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7/24/2019 Termal Fisika
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FISIKA TERMALBAGIAN 2
FISIKA THERMAL (2)yusronsugiarto.lecture.ub.ac.id
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Ekspansi Thermal
Gas Ideal
Persamaan GasTeori kinetik
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Ekspansi termal dari benda padat dan cair
Fenomena terjadinya peningkatan volume dari suatu materi
karena peningkatan temperatur disebut dengan ekspansi termal.
Ekspansi termal adalah konsekuensi dari perubahan average
separation antar atom dalam suatu materi.
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• Benda akan memanjangbila temperaturnyameningkat
• Pertambahan panjangproporsional terhadapperubahan temperatur
DL/L = a DT atau
DL = a L DT atau
dL
dT a L
L DL
L DL
DL/L = aDT
DL/L = aDT
Ekspansi Linier
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Contoh:
• Suatu penggaris besi dikalibrasi pada suhu 20 °C. Hitung errordalam pengukuran jika penggaris tersebut digunakan untuk
mengukur sepanjang 500 mm pada temperatur 45 °C. abesi=
1,2x10-5 C-1
DL/L = a DT
DL = L a DT
DL = 500 x10-3 m x 1,2 x10-5 C-1 x 25 °C
DL = 1,5x10-4 m = 0,15 mm
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Ekspansi Volume
• Setiap sisi panjang berubah dari Lmenjadi L+DL = L + La DT
• Volume awal = L3
• Volume baru (V+DV )
= (L +DL)3 = (L + La DT)3
= (L (1+a DT ))3
= L3
+ (1+ a DT)3
= V [1+ 3a DT+ 3(a DT) 2 +(a DT)3]
L DL
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Ekspansi Volume (lanjutan)
• V+DV = V [1+ 3a DT+ 3(a DT) 2
+(a DT)3
]DV / V = [3a DT+ 3(a DT) 2 +(a DT)3]
• Karena a DT < 1 untuk nilai DT < 100 °Cmaka nilai 3(a DT) 2 dan (a Dt )3 dapat
diabaikan. Sehingga:DV / V = 3a DT
DV = 3a V DT
• 3a b → DV = b V DT
L DL
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Unusual Behavior of Water
Cairan umumnya akan meningkat volumenya denganpeningkatan temperatur. Kecuali untuk air dingin sepertiditunjukkan pada gambar.
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Ekspansi Thermal
Gas Ideal
Persamaan GasTeori kinetik
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Ideal Gas
An ideal gas is an idealized model for real gases that have
sufficiently low densities.
The condition of low density means that the molecules of the gas
are so far apart that they do not interact (except duringcollisions that are effectively elastic).
The ideal gas law expresses the relationship between the
absolute pressure (P) , the Kelvin temperature (T) , the volume
(V), and the number of moles (n) of the gas.
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Misal gas ideal didalam wadah silinder yang
volumenya dapat divariasikan dengan piston yangdapat bergerak. Jika diasumsikan sistem tertutup,
maka massa (atau jumlah mol) gas tetap konstan.
Pada sistem tersebut dapat diperoleh informasi:
Pada temperatur konstan, tekanan berbandingterbalik dengan volume (Hukum Boyle)
Pada tekanan konstan, volume berbanding lurus
dengan temperatur (Hukum Charles)
Pada volume konstan, tekanan berbanding lurus
dengan temperatur (Hukum Gay-Lussac)
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Ekspansi Thermal
Gas Ideal
Persamaan GasTeori kinetik
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Amount of Gas
• Better to describe gas in terms of number of moles.
• The number of moles n contained in any sample is the
number of particles N in the sample divided by the number
of particles per mole N A (Avogadro's number):
A N
N n
The number of moles contained in a sample can also be found
from its mass.
M
mn
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Avogadro’s Constant
• One mole of any gas
contains the same number of
particles. This number is
called Avogadro’s constantand has the symbol NA. The
value of NA is 6.02 × 1023
particles per mole.
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Avogadro’s Law
nV
The most significant consequence of
Avogadro's law is that the ideal gas
constant has the same value for all
gases. This means that the constant is
given by:-
constant
22
22
11
11 nT
V p
nT
V p
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Deriving Ideal Gas Equation
• From Boyle’s Law:
• From Charles’s Law:
• From Avogadro’s Law:
• Combining these three:
• Rewriting using the gas
constant R:
pV
1
p
nT V
nV
T V
nRT pV
p
nT RV
Therefore:-
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PV Diagram in Thermodynamic Process
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Ekspansi Thermal
Gas Ideal
Persamaan GasTeori kinetik
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Kinetic Theory of Gases
The pressure that a
gas exerts is caused bythe impact of its
molecules on the walls
of the container.
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Consider a gas molecule colliding
elastically with the right wall of thecontainer and rebounding from it.
The force on the molecule is obtained
using Newton’s second law as follows:
D
D
t
P F
( (
L
mv
v L
mvmv F
2
2
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The force on one of the molecule:
According to Newton's law of action–reaction, the force on the
wall is equal in magnitude to this value, but oppositely directed.
The force exerted on the wall by one molecule:
L
mv2
( ( L
mv
v L
mvmv 2
2
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If N is the total number of molecules, since these particles move
randomly in three dimensions, one-third of them on the average
strike the right wall. Therefore, the total force is:
Vrms = root-mean-square velocity.
L
vm N F rms
2
3
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Pressure is force per unit area, so the pressure P acting on a
wall of area L2 is
Since the volume of the box is V = L3, the equation above
can be written as
3
2
23 L
vm N
L
F P rms
2
2
1
3
2
rmsvm N PV
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Contoh soal:
1. Berapa banyaknya massa Cl2
(gram) yang dapat disimpan
dalam suatu wadah (kontainer) dengan volume 10 L pada
suhu 30 °C dan tekanan 1000 kPa?
2. Pada suhu 150 °C dan tekanan 100 kPa, sebuah senyawa
dengan massa 2,506 gram memiliki volume 1 L. Hitungmassa molar senyawa tersebut.
3. Berapa vrms dari atom helium yang mengisi sebuah balon
dengan diameter 30 cm pada suhu 20°C dan tekanan 1
atm? (diketahui jumlah atom helium dalam balon
sebanyak 3,54 x 1023 dan massa atom helium 6,64 x 10-24 g)
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FISIKA TERMALBAGIAN 2
TERIMA KASIH Yusron sugiarto
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