topic 3 thermal energy and heat

7/29/2019 Topic 3 Thermal Energy and Heat

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. 1

Temperature: measures the thermal energy of an object.

Heat: flow of thermal energy between objects with different

temperatures.

Types of Heat Transfer

conduction, convection, radiation

R-values for home insulation

Temperature and Phase ChangesHeating water and hurricane example

Thermal Efficiency of Heat Engine

Why thermal energy is "low-quality" energy!

Topic 3: Thermal Energy and Heat
http://www.suu.edu/faculty/colberg/Hazards/Hurricanes_Noreasters/Hurricane_Anim_2.html


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. 2

Motivation: Coal-fired Power Plant

Order the types of energy that occur in a coal-fired power plant from

start-to-finish. (4-digit answer)

(a) electrical (b) chemical (c) thermal (d) mechanicalWhat approximate percentage of the original energy in the coal is

"lost to heat" in a typical power plant?

(a) 25% (b) 60% (c) 95%


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. 3

What is Temperature?

Thermometer

The molecules in hot air move than in cold air.

(a) slower (b) faster


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. 4

Celsius Scale

40 C = 104 F

35 C = 95 F30 C = 86 F

25 C = 77 F

20 C = 68 F

15 C = 59 F

10 C = 50 F

5 C = 41 F

0 C = 32 F

REALLY hot dayH2O Boils

100 C

H2O

Freezes0 C 32 F

212 F

Room Temp.

Water Freezes

Which temperature scale is more sensitive?

(a) Fahrenheit (b) Celsius


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Conduction: Vibrations of atoms/molecules transfer heat.

(Atoms/molecules do NOT move from one region to another).

Convection: Air molecules move from hotter to colder regions.

Radiation: Objects radiate infrared electromagnetic waves, where hotter

objects radiate more energy.

Convection

Conduction

Radiation


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Choose the type of heat transfer for each of the following examples:

When the sun shines through windows into a room and the room

temperature rises.

When hot air rises to higher altitudes.

When the Earth's atmosphere is warmed by the sun's rays.

(1) conduction (2) convection (3) radiation (3-digit answer)


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. 7

Heat Transfer and R-values

The R-value of a material is calculated from how it transfers heat.

R-values depend on the type and thickness of material.If the thickness doubles, then the R-value doubles.

R-values of wall components are added together.

To make a better insulated wall, you should build it with material that

has a ________ R-value.

(a) higher (b) lower


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. 8

Total R-value of a Wall

What is the total R-value for this wall? (2-digit answer)

Sheetrock

R = 0.5

Plywood +

ShinglesR = 1.5

Insulation

R = 20


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Temperature Changes for Water

Specific heat (heat capacity) measures how much heat energy is required

to increase the temperature of 1 kg of a substance by 1 C (or 1 K).

It takes 1 kcal to heat 1 kg ofliquid water by 1 C.

It takes about 0.5 kcal to heat 1 kg of solid ice or gas vapor by 1 C.

Which water phase has a higher heat capacity?

(a) liquid (b) gas vapor

Which water phase will have a smaller temperature increase if you add

10 kcal of heat?

(a) liquid (b) gas vapor
http://en.wikipedia.org/wiki/Energyhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Energy


10/31. 10

Temperature Changes for Water

If you heated 0.3 kg of water (volume of a soda can) from 20 C to

100 C, then what is the heat added in kcal (same as food calories)?

Remember that it takes 1 kcal to heat 1 kg of liquid water by 1 C.

If you heated 4 kg of water (~1 gallon) using 160 kcal of heat energy,

then what would be the temperature increase of the water (C)?


11/31. 11

Phase Changes

Watch this animation to review states of matter of water and

the energy changes associated with changing state.


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Phase Changes for Water

ICE: Molecules vibrate in a crystal structure.

Liquid: Molecules move but are still attracted toeach other.

Gas/Vapor: Molecules move fast and are far apart.

Which phase is less dense? (a) Ice (b) Liquid Attraction due to

charged atoms.

H+ H+

O


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What happens when ice cubes at 0 C are heated to 105 C?

