Thermodynamics

ThermodynamicsHeat and Temperature, Thermal

Properties of Matter, The First Law of Thermodynamics, The Second Law of

Thermodynamics, Heat Engines, Internal-Combustion Engines,

Refrigerators, Carnot Cycle, Entropy

Physics vocabulary

• It is a thermodynamics process with no heat transfer into or out of the system.

• Adiabatic• What is a constant-

temperature thermodynamic process?

• Isothermal• What is a thermodynamic

process at constant atmospheric pressure?

• Isobaric• It is a constant-volume

thermodynamic process• Isochoric

Example

• A surveyor uses a steel measuring tape that is exactly 50.000m long at a temperature of 20°C. What is its length on a hot summer day when the temperature is 35°C? α= 1.2 x10-

3/C°

Example

• Feed a cold, starve a fever: During a bout with the flu, an 80-kg man ran a fever of 2.0°C above normal, that is a body temperature of 39.0°C. Assuming that the body is mostly water, how much heat is required to raise his temperature by that amount? cwater= 4190J/KgK

TEMPERATURE AND HEAT

Thermal Equilibrium, Temperature Scales, Thermal Expansion, Quantity of Heat, Calorimetry and Phase Changes, Mechanisms of Heat transfer

Temperature and Thermal Equilibrium

• Temperature is a measure of hotness or coldness.

• When an interaction causes no further change in the system, then it is in the state of thermal equilibrium.

• Two systems are in thermal equilibrium if and only if they have the same temperature.

Zeroth Law of thermodynamics

If one system is initially in thermal equilibrium with two other

systems, then, these two other systems are also in thermal

equilibrium with each other.

Thermometric Scales

Thermal Expansion

Linear expansion

Volume expansion

Thermal Stress

Quantity of Heat (Q)

Energy transfer that takes place solely because of temperature difference is called

heat flow or heat transfer, and energy transferred in this way is called heat.

Quantity of Heat (Q)

Calorie (cal) is defined as the amount of heat required to raise the temperature of one gram of water from 14.5°C- 15.5°C

Conversion factors for the Quantity of Heat

1 cal = 4.186J1 kcal =1000 cal = 4186 J 1 Btu= 778 ft.lb = 252 cal = 1055 J

Specific Heat Capacity (C)

The amount of heat needed to raise one kilogram of a substance to 1C°

Phase Changes

• The term phase is used to describe the specific state of matter.• A transition from one phase to

another is called a phase change or phase transition.

Phase Change

• Heat of fusion (Lf)- the heat required per unit mass in changing solid to liquid

• Heat of vaporization (Lv)- the heat required per unit mass in changing liquid to gas

• Heat of combustion (Lc) - the heat required per unit mass in complete combustion of one gram of gasoline

Phase Change

Solid Liquid GasLf Lv

Mechanisms of Heat transfer

• Conduction occurs between a body or between two bodies in contact.• Convection depends on motion of

mass from one region of space to another.• Radiation is heat transfer by EM

radiation

Example

• A surveyor uses a steel measuring tape that is exactly 50.000m long at a temperature of 20°C. What is its length on a hot summer day when the temperature is 35°C? α= 1.2 x10-

3/C°

• 50.009m

Example

• Feed a cold, starve a fever: During a bout with the flu, an 80-kg man ran a fever of 2.0°C above normal, that is a body temperature of 39.0°C. Assuming that the body is mostly water, how much heat is required to raise his temperature by that amount? cwater= 4190J/Kg.K

• 6.7x105J= 133kcal

THERMAL PROPERTIES OF MATTER

Molecular Properties of Matter, KMT, Heat Capacities, Phases of Matter

KMT1. Gases are considered to be

composed of minute discrete particles.

2. The molecule in a container are believed to be in ceaseless motion during which they collide with each other and the walls of the container

KMT3. The molecular collision is perfectly

elastic.4. The molecules average KE is

proportional to any given absolute temperature.

