THERMODYNAMICS ENGR 103 Lecture 11
Chapter 6
Introduction
Pressure
Temperature
Energy
Day 11 – Student Outcomes
Students should be able to:
• Identify the 1st and 2nd laws of Thermodynamics
• Pressure – pick up from lecture 9 • Knowing what atmospheric pressure is
• Familiar with units of pressure
• Understanding the difference between gauge, absolute, vacuum pressures and how to go back and forth
• Temperature – knowing the difference between a temp reading and temp change • Unit conversations for each above
• Knowing the difference between the relative and absolute scale
• Energy – definition (capacity to do work) • Potential Energy – stored energy
• Spring – formula and how to use it
• Gravitational – formula and how to use it
• Kinetic Energy – motion – formula and how to use it
• What internal energy – translation, rotation, vibration, sensible energy, latent energy – is
Thermodynamics – the science of energy transformation and utilization.
Pressure
Temperature
Energy
Work
Heat
Power & Efficiency
Common thermodynamic engineering
systems
• Cooking with a microwave, oven, toaster, stove
• Heating and cooling systems
• Internal combustion and diesel engines
• Turbines, pumps, and compressors
• Heat exchangers, cooling towers
• Solar panels, wind turbines and wave energy
convertors
• Your body
• Fire
First Law of Thermodynamics
• Energy can be converted from one form to another, but
total energy remains constant (energy cannot be created
or destroyed).
http://www.googlelearn.com/first-law-of-thermodynamics/
Second Law of Thermodynamics
• Second Law of Thermodynamics – Energy conversions
naturally occur in one direction, from high quality to lower
quality states.
http://cheezburger.com/5720232192
http://ffden-
2.phys.uaf.edu/212_spring2011.web.dir/Mitchell
_Titus/2nd_and_3rd_Law.html
Pressure
Normal force exerted per unit area by a fluid
Where,
P = pressure
F = normal force
A = area
SI units = N/m2 = Pascal [Pa]
US = lbf/in 2 = psi
also bar, mm Hg, in Hg, torr (see unit conversion sheet)
A
F P
Pressure Example # 1
Who/what exerts more pressure on the ground: an
elephant or a 100 pound woman wearing high heels?
An elephant will have at least 2 feet on the ground at all
times. A typical surface area for the bottom of its foot is 40
square inches. Male African elephants are the largest
surviving terrestrial animal and can reach a weight of
15,000 lbf.
𝑃 = 𝐹
𝐴 = 15,000 𝑙𝑏𝑓
2 ∗ 40 𝑖𝑛2 = 190 𝑝𝑠𝑖
Pressure Example #1
Who/what exerts more pressure on the ground: an elephant or a woman wearing high heels?
A typical woman’s stiletto has a maximum diameter of 0.4 inches. At some points in a person’s gait, all weight is on the heel of one shoe.
𝑃 = 𝐹
𝐴 =
100 𝑙𝑏𝑓 𝜋 4 ∗ 0.4 𝑖𝑛 2
= 800 𝑝𝑠𝑖
A 100 pound woman in stilettos exerts far more pressure on the ground than the largest elephant!
Pressure – Types & Relationships
Atmospheric [Patm] – pressure exerted at a certain location
Standard atmospheric pressure = 1 atm (at sea level)
= 14.696 lbf per square inch [psi]
= 101,325 Pascal [Pa]
*Assume standard atmospheric pressure = 1atm unless stated
otherwise.
Pressure – Types & Relationships
• Gauge [Pgauge] – the pressure that uses atmospheric
pressure as the reference; difference between absolute
pressure and local atmospheric pressure
• Absolute [Pabs] – pressure referenced to a perfect vacuum
• Vacuum [Pvac] – a pressure below atmospheric pressure
Pgauge = Pabsolute – Patmospheric
Pvacuum = Patmospheric – Pabsolute
Pressure – Types & Relationships
Pgauge = Pabsolute – Patmospheric
Pvacuum = Patmospheric – Pabsolute
Use this
for pressures
above
atmospheric
pressure
Use this
for pressures
below
atmospheric
pressure
Pressure Example # 2
The local atmospheric pressure in Denver is 83.4 kPa. If a car
tire is inflated to a gauge pressure of 35 psi, what is the
absolute pressure in kPa?
Pgauge = Pabsolute – Patmospheric → Pabsolute = Patmospheric + Pgauge
Conversion of gauge pressure:
Solve for absolute pressure:
Pressure Example #3
Express the following absolute pressures as either gauge
or vacuum pressure as appropriate: a) 17.00 psi and b)
70.0 kPa.
Temperature
• Temperature – measure of atomic and molecular kinetic
energy of a substance
• Temperature difference
• Indicator of heat transfer
• Capacity to perform work (heat engines)
Temperature – measure of atomic and molecular kinetic energy of a substance.