Select the steps in the order that they occur. (4-digit answer)

(a) Ice at 0 C melts.

(b) Water vapor heats up from 100 to 105 C.

(c) Liquid water heats up from 0 C to 100 C.

(d) Liquid water at 100 C evaporates and becomes a gas (vapor).

Do you think each step above requires the same quantity of heat?

(a) yes (b) no


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During phase changes, the temperature remains constant until enough

energy has been added to complete a phase transition.

Which phase change requires more energy?

(a) ice-to-liquid (b) liquid-to-vapor

Temperature vs. Heat Added for 1 kg H2O

Liquid to Vapor

Liquid

Vapor

0 C

100 C

Temp.

Heat Added0 80 kcal 180 kcal 720 kcal

Ice toLiquid

change = 540 kcal


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Phase & Temperature Changes for Water

Rank the energy changes for these steps from highest to lowest, i.e.,

the first choice will have the highest energy change. (3-digit answer)

(a) Step 1: Ice at 0 C melts to become liquid at 0 C.

(b) Step 2: Liquid water heats up from 0 C to 100 C.

(c) Step 3: Liquid water at 100 C evaporates to vapor at 100 C.

Liquid to VaporLiquid

Vapor

0 C

100 C

T

emp.

Heat Added0 80 kcal 180 kcal 720 kcal

Ice toLiquid

change = 540 kcal

Step 1Step 2

Step 3


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Phase & Temperature Changes for Water

It takes 1 kcal to raise the temperature of 1 kg of water by 1 C, but only

0.5 kcal to raise the temperature of 1 kg of ice or water vapor by 1 C.

Also, it takes 80 kcal to melt 1 kg of ice sitting at 0 C,

and 540 kcal to evaporate 1 kg of liquid water sitting at 100 C.

How many kcal are required to evaporate 2 kg of liquid water at 100 C

How many kcal are required to raise the temp. of2 kg ice by 20 C?


17/31. 17

Water Evaporation and Condensation

The energy required to evaporate 1 kg of water is _______ the

energy released when 1 kg of vapor condenses into liquid.Assume that the water is at the same initial and final temperatures.

(a) less than (b) equal to (c) greater than

Condensation =

Liquid droplets

Evaporation =

Humid Air

& C f C


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Phase & Temperature Changes for Cooling Water

A sample of water vapor at 110 C is cooled to 90 C.

The overall process is: (a) endothermic (b) exothermic

In what ORDER do the steps occur for this process? (3-digit answer)(a) cooling of liquid water

(b) cooling of water vapor

(c) condensation of water vapor to liquid water

Rank the ENERGY changes for these steps from highest to lowest.(3-digit answer)

(a) cooling of liquid water by 10 C

(b) cooling of water vapor by 10 C(c) condensation of water vapor to liquid water

H i W E i d C d i


19/31. 19

Hurricanes: Water Evaporation and Condensation

What features of the hurricane did you see?

Hurricane Wilma movie (2005)

H i N C diti
http://www.nasa.gov/mpg/149564main_wilma_sst.mpg


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Hurricanes: Necessary Conditions

WARM ocean water temperatures (> 80 F)! (stores ENERGY!!)

LOW PRESSURE core (creates circulation of moist air).

Hurricane Katrina (2005)

red = 85 to 95 F

H i Ai M t


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Hurricanes: Air Movement


Hurricane Animation

of Air Movement

Why does a hurricane have a spiral or rotating shape?

(a) Coriolis force (due to Earth's rotation) (b) Meniscus force

H i POWER!


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Hurricanes = POWER!

Energy released via RAIN (condensation)

In one day, assume about rain falls over a

400-mile radius (~20 trillion kg H2O).

Multiply by the heat released during

condensation (600 kcal/kg or 2500 kJ/kg)

Equals power of about 600 TW !!!

How many times greater is this hurricane

power than worldwide power consumption?

(a) 2x (b) 10x (c) 40x

Grey (1981) via Chris Landsea (NOAA) - PBS NOVAscienceNOW site

600 TW !!!

Th d i


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Thermodynamics

Laws of Thermodynamics

1. Energy is conserved.

Energy can change forms, but is neithercreated nor destroyed.