5. All forces of attraction are negligible due to rapid molecular separation.

Gas Laws

• Boyle’s Law:P1 x V1 = P2 x V2

• Charles’ Law: V1/T1 = V2/T2

• Gay-Lussac’s Law: P1/T1 = P2/T2

Ideal Gas law

Emphasizes on the amount of substance and its effect on the

volume of a gas, represented by Avogadro’s law

n α V if PV α RT, then PVα nRT

Graham’s law of diffusion

The rate of flow of a gas molecule is inversely proportional to the square root of its density or its molecular weight, at constant temperature and

pressure.

r1/r2 = √ (d2/d1)

r1/r2 = √ (MW2/MW1)

Example

• How much faster does H2 travel than O2 molecule at the same temperature and pressure if molecular weights of H2 and O2 are 2g/mole and 32g/mole respectively?• H2 molecules travel 4times

faster than O2 molecules

Root-mean-square speed of a gas molecule

Where k is the ratio of R to NA (Boltzman constant)= 1.381x10-23 J/molecule.K

FIRST LAW OF THERMODYNAMICS

Water is heated, then boils;the expanding steam does work

to propel the locomotive

Explain the thermodynamic process

in making popcorn

First Law of Thermodynamics

Internal energy is the change in initial and final energies of the system

Internal energy is the sum of heat exchange between the system and the surroundings and W done on or by the systemUsed in

some sources

Sign Convention

• W done on the system: +• W done by the system: -• Heat added to the system: +• Heat released by the system: -

Used in some

sources

First Law of Thermodynamics

Internal energy is the change in initial and final energies of the system

Internal energy is the sum of heat exchange between the system and the surroundings and W done on or by the system

Sign Convention

• W done on the system: -• W done by the system: +• Heat added to the system: +• Heat released by the system: -

Adiabatic Process

Q=0 U2-U1= -W

no heat transfer into or out of the system

Isochoric Process

W= 0 U2-U1= Q

it does no work

Isobaric Process

W,Q,∆U≠0

p(V2-V1 )= W

Isothermal Process

W,Q,∆U≠0

Q = WIf ∆U=0 (Ideal gas)

SECOND LAW OF THERMODYNAMICS

Heat engines, Internal-Combustion Engines, Refrigerators, Carnot Cycle, Entropy

Explain the thermodynamic process

in this picture

Second Law of Thermodynamics

a general principle which places constraints upon the direction

of heat transfer and the attainable efficiencies of heat engines

In so doing, it goes beyond the limitations imposed by the first law of thermodynamics.

Second Law of Thermodynamics: heat engines

Second Law of Thermodynamics: heat

enginesThe most efficient heat engine cycle is the Carnot

cycle, consisting of two isothermal processes and two adiabatic processes.

the Carnot efficiency: the processes involved in the

heat engine cycle must be reversible and

involve no change in entropy

Carnot Engine

The 4-stroke engine cycle

The 4-stroke engine cycle

• Intake stroke: the inlet valve is open and a fresh fuel-air mixture is pulled into the cylinder by the downward motion of the piston.

The 4-stroke engine cycle

• Compression stroke: the piston moves upward, compressing the mixture. The temperature and pressure increase. Prior to the piston reaching the top of its travel, the spark plug ignites the mixture and a flame begins to propagate across the combustion chamber.

The 4-stroke engine cycle

• Expansion/Power stroke: the flame continues its travel across the combustion chamber. The high pressure in the cylinder pushes the piston downward. Energy is extracted from the burned gases in the process.

The 4-stroke engine cycle

• Exhaust stroke: the hot products flow rapidly out of the cylinder because of the relatively high pressure within the cylinder compared to that in the exhaust port.

Second Law of Thermodynamics:

RefrigeratorsIt is not possible for heat to flow from a colder body to a warmer body

without any work having been done to accomplish this flow.

Second Law of Thermodynamics:

RefrigeratorsEnergy will not flow spontaneously

from a lower temperature object to a higher temperature object.

“second form” or Clausius Statement

Second Law of Thermodynamics: Refrigerators

2 vs 7

Second Law of Thermodynamics: Entropy

• In any cyclic process the entropy will either increase or remain the same.

• Entropy is a measure of the amount of energy which is unavailable to do work.

• Entropy is a measure of the multiplicity of a system.

• Entropy is a measure of the disorder of a system.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html