Temperature Thermal Expansion Ideal Gas Law Heat Transfer ...Lwillia2/41/41Ch17_s14.pdfPhase Change:...

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•Temperature

•Thermal Expansion

•Ideal Gas Law

•Kinetic Theory

•Heat

•Heat Transfer

•Phase Changes

•Specific Heat

•Calorimetry

Zeroeth Law

• Two systems individually in thermal

equilibrium with a third system (such as a

thermometer) are in thermal equilibrium

with each other.

• That is, there is no flow of heat within a

system in thermal equilibrium

1st Law of Thermo

• The change of internal energy of a system

due to a temperature or phase change is

given by (next chapter):

Temperature Change: Q = mcT

Phase Change: Q = mL

• Q is positive when the system GAINS heat

and negative when it LOSES heat.

2nd Law of Thermo

• Heat flows spontaneously from a substance

at a higher temperature to a substance at a

lower temperature and does not flow

spontaneously in the reverse direction.

• Heat flows from hot to cold.

• Alternative: Irreversible processes must

have an increase in Entropy; Reversible

processes have no change in Entropy.

• Entropy is a measure of disorder in a system

3rd Law of Thermo

It is not possible to

lower the

temperature of any

system to absolute

9( ) ( ) 32

5T F T C

5( ) ( ) 32

9T C T F

( ) ( ) 273.15T K T C

Temperature is measured by a thermometer.

Kelvin is the Absolute Scale.

What is "room temperature" (68 degrees F) in Celsius and Kelvin?

5( ) ( ) 32

9T C T F

( ) ( ) 273.15T K T C

568 32

9 20 C

293.15K

Do book quiz 2!

30 is HOT.

20 is NICE.

10 is CHILLY.

Zero is ICE!

•Heat Energy is a flow of energy from hotter to colder because of a

difference in temperature. Objects do not have heat. [Heat] = Joule

• Internal Energy of a system is a measure of the total Energy due to

ALL random molecular motions INTERNAL of the system

(Translations KE, Rotational KE, Vibrational KE) and internal

POTENTIAL energies due to interactive forces (electromagnetic,

strong, weak, gravitational) Objects have energy.

•Mechanical Energy is due to the kinetic and potential energies of

the system itself in an external reference frame.

•Mechancial Equivalent of Heat: mechanical energy converted to

heat energy by doing work on the system: 1.000 kcal = 4186J

A 10,000 kg truck applies the

brakes and descends 75.0 m at

a constant speed, causing the

brakes to smoke as shown. If

the brakes have a mass of

100.00 kg and a specific heat of

800 J/kgC, calculate the

temperature increase of the

brakes. truck brakesm gh m c T

92.0truck

brakes

m ghT C

•Heat Energy is a flow of energy from hotter to colder because of a

difference in temperature. Objects do not have heat. [Heat] = Joule

•Heat Energy entering or leaving a system will cause either a

Temperature Change: Q = mcT or a Phase Change: Q = mL

• The change of internal energy of a system due to a temperature or phase change is given by:

Temperature Change: Q = mcT

Phase Change: Q = mL

• Q is positive when the system GAINS heat and negative when it LOSES heat.

Specific Heat: Thermal Inertia

The Specific Heat of a substance is the amount of Energy it

requires to raise the temperature of 1 kg, 1 degree Celsius.

Q mc T 0

m T kg C

•The higher the specific heat, the more energy it takes and

the longer it takes to heat up and to cool off.

•The lower the specific heat, the less energy it takes and the

quicker it takes to heat up and cool off.

•Substances with HIGH specific heat STORE heat energy

and make good thermal moderators. (Ex: Water, Oceans)

Some Specific Heat Values

More Specific Heat Values

Specific Heat

Why does water have such a high

specific heat?

Heat goes into other modes of energy so

that temperature changes slowly.

glycerin

Q mc T

How much heat is required to raise the temperature of a

0.750kg aluminum pot containing 2.50kg of water at 30ºC to

the boiling point?

Al Al w wQ m c T m c T

.75 (900 / ) 2.5 (4186 / ) (70 )kg J kg C kg J kg C C

Al Al w wm c m c T

57.798 10Q x J

Phase Change Q mL

•A change from one phase to another

•A phase change always occurs with an exchange of energy!

•A phase change always occurs at constant temperature!

Sample Latent Heat Values

Phase Change

Energy goes into the system and breaks molecular bonds..

Energy is given up by the system by forming molecular bonds

Melting: Energy goes into the system and breaks molecular bonds..

