THERMODYNAMICS

Department of Physics, Faculty of Science

Jazan University, KSA

Part-1

222 PHYS

Thermodynamics is the study of the effects of work, heat, and energy on a system

Thermodynamics is only concerned with macroscopic (large-scale) changes and

observations

Thermodynamics

• Thermodynamic system: a quantity of fixed mass under investigation,

Definitions

• System boundary: interface separating

system and surroundings

• Universe: combination of system and

surroundings.

• Surroundings: everything external to the system

Definitions

Systems can be classified as:

• Open: mass and energy can be transferred between system and surroundings

• Closed: energy can transfer but not mass

• Isolated: Neither energy nor mass can be transferred between system and

surroundings

A Closed system (a control mass) consists of a fixed amount of mass, and no

mass can cross its boundary. That is, no mass enters or leave a closed

system.

such as, Piston-cylinder device

An Open system (or a control volume ）is a properly selected region in space.

Both mass and energy can cross the boundary of a control volume.

such as, A Water heater, a turbine and a compressor, etc

Definitions

Intensive properties: are those independent of the size of a system as

well as its Temperature (T), Pressure (P) and concentration and density

Extensive properties: are those whose values depend on the size or

extent of the system

Sum of the properties of the system’s components.

•Depends on the size of the system

•Volume (V), Area (A), # of moles (n), mass(m), internal energy and

enthalpy

Two important interactions between the system and its surroundings:

• heat can cross into the system (our potato can get hot), and

• work can cross out of the system (our potato can expand).

All of thermodynamics can be expressed in terms of four quantities

Temperature (T) - Internal Energy (U) - Entropy (S) - Heat (Q)

Thermodynamics

provides a framework of relating the macroscopic properties of a system to one

another.

Change where system is always in thermal equilibrium:

{ reversible process}

Change where system is not always in thermal equilibrium:

{irreversible process}

Examples of irreversible processes:

• Free expansion - melting of ice in warmer liquid - frictional heating

Types of the questions that Thermodynamics addresses:

• How does an engine work? What is its maximum efficiency?

• How does a refrigerator work? What is its maximum efficiency?

• How much energy do we need to add to a kettle of water to change it to steam?

Properties of a pure substance

Pure substance: a material with homogeneous and invariable composition.

• Pure substances can have multiple phases, It may exist in more than one phase,

but the chemical composition is the same in all phases.

• An ice-water mixture is still a pure substance.

• An air-steam mixture is not a pure substance.

(air, being composed of a mixture of N2, O2, and other gases,

is formally not a pure substance).

Phases of matter

Gas - very weak

intermolecular forces,

rapid random motion

Liquid - intermolecular

forces bind closest neighbours

Solid - strong

intermolecular forces

low

temp

high

pressure

high temp

low pressure

Property

Definition

Symbol

S.I.Unit

Volume

Volume of a substance

V

m3

Internal Energy

The translational, rotational and

vibrational kinetic energy of a

substance

U

Joules (J)

Enthalpy

U + PV

H

Joules (J)

Entropy

The entropy is a measure of the

lack of structure or the amount of

disorder in a system

S

Joules/Kelvin (J / K)

Thermodynamics •“set of tools” that describes the macroscopic properties of equilibrium systems •entirely empirical science •based on four laws

The zeroth law of thermodynamics deals with thermal equilibrium and provides a means of measuring temperature.

The first law of thermodynamics: deals with the conservation of energy and introduces the concept of internal energy. The second law of thermodynamics: dictates the limits on the conversion of heat into work and provides the yard stick to measure the performance of various processes. It also tells whether a particular process is feasible or not and specifies the direction in which a process will proceed. As a consequence it also introduces the concept of entropy. The third law of thermodynamics: defines the absolute zero of entropy.

Zeroth law of thermodynamics

When two bodies have equality of temperature with a third body, then they have equality of temperature

If A and C are at thermal equilibrium, i.e. at the same temperature,

and B and C are at thermal equilibrium, then it follows that A and B

are at thermal equilibrium, i.e. at the same temperature.

Heat

Sign of Q : Q > 0 system gains thermal energy Q < 0 system loses thermal energy

The term Heat (Q) is properly used to describe the thermal

energy transferred into or out of a system from a thermal reservoir.

Q U

Heat is a transfer of energy. It is NOT energy!!

Heat is a form of energy.

It is a scalar quantity.

It is measured in joules (J).

When we heat a substance it

becomes hotter

or changes state.

