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

http://youtu.be/icXlTShblj0

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 ?

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