Relative
• (SI) Celsius [ºC]
• (AES, USCS) Fahrenheit [ºF]
Absolute
• (SI) Kelvin [K] – note lack of
degree symbol
• (AES, USCS) Rankine [ºR]
Boiling point of water
100 °C = 212 °F
Freezing point of water
0 °C = 32 °F
Room temperature
21 °C = 70 °F
Absolute zero
0 K = – 273 °C
0 °R = – 460 °F
Temperature Conversions
Converting a Measured Temperature
Relating F to C:
T [F] = 1.8 T [C] + 32
Relating C to F:
T [C] = (T [F] – 32) / 1.8
Relating K to C:
T [K] = T [C] + 273
Relating R to F:
T [R] = T [F] + 460
Converting a Change in Temperature and Material properties (scalar).
1 C ≡ 1.8 F
1 C ≡ 1 K
1 F ≡ 1 R
1 K = 1.8 R
Used for specific heat, thermal conductivity, convection coefficient, coefficient of expansion, etc. (see Unit Conversion Sheet).
Energy – capacity to do work.
• Potential Energy – the stored energy of the position
possessed by an object.
• Elastic potential energy – spring or elastic solid – PEspring = ½ kx 2
• Gravitational potential energy – energy that a system possess by
virtue of its elevation with respect to a reference in a gravitational
field – PEgravitational = mgz
http://www.thinkenergytypes.com/potential-energy.html
Potential Energy Spring Example # 4
http://www.chegg.com/homework-help/questions-nd-answers/note-question-
asking-length-block-travels-relative-equilibrium-position-spring-compressed–
q2638741
How much potential energy does
the spring have in the diagram
shown?
k= 14 N/m
Potential Energy Gravitational Example # 5
m= 100 kg
Energy – capacity to do work.
Kinetic Energy – the energy that a system possess
as a result of its motion with respect to a reference
frame.
KE = ½ mv2
http://www.sciencelearningspace.com/
Kinetic Energy Example # 6
Kenyan Drake (210 lbf) can run the 40 in 4.34 s.
Cam Robinson (326 lbf) can run the 40 in 5.28 s.
Who has the highest average kinetic energy while running
the 40?
http://www.bing.com/images/search?q=kenyan+drake+images&view http://www.bing.com/images/search?q=cam+robinson+images&view
Energy – capacity to do work.
Internal Energy – the sum of all the microscopic forms of energy, U.
1. Molecules translate, rotate, and vibrate
2. Electrons orbit and spin, nucleus spins
3. Sensible energy – the energy required to change the temperature of a system
Q = m cp ΔT
• Q – change in energy (for a ΔT)
• cp – specific heat (kJ/kg·K)
• m – mass
• ΔT – change in temperature
Internal Energy – Continued
4. Latent Energy – the amount of energy required to
produce a phase change
Qf = m ΔHf
• ΔHf – heat of fusion
Qv = m ΔHv
• ΔHv – heat of vaporization
• If a substance undergoes a phase change during a change in
temperature the energy required is Q plus the Latent Energy
Internal Energy Example # 7
How much energy does it take to heat up 27 kg of water
from 3°C to 92°C? (cp of liquid water is 4.187 kJ/kg·K)
How do these different types of energy relate?
The 1st Law of Thermodynamics!
More next time…
Practice Problems #1 – Temperature
A) Which of the following is
the highest temperature?
a) 150 °C
b) 150 K
c) 150 °R
Compare all in units of °F
B) Which of the following has the highest change in temperature?
a) 5 lbm / °F
b) 3 kg / K
c) 3 lbm / °R
Compare all in units of kg/°C
Problem Problems #1 – Solution
1) Which of the
following is the highest
temperature?
a) 150 °C = 302 °F
b) 150 K = -190 °F
c) 150 °R = -310 °F
2) Which of the following
has the highest change
in temperature?
a) 5 lbm/°F = 4.08 kg/ °C
b) 3 kg/K = 3 kg/°C
c) 3 lbm/°R = 2.45 kg/°C
Practice Problems
• Page 193, problems 1 – 5
To do: • Iso and Ortho (33) plates due Friday, 9-18
• Read Ch. 6.4-5
• HW#11 – due Wed, 9-16
#1: http://youtu.be/icXlTShblj0
Watch this video. As an experiment, you press on 1 nail and determine your pain threshold is 6 lbf for that 1 nail. Design a bed of nails you would be willing to lay down on if your weight is assumed to be 150 lbf. State all assumptions.
#2: A vertical, frictionless piston-cylinder device contains a gas. The piston has a mass of 3 kg and a radius of 5 cm. A downward force of 75 N is applied to the piston. If the atmospheric pressure is 102.2 kPa, find the absolute pressure inside the cylinder. Use an appropriate SI prefix in your answer. Continued on next page Figure for Problem #2
To do (continued):
• HW#11 – due Wed, 9-16 (continued)
#3: A pressure gauge in a UA lab reads 16.8 psi. Can this pressure be
vacuum pressure? Why or why not? If you think it can, convert to
absolute pressure. If you think it can’t, assume that it is gauge
pressure, and convert to absolute pressure.
#4: Typical ocean water freezes at 28.4°F. What is this temperature in
°R, °C, and K?
#5: The specific heat of aluminum is 0.215 𝐵𝑡𝑢
𝑙𝑏𝑚−℉ . What is the specific
heat of aluminum in 𝐵𝑡𝑢
𝑙𝑏𝑚−°𝑅 ,
𝐽
𝑘𝑔−℃ , and
𝐽
𝑘𝑔−𝐾 ?