The old metric temperature scale, Celsius (◦C), was defined so that

0 ◦C is the freezing point of water, and

100 ◦C is the boiling point of water.

The transformation between temperature scales is as follows

Heat always flow from an object with excess energy to an object with an absence of energy.

Heat flows from hot to cold. 1 cal = 4.186 J

Convert calories to Joules

O[ ] [ ] 273.15T K T C O 5[ ] ( [ ] 32)

9T C T F

The Absolute (Kelvin) Temp. Scale

The absolute (Kelvin) temperature

scale is based on fixing T of the triple

point for water (a specific T = 273.16 K

and P = 611.73 Pa where water can

coexist in the solid, liquid, and gas

phases in equilibrium).

TPP

PKT 16.273 - for an ideal gas constant-

volume thermoscope

absolute zero

T,K

P PTP

273.16

PTP – the pressure of the gas in a

constant-volume gas thermoscope

at T = 273.16 K

0

Heat Capacity

• The heat capacity is the energy required to raise the temperature of an object by 1oC.

• The heat capacity depends on the mass of the object and the ability of the object to resist the flow of heat into or out of the object.

Q = mcxΔT

• The constant cx is called the specific heat of substance x, (SI units of J/kg·K)

1 dQ dqc

m dT dT

Specific heat capacity

specific heat”, is the heat capacity per unit mass

m – mass [kg] C – Heat capacity [J/oC] c – Specific heat [J/kgoC]

0limT

Q dQC

T dT

Heat capacity

Because the differential dQ is inexact, we have to specify under what conditions heat is added.

Or, more precisely, which parameters are held constant. This leads to two important cases:

• the heat capacity at constant volume, CV

• the heat capacity at constant pressure, Cp

andV P

V P

dQ dQC C

dT dT

( isothermic, C = ,

adiabatic, C = 0 )

3 5Monatomic: ;

2 2

5 7Diatomic: ; ;

2 2

V P

V P

R Rc c

R Rc c

A common use of the heat equation is to determine the final temperature of a mixture of two different objects at different initial temperatures.

coldhot QQ

222111 TmcTmc

This is necessary to compensate for the different directions of heat flow.

222111 TTmcTTmc ff

2222211111 TmcTmcTmcTmc ff

2221112211 TmcTmcTmcTmc ff

2221112211 TmcTmcmcmcTf

2211

222111

mcmc

TmcTmcT f

This would be the final temperature of a mixture of two materials with different mass, specific heats and initial temperatures.

This expression is not valid if there is a phase change at any time during the process.

Specific Heat values for selected substances.

Heat and Latent Heat

Latent heat is defined as the heat absorbed or released when a substance

changes its physical state completely at constant temperature

1) Latent heat fusion

2) Latent heat of vaporization

Latent heat fusion is the heat absorbed when a solid melt at constant

temperature

Latent heat of vaporization is the heat absorbed

when a liquid changes into vapor at constant

temperature

Latent Heat The amount of heat required for a phase transition is given by:

mLQ Q – Heat [J] L – Latent Heat [J/kg] M – mass [kg] Ckg

J

Cg

calcwater

41841

Ckg

J

Cg

calcice

20925.0

Ckg

J

Cg

calcsteam

20105.0

kg

Jx

g

calL mf

5

/ 103.380

kg

Jx

g

calL cv

6

/ 1026.2540

Example1-1: 10 kg of ice at 0oC is dropped into a steam chamber at 100oC. How much steam is converted to water at 100oC?

hotcold QQ

csteamwaterOHOHfice LMTmcLm 22

c

waterOHOHfice

steamL

TmcLmM

22

kgM steam 3.3

The negative sign on the mass is acceptable here since it represents the mass of steam that was lost (converted to water).

Heat and Latent Heat

• Latent heat of transformation L is the energy required for 1 kg of substance to undergo a phase change. (J / kg)

Q = ±ML

• Specific heat c of a substance is the energy required to raise the temperature of 1 kg by 1 K. (Units: J / K kg )

Q = M c ΔT

• Molar specific heat C of a gas at constant volume is the energy required to raise the temperature of 1 mol by 1 K.

Q = n CV ΔT

If a phase transition involved then the heat transferred is

Q = ±ML+M c ΔT

Latent heat and specific heat

• The molar specific heat of gasses depends on the process path

• CV= molar specific heat at constant volume

• Cp= molar specific heat at constant pressure

Cp= CV+R (R is the universal gas constant)