Termodinamika
189
BC6C52B:6252=29ABC6C6CA2?C:2D2EABC6CA6?8632?82? :CD6B2= A252 CE9E
252 ABC6C :?: 82C 6?H6B2A 2=B 52B:
B6C6BF:B 36BCE9E D:?88: 52? 6=2E2? EC292
BC6C 52B: 6 252=29ABC6CA6?8632?82?25:232D:,6=22ABC6CCE9E82CDEBE?
52B: 6?;25:
C23:= 6=2E2? EC292 BC6C 52B: 6
252=29ABC6CA62A2D2?:CD6B2=A252CE9E 2=2ABC6C:?:82C
6=6A2C 2=B 6 B6C6BF:B 36BCE9E B6?529
52? 6=2E2? EC292 BC6C 29:B 52B: 6 252=29 ABC6C A62A2D2? 25:232D:
,E9E ?2: 52B: 6
C23:= 6=2E2? EC292 ,:=EC2B?D252=2952C2B52B:6C:?5:6C6=H2:DE6C:?H2?8A2=:?8
:562=H2?8C6=2?;ED?H25:C63ED6C:?2B?D,6=22ABC6CC:=EC2B?D 36B=2?8CE?8 82C 6?6B:2 2=B
52B: B6C6BF:B 36BCE9E D:?88: 52? 6=6A2C 2=B
6 B6C6BF:B 36BCE9E B6?529 .C292 H2?8 5:=2E2? =69 82C C6CE2: 56?82? EE -6B5:?2:2 252=29
2D2E O
O O
6D6B2?82? 2=B H2?8 5:C6B2A 52B: B6C6BF:B H2?8 36BCE9E D:?88:
2=B H2?8 5:=6A2C 6 B6C6BF:B H2?8 36BCE9E B6?529
3. Efisiensi Mesin
6B92D:2?.2
232BD6BC63ED6?E?;E2?C:=EC2=B 5: 52=2 C63E29 6C:? 6B32?5:?82? 2?D2B2 36C2B?H2 EC292
H2?8 52A2D 5:=2E2? =69 C:CD6 D6B9252A 2=B
H2?8 5:C6B2A 52A2D 6?6?DE2?67:C:6?C:CE2DE6C:?,642B22D62D:C67:C:6?C:6C:?52A2D
5:DE=:C2? C63282: 36B:ED P
O
a. Mesin Carnot
7:C:6?C: 6C:? 2B?D 5:A6B=69 56?82? 42B2 6?882?D: EC292
56?82? 23. 9
C69:?882 23. 9
6?;25: P
2D2E
P O
-6=29 5:3292C 329G2 E?DE CE2DE 82C :562= 6?6B8: 52=2 C632?5:?8 56?82? CE9E ED=2
C69:?882 E?DE 6C:? 2B?D 23. 9
52A2D 5:DE=:C C63282: 36B:ED
P O
6?882?D:2? 36C2B2? 2=B 6?;25: CE9E ED=2 52=2
6?6?DE2? 67:C:6?C: C63E29 6C:? 36B52C2B2? A252 6?6B8: 52=2 C632?5:?8 56?82? CE9E 6=F:? 6?E?;E2? 329G2
O 6D6B2?82?
CE9E B6C6BF:B D:?88: CE9E B6C6BF:B B6?529
Gambar 8.11
Siklus mesin ideal Q
1
dan suhu fungsi reservoir diubah seluruhnya
menjadi usaha W dengan efisiensi 100.
T
1
Q
1
W = Q
1
Q
2
= 0
Kata Kunci
• efisiensi mesin
Gambar 8.12
Perubahan kalor menjadi kerja: Q
1
= kalor masuk; Q
2
= kalor dilepaskan; W = usaha yang dilakukan;
T
1
= suhu reservoir tinggi; dan T
2
= suhu reservoir rendah. T
1
Q
1
Q
2
W
T
2
Mudah dan Aktif Belajar Fisika untuk Kelas XI
190
b. Mesin Otto
6C:?DD6BEA22?6C:?2=BH2?8AB:?C:A6B;2?H236B52C2B2? C:=EC DD 252 C:=EC DD D6B52A2D 5E2 ABC6C 25:232D: 52? 5E2
ABC6C :C9B: 6B92D:2? .2 252.2 O52?O6BEA22?ABC6C25:232D:C652?82?
O52?O6BEA22?ABC6C:C9B:52AE? 252=292=BH2?8
2CE52=2C:CD6C652?82? 252=292=BH2?86=E2B5:=6A2C2?
