Thermodynamics and Efficiency

Temperature and heat

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

of Heat transfer

When a hot object warms a

cooler object, does temp. flow

between them? Are the

temperature changes of the

two objects equal?

Image from http://image.slidesharecdn.com/chapter-13-powerpoint-1232611904984111-3/95/ap-physics-chapter-13-powerpoint-10-728.jpg?cb=1232611961

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.

Thermometric Scales

Thermal Expansion

Linear expansion

Volume expansion

Linear expansion

https://dagnygromer.files.wordpress.com/2008/05/hyrail.jpghttps://littlekiwi83.files.wordpress.com/2012/08/dsc_0123.jpg

Volume expansion

http://www.kshitij-iitjee.com/Study/Physics/Part3/Chapter19/2.jpg

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 = 4.186J

1 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°

Specific Heat Capacity (c)

Substance Specific heat capacity (J/kgK

or J/kg C°)

Water 4190

Vegetable oil 2000

Copper 390

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

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 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

Example A 0.50kg of water is initially at 30°C. What is its final

temperature after absorbing 16,200J of heat? Specific

heat capacity of water is 4190 J/kgK.

Example A certain amount of water is initially at 30°C. After

gaining 16,200J of heat, its final temperature is now

39°C. What is the mass of the water (in kg)? Specific

heat capacity of water is 4190 J/kgK.

Example A 0.50kg of water is initially at 30°C. What is the

amount of heat transferred if its final temperature is

0°C? Is heat gained or lost? Specific heat capacity of

water is 4190 J/kgK.

Example In a serving of ice cream and cake, you have gained

500 calories. To compensate, you decided to work by

climbing the stairs. If your mass is 60kg, how much

total height (in m) must you climb?

Example A Physics student consumed a 0.23kg of C2 (mostly

water) with 115 calories per 1.15 serving. How much

work must you perform to remove these calories?

FIRST LAW OF THERMODYNAMICSEnergy transformation

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

Thermal energy vs Heat

TE the energy a substance or system is related to its

temperature

Heat is transferred between substances or systems

due to a temperature difference between them

http://www.energyeducation.tx.gov/energy/section_1/topics/forms_of_energy/thermal_energy.html

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

Does the system expand or

compress?

True or False? 𝑃∆𝑉 ≠ 0

Isobaric Process

W,Q,∆U≠0

p(V2-V1 )= W

Image retrieved on Jan. 19, 2015. http://cnx.org/resources/9a61204de34230d2dd1b2e8be169e177/Figure_16_02_04a.jpg

Isothermal Process

W,Q,∆U≠0

Q = WIf ∆U=0 (Ideal gas)

Fuel

The Human Body as a Machine

Serving size

Total fat

Sugars


Serving size

Total fat

Sugars

1 g = 9 Cal

1 g = approx. 4.38 Cal

Protein 1 g = 4 Cal

It pays to read the

nutritional facts.

Serving size

Total fat

Sugars


Serving size

Total fat

Sugars

1 g = 9 Cal

1 g = approx. 4.38 Cal

Protein 1 g = 4 Cal

It pays to read the

nutritional facts.

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 engines

The 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


I. Intake stroke.


II. Compression stroke


III. Expansion/Power

stroke


IV. Exhaust stroke

Second Law of Thermodynamics: Refrigerators

It 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.


Energy will not flow spontaneously from a

lower temperature object to a higher temperature

object.

“second form” or Clausius Statement


Second Law of Thermodynamics: Steam Engines vs Refrigerators

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Second Law of Thermodynamics: Steam Engines vs Refrigerators

http://i.stack.imgur.com/vtb7E.png

http://www.proepowersystems.com/Engine_files/image008.gif

https://charchitecture.files.wordpress.com/2011/11/3refrig7.jpg

𝑒 =𝑊

𝑄𝐻= 1 +

𝑄𝐶𝑄𝐻

= 1 −𝑄𝐶𝑄𝐻

efficiency

https://realizeengineering.files.wordpress.com/2014/08/refrigeration-cycle.jpg

ExampleA gasoline engine in a large truck takes in 10,000 J of heat

and delivers 2,000 J of mechanical work per cycle. The heat is

obtained by burning gasoline. a) What is the thermal efficiency

of the engine? b) How much heat is discarded per cycle? c) if

the engine goes through 25 cycles per second, what is its

power output in Watts?

ExampleA certain steam turbine is able to transform 1,500,000

J of mechanical energy to 1,000,000 J of electrical

energy every second. a) How much energy is wasted

every second? b) What is the efficiency of the turbine?

ExampleWhat is the efficiency of a motor that is fuelled by

2,500 J of energy that can lift a 30 kg mass to a height

of 6 m from where the mass was?

Geothermal Power Plant

https://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/Diagram_VaporDominatedGeothermal_inturperated_version.svg/2000px-Diagram_VaporDominatedGeothermal_inturperated_version.svg.png

http://www.ucsusa.org/sites/default/files/images/2014/12/energy-renewable-geothermal-plant-nesjavellir-power-station-iceland.jpg

Source: http://www.southwestclimatechange.org/files/cc/figures/icecore_records.jpg, Image by Unknown

http://www.southwestclimatechange.org/files/cc/figures/icecore_records.jpg

http://media.pennlive.com/opinion/photo/climate-change-c1279739ffbefdc5.jpg

Climate change

Rising sea levels

Extreme heat events

Extreme stormsCO2

https://www.organicconsumers.org/sites/default/files/12309025826_94f29d3db9_o_0.jpg

Risks

to agriculture

natural resources

http://www.ecy.wa.gov/climatechange/whatis.htm

to human health

Thermodynamics and Efficiency

Education