT PRENOS TOPLOTE OD HLADINJEG TIJELA NA TOPLIJE.
ENTROPIJA ENTROPIJA ODREĐUJE SMJER ODVIJANJA TERMODINAMI ČKOG PROCESA. TOPLOTA SE PRIRODNO KREĆE OD TOPLIJEG KA HLADNIJEM. Prirodni smjer pretvaranja energije MEHANIČKA ENERGIJA
→ TOPLOTNA ENERGIJA
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δ Q p
∫ T dS =
δ Q p
≤0
δ Q p T povratni ciklus
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ENTROPIJA KLAUZIJUSOVA KLAUZIJUSOVA TEOREMA
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δ Q
Za povratne sisteme dS = T
Za nepovratne sisteme
dS ≥
δ Q T
Kod nepovratnih procesa uvjek imamo pove ćanje entropije. I zakon termodinamike
δ Q = dU + δ W II zakon termodinamike
δ Q = TdS Kombinacijom dobivamo
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x
S x − S0 =
dU
x
p
∫ 0 T + ∫ 0 T dV
Toplota koja se prenosi u povratnom procesu može se izraziti S2
Q12 =
∫ TdS
S1
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Unutrašnja energija
dU = ncv dT Entropija dS =
dU T
+
p T
dV = ncV dT + nR
dV V
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Nernstova teorema (III zakon teromdinamike) Pri apsolutnoj nuli i entropija bilo kojeg tijela teži nuli.