=69 C:CD6 7:C:6?C: 52B: C:=EC DD 5:?H2D22? 56?82? A6BC222? O
,:=EC DD 32?H2 5:2A=:2C:2? A252 6C:? DB 322B
c. Mesin Diesel
6C:? 5:6C6= 6BEA22? 6C:? 2=B H2?8 AB:?C:A 6B;2?H2 6?88E?22?C:=EC5:6C6=252C:=EC5:6C6=D6B52A2D5E2ABC6C25:232D:
C2DE ABC6C :C32B: 52? C2DE ABC6C :C9B: 6B92D:2? .2 BC6C 52B: 6 6BEA22? ABC6C :C32B: O 52? O
6BEA22?ABC6C25:232D:52?ABC6CO6BEA22?ABC6C:C9B: B:?C:A6B;26C:?5:6C6=92A:BC2256?82?6C:?DD2B6?2C6E2
ABC6CA2526C:?5:6C6=C2256?82?6C:?DD64E2=:ABC6C:C32B: 7:C:6?C: 3636B2A2 6C:? 5:DE?;E2? A252 -
p
V A
Q
1
B
C Q
2
D
Gambar 8.13
Dua proses adiabatik dan dua proses isokhorik pada siklus Otto.
- 7:C:6?C:636B2A26C:?
33
6C:?3:=36?C:? O
6C:?5:6C6= O
-EB3:?E2AA632?8:D?E=:B -EB3:?E2AA632?8:D32DE32B2
33
Gambar 8.14
Siklus diesel p
V A
Q
1
B
C Q
2
D
,63E29 6C:? 2B?D 6:=:: 67:C:6?C:
:2 CE9E B6C6BF:B D:?88: D6?DE2?=2936C2B?H2CE9EB6C6BF:BD:?88:282B67:C:6?C:6C:?6?;25:
56?82?
2?882A2?CE9EB6C6BF:BB6?529D:526?82=2:A6BE3292? 7
6BD22D6?DE2?CE9EB6C6BF:BB6?529 A252
O O
65E2D6?DE2?CE9EB6C6BF:BD:?88: 56?82?6A6B8E?22?
H2?8D6=29 5:9:DE?8D25:E?DE
O O
O 25:CE9EB6C6BF:BD:?88:H2?85:3EDE92?252=29
Contoh
8.8
Pembahasan Soal
Sebuah mesin Carnot yang menggunakan reservoir suhu tinggi
800 K, memiliki efisiensi 40. Agar efisiensinya naik menjadi 50, suhu
reservoir suhu tinggi dinaikkan menjadi ....
a.
900 K d.
1.180 K b.
960 K e.
1.600 K c.
1.000 K
Soal UMPTN Tahun 1990 Pembahasan:
Keadaan 1: = 40
T
1
= 800 K = 1 –
2 1
T T
40 = 1 –
2
800 K T
T
2
= 480 K Keadaan 2:
= 50 T
2
= 480 K = 1 –
2 1
T T
50 = 1 –
1
480 K T
T
1
= 960 K
Jawaban b
Termodinamika
191
,63E296C:?2B?DH2?86?88E?22?B6C6BF:B CE9E D:?88:
6:=:: 67:C:6?C:
.?DE
6?2:2?67:C:6?C:?H26?;25:
36B2A256B2;2D CE9EB6C6BF:BD:?88:92BEC5:?2:2?
6C:? 2B?D H2?8 36BCE9E B6C6BF:B D:?88:
L 6:=::67:C:6?C:
7:C:6?C:52B:6C:?D6BC63ED
5:D:?82D2? 6?;25:
6B2A2 56B2;2D CE9E B6C6BF:BD:?88:92BEC5:?2:2?
6C:? 2B?D 6?6B:2 2=B 52B: B6C6BF:B H2?8 36BCE9ED:?88:L52?6=6A2C2??H2A252CE9E
B6?529L-6?DE2?67:C:6?C:6C:?2B?DD6BC63ED 6C:? A6?5:?8:? BE2?82? 6?H6B2A 2=B C636C2B
;E=652=2G2DEC6?:2CE9EBE2?82? 22?5:A6BD292?2?C636C2BLC652?82?CE9E
Tes Kompetensi Subbab
C
2+,--.5,5-4 =:?8E?82?D6A2DA63E2?82?2=B252=29
L
6B2A22952H2=:CDB:H2?85:3EDE92? ,63E296C:?6?H6B2A2=BC636C2B
E=652B:
C63E29B6C6BF:B36BCE9E
52?63E2?8?H2 C636C2B
;E=6A252CE9E
2B:52D2D6BC63ED
D6?DE2?=29 2
67:C:6?C:6C:?52? 3
EC292H2?852A2D5:=2E2? ,63E296C:?2B?D6:=::67:C:6?C:
:2CE9E
B6C6BF:BD:?88:
D6?DE2?=2936C2B?H2CE9E B6C6BF:B D:?88: 282B 67:C:6?C: 6C:? 6?;25:
56?82? 2?882A2? CE9E B6C6BF:B B6?529 D:52 6?82=2:A6BE3292?