2. Disorder (or entropy) increases for a

spontaneous process.

Which is higher quality energy?

(a) electrical (b) thermal

Can 100 kJ of thermal energy be

converted into 100 kJ of electricity using

a "heat engine"?

(a) yes (b) no

High QualityEnergyLow Entropy

Low QualityEnergyHigh Entropy

H t E i d M i Effi i


24/31. 24

Heat Engine and Maximum Efficiency

A heat engine extracts mechanical or

electrical energy when material flows

between hot and cold reservoirs.

Maximum efficiency e of heat engine:

Temp. in Kelvin (add 273 to temp. in C)

hot

cold

T

T1e LESS than 1(or 100%)

To increase the efficiency of a heat engine, the temperature difference

between the hot and cold reservoirs should be: (a) smaller (b) larger

C l l ti f M i Effi i


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Calculation of Maximum Efficiency

Calculate the maximum efficiency of an engine operating between

27 C and 227 C. (Format = 0.XX )

T in Kelvin (add 273 to C)

If this engine extracted a total energy of 200 MJ from fuel, then how

many MJ of energy could be used as mechanical energy?

hot

cold

T

T1e

H t E i i C


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Heat Engine in a Car

Order the types of energy in a car engine from start-to-finish.

(a) thermal (b) chemical (c) mechanical (3-digit answer)

Only 15% of gas energy isused to move the car!

Tire Losses = 5%

Braking

Losses5%

Air Resistance

5%

Friction = 3%

Trans-mission,

Drivetrain18%

Kinetic

Energy15%

Accessories2%

Engine Friction = 18%

ThermodynamicLosses = 62%

Energ "Losses" for Car


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Energy "Losses" for Car

Rank from highest to lowest the following energy losses for an

automobile: (3-digit answer)

(a) Thermodynamic losses of a heat engine(b) Air resistance OR braking OR tire resistance (all about equal)

(c) Engine friction

Tire losses5%

BrakingLosses

5%

Air Resistance5%

Trans-

mission,Drivetrain

18%

KineticEnergy15%

Accessories2%

Engine Friction18%

Thermo Losses = 62%

Car Energy Efficiency and Safety


28/31. 28

Car Energy Efficiency and Safety

Fuel efficiency usually requires lighter, less powerful cars.

"Weight may be desirable in a steam roller, but nowhere else!" - H. FordBUT, what about safety?

What is more important to your safety in a car crash?

(a) weight of car (b) size of car

Car Safety


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Car Safety

Nationwide crash data confirm that SIZE confers SAFETY !

Therefore, we need moderately sized cars that are also lighter!

What is size related to that is important in a crash?(a) stopping distance (b) braking efficiency (c) crumple zone

Extrafatalitiesperyear

Same SizeReduceWEIGHT

Reduce SIZESame

Weight

350 more fatalities

600 fewer fatalities

Weight Reduction of Cars


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Weight Reduction of Cars

Lightweight carbon-composite crush structures can absorb 6 to 12 times

as much energy per kg as steel!

Have you ever been involved in a car accident that resulted in some

body damage to the car? (a) yes (b) no

Damaged Golf

UndamagedMcLaren

Eco Driving to Save Energy!


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Eco-Driving to Save Energy!

To increase gas mileage, drivers are starting to practice eco-driving.

Boost your gas mileage 20 to 40% by utilizing these techniques:

Moderately accelerate from a stop.

Try to avoid braking by coasting to traffic lights.

Drive about 60 mph on the highway. (lower air resistance!)

Do you minimize braking by anticipating traffic stops and lifting up

on the accelerator? (a) yes (b) no

Do you have other ideas about how to improve gas mileage?

Be a "feather foot" driver!!
http://upload.wikimedia.org/wikipedia/commons/4/40/Feather.svghttp://upload.wikimedia.org/wikipedia/commons/4/40/Feather.svghttp://theinspirationroom.com/daily/print/2008/8/volvo_start.jpg

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