Thermodynamics and Efficiency
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Transcript of 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
The Human Body as a Machine
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
The Human Body as a Machine
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
The 4-stroke engine cycle
I. Intake stroke.
The 4-stroke engine cycle
II. Compression stroke
The 4-stroke engine cycle
III. Expansion/Power
stroke
The 4-stroke engine cycle
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.
Second Law of Thermodynamics: Refrigerators
Energy will not flow spontaneously from a
lower temperature object to a higher temperature
object.
“second form” or Clausius Statement
Second Law of Thermodynamics: Refrigerators
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://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