Prof. Siyoung Jeong

Thermodynamics I

MEE2022-02

Spring 2014

Chapter 1

Introduction

Fundamentals of Thermodynamics

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Chapter 1. Introduction

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• Thermodynamics : Science of energy and

entropy

- Science of heat and work and properties related

to heat and work

• Basis for diverse scientific disciplines

- Physics, chemistry, biology, etc.

• Applied to various fields

- Mechanical engineering, chemical engineering,

aerospace engineering, etc.

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1.1 A Thermodynamic system and the control volume

• Schematic diagram of a steam power plant

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• Schematic diagram of a refrigerator

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• Thermodynamic system

- System / Surroundings

- Control mass (no mass flow)

- Control volume (more general)

- Open / closed systems

- System boundary / Control surface

- Isolated system : No influence from surroundings

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Control mass Control volume

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1.2 Macroscopic versus microscopic points of view

• Macroscopic vs. Microscopic

25 mm

Atmospheric P, T

10^20 atoms

6*10^20 equations!

Classical Thermodynamics Statistical Thermodynamics

Continuum

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1.3 Properties and state of a substance

• Phase : Gas, liquid, solid

• A substance may exist in in various states (T, p, etc.).

• Thermodynamic property : observable & macroscopic

- Intensive : pressure, temperature, density, etc.

- Extensive : mass, volume, etc.

- Specific property : property/mass,

intensive property, ex) specific volume

• Thermodynamic equilibrium - Thermal equilibrium : T

- Mechanical equilibrium : P

- Chemical equilibrium : μ (chemical potential)

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1.4 Processes and cycles

• Process : Path of state change

• States during a process?

- Quasi-equilibrium process : Defined ideally but

applied to many practical cases

- Nonequilibrium process

• Iso-X process : Constant X

- Isothermal : T=const.

- Isobaric (isopiestic) : P=const.

- Isochoric : V=const.

• Cycle : Initial state = Final state

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1.5 Units for mass, length, time, and force

• SI units : Metric International System

- Time : second [s]

- Length : meter [m]

- Mass : kilogram [kg]

cf) mole [mol] : 12 g of carbon-12

- Force : Newton [N]

• English Engineering Units

- ft, in, lbm, lbf, …

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Chapter 1. Introduction

Ex. 1.1

What is the weight of a 1 kg mass at an altitude where the local acceleration of gravity is 9.75 m/s2?

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1.6 Specific volume and density

• v , Specific volume = volume / mass [m3/kg]

r , Density = mass / volume [kg/m3]

• Continuum concept :

• Molal property : [m3/kmol] , [kmol/m3]

m

Vv

VV

lim

v r

Both intensive properties }

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Ex. 1.2 A 1 m3 container, shown in Fig. 1.9, is filled with

0.12 m3 of granite, 0.15 m3 of sand, and 0.2m3 of

liquid 25℃ water; the rest of the volume, 0.53 m3,

is air with a density of 1.15 kg/m3. Find the

overall (average) specific volume and density.

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1.7 Pressure

• Continuum concept

• Unit : 1 Pa = 1 N/m2

1 bar = 105 Pa = 0.1 MPa

1 atm = 101325 Pa ~ 1 bar

A

FP n

AA

lim

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• Absolute vs. gauge pressure

gHPPP

gHPP

AgHAPmgAPAP

B

B

B

r

r

r

0

0

00

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0

00

Fluid with a constant density:

Fluid with a varying density: ( )

B

H

B

P P P gH

P P P dP z gdz

r

r

0P P

P

P dP

BP P

0z

z

z dz

z H

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Ex. 1.3 The hydraulic piston/cylinder system shown in

Fig. 1.11 has a cylinder diameter of D = 0.1 m

with a piston and rod mass of 25 kg. The rod has

a diameter of 0.01 m with an outside atmospheric

pressure of 101 kPa. The inside hydraulic fluid

pressure is 250 kPa. How large a force can the

rod push with in the upward direction?

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Chapter 1. Introduction

Ex. 1.4

A mercury barometer located in a room at 25 ℃ has a height of 750 mm. What is the atmospheric

pressure in kPa?

Ex. 1.5

A mercury (Hg) manometer is used to measure the pressure in a vessel as shown in Fig. 1.13. The

mercury has a density of 13,590 kg/m3, and the height difference between the two columns is

measured to be 24 cm. We want to determine the pressure inside the vessel.

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Ex. 1.6

What is the pressure at the bottom of the 7.5 m tall storage tank of fluid at 25 ℃ , shown in Fig.

1.15? Assume that the fluid is gasoline with atmospheric pressure 101 kPa on the top surface.

Repeat the question for the liquid refrigerant R-134a when the top surface pressure is 1 MPa.

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Ex. 1.7

A piston/cylinder with a cross-sectional area of 0.01 m2 is connected with a hydraulic line to another

piston/cylinder with a cross-sectional area of 0.05 m2. Assume that both chambers and the line are

filled with hydraulic fluid of density 900 kg/m3 and the larger second piston/cylinder is 6 m higher

up in elevation. The telescope arm and the buckets have hydraulic piston/cylinders moving them, as

seen in Fig. 1.16. With an outside atmospheric pressure of 100 kPa and a net force of 25 kN on the

smallest piston, what is the balancing force on the second larger piston?

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1.8 Energy

• Microscopic view

- Intermolecular (potential) energy

- Molecular kinetic energy

- Intramolecular energy

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1.9 Equality of temperature

• “Sense of hotness or coldness”

However, difficult to rigorously define temperature

• Two objects in thermal contact for a long time

→ No change → thermal equilibrium

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1.10 The zeroth law of thermodynamics

• Zeroth law

TA = Tthermometer

TB = Tthermometer

∴ TA = TB

Basis of temperature measurement

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• Fahrenheit, Celsius

- Celsius: formerly know as centigrade

• Absolute temperatures

K = °C + 273.15

R = F + 459.67

Fahrenheit Celsius

Ice pt 32 0

Steam pt 212 100

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1.11 Temperature scales

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1.12 Engineering applications

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