D. Hukum II Termodinamika
EE -6B5:?2:2 6?H2D22? D6?D2?8 662=2? 6?6B8: H2:DE 6?6B8:D:5252A2D5:4:AD22?2D2E5:EC?292?6=2:?2?92?H252A2D5:E329
52B: C2DE 36?DE 6?6B8: 6 36?DE 6?6B8: =2:??H2 EE -6B5:?2:2 D:52632D2C:3282:2?2A6BE3292?6?6B8:D6BC63ED36B=2?8CE?82:?92=?H2
56?82?EE-6B5:?2:2H2?86:=::32D2C2?32D2C2?D6BD6?DE
,63E29 6C:? 52A2D 36B86B2 5:C63232? 252?H2 6?6B8: 2=B H2?8 5:36B:2?A2526C:?D6BC63EDC642B2D6BEC6?6BECD6D2A:C632=:?H2D:52
E?8:? 6?6B8: 86B2 EC292 52A2D 636B:2? 2=B C642B2 D6BEC 6?6BEC 252 6?H2D22??H2 D:52 252 C63E29 6C:? AE? H2?8 366B;2
6?H6B2A 6?6B8: 2=B 52? 6?8E329 C6=EBE9?H2 6?;25: EC292
6B:ED:?:92=92=H2?836B2:D2?56?82?EE-6B5:?2:2 H2?8 6BEA22? 6C:AE=2? 52B: A6?822D2?A6?822D2? H2?8
5:=2E2? =69 -6 -, 52? -5353 6?EBED 6=F:? 52? =2?4 D:52 E?8:? 63E2D 6C:? H2?8
366B;2 52=2 CE2DE C:=EC 6?6B:2 2=B 52B: C2DE B6C6BF:B 52? 6?8E329 C6=EBE9 2=B D6BC63ED 6?;25: EC292
6?EBED=2EC:ECD:52E?8:?63E2D6C:?H2?8366B;252=2 C2DEC:=EC6?823:=2=B52B:B6C6BF:BH2?86:=::CE9EB6?529
52? 636B:2??H2 A252 B6C6BF:B H2?8 6:=:: CE9E D:?88: D2?A2 66B=E2? EC292 52B: =E2B
6C:?2B?DH2?8366B;25:2?D2B2B6C6BF:BCE9E 52?B6C6BF:B
H2?8 36BCE9E 6?892C:=2? 67:C:6?C: H2?8 D:?88:
25: 6?EBED 6=F:? 52? =2?4 D:52 E?8:? CE2DE 6C:? 92?H2 6:=:: C63E29 B6C6BF:B 52? D:52 E?8:? C63E29 6C:? 6:=::
67:C:6?C:
:267:C:6?C:
2=BH2?85:C6B2A52B:=:?8E?82??H2 22?5:E329C6=EBE9?H26?;25:EC292.C2926C:?C6242:?:5:C63ED
6C:? ,-,00+;6?:C65E26?H2D22??H2D:522526C:?C6A6BD:
:DE G2=2EAE? 2C:9 66?E9: EE -6B5:?2:2
Tokoh
Nikolaus Otto
Pada 1876, seorang warga negara Jerman, Nikolaus Otto menjadi orang
pertama yang membuat dan menjual mesin 4 tak yang kemudian menjadi
dasar pembuatan kebanyakan mesin. Ia menamakan mesinnya Silent Otto
karena mesin tersebut mampu bekerja tanpa menimbulkan
kebisingan. Salah satu ciri mesin 4 tak adalah tekanan kompresinya. Jika
bahan bakar berupa gas yang dimampatkan, akan lebih banyak
energi yang dilepaskan. Gagasan ini pertama kali dikembangkan oleh
Alphone Beau de Rochas 1815– 1891 yang berkewarganegaraan
Prancis, tetapi justru Otto yang menyukseskan ide tersebut.
Sumber: Jendela Iptek: Energi, 1997