Freezing: Energy is given up by the system by forming molecular bonds

Phase Change: Melting & Freezing

•Melting: Solid to Liquid @ the melting temperature

•Melting is a cooling process

•Freezing: Liquid to Solid @ the melting temperature

•Freezing is a warming process.

Why do farmers spray peaches with water to

save them from frost?

Freezing is a warming process!

If you were in an igloo on a freezing

night. You would be warmed more by

a) a bucket of ice melting.

b) a bucket of water freezing

c) the same either way.

d) neither - are you nuts?

Phase Change: Evaporation •Takes place at the surface of a liquid due to escaping

molecules.

•Occurs at all temperatures

•Evaporation occurs when water vapor pressure in the liquid

exceeds the pressure of water vapor in the surrounding air.

•Evaporation is a cooling process.

Evaporation is a Cooling Process

Phase Change: Boiling •Boiling is evaporation under the surface of the liquid.

•Liquid boils at the temperature for which its vapor pressure

exceeds the external pressure (mostly atmospheric pressure.)

•Boiling point depends on temperature AND pressure:

•@ 1 atm, bp of water is 100ºC, @ 5atm, bp of water is 374

•Boiling is a cooling process.

•At low pressures, liquids are boiled (‘freeze-dried’) into

solids.

Phase Change: Condensation

•Gas molecules condense to form a liquid.

•Condensation is a warming process

•Why is a rainy day warmer than a cloudy or clear day in

winter?

•Why do we feel uncomfortable on a muggy day?

Condensation is a Warming Process

Phase Change: Humidity

•Vapor is the gas phase of a substance below its boiling

temperature.

•Air can ‘hold’ only so much water vapor before it becomes

saturated and condensation occurs. Humidity is a measure of

vapor density.

•Warm air can hold more water vapor. More condensation occurs

at cooler temperatures because the molecules are moving slower.

Slow moving water molecules coalesce upon collision.

Windward: Wet

Leeward: Dry

Pushed

Cools and condenses at Top

Stormy Weather

When warm air rises, it expands and cools.

The water vapor in the air soon condenses

into water droplets, which form clouds and

eventually these droplets fall from the sky as

Phase Change:Sublimation

The conversion of a solid directly to a gas & visa versa

Examples: snowflakes, Moth Balls, dry ice

Phase Change: Triple Point

A temperature and pressure at which all three

phases exist in equilibrium.

Freezing-Melting Evaporation

-Condensation

Sublimation

Lines of

equilibrium

Phase Change

Phase change occurs at a Constant Temperature!

Latent Heats of: Fusion & Evaporation Lf, Lv

334 / solid-liquid

2256 / liquid-gas

L kJ kg

Water:

Phase Change: Water

How much steam @ 100 °C does it take to melt 1kg of ice at -30 °C?

2256 /

2090 /

4186 /

L kJ kg

c J kg C

•How much energy is needed to raise the ices to 0 °C

•How much energy is needed to melt 1kg of ice?

•How much energy is given up by the steam?

•What happens to the steam that is melting the ice?

Phase Change: Water

2256 /

2090 /

4186 /

L kJ kg

c J kg C

How much energy is needed to raise the ices to 0 °C

1 1 (2090 / )(30 )Q kg J kg C C

62700J

Phase Change: Water

2256 /

2090 /

4186 /

L kJ kg

c J kg C

How much energy is needed to melt 1kg of ice?

2 334Q kJ

1 (334 / )kg kJ kg

1 62700Q J

2 334Q kJ

Phase Change: Water

2256 /

2090 /

4186 /

L kJ kg

c J kg C

1 62700Q J

2 334Q kJ

•How much energy is given up by the steam?

•What happens to the steam that is melting the ice?

397totalQ kJ

The First Law of Thermodynamics

• The First Law of Thermodynamics is a special case of the Law of Conservation of Energy

– It takes into account changes in internal energy and energy transfers by heat and work

• Although Q and W each are dependent on the path, Q + W is independent of the path

intE Q W

Work in Thermodynamics • Work can be done on a deformable

system, such as a gas

• Consider a cylinder with a moveable

piston

• A force is applied to slowly compress the

– The compression is slow enough for

all the system to remain essentially in

thermal equilibrium

– This is said to occur quasi-statically

ˆ ˆ dW d F dy Fdy PA dy PdV F r j j

dW PdV

• Interpreting dW = - P dV

– If the gas is compressed, dV is negative and the

work done on the gas is positive

– If the gas expands, dV is positive and the work

done on the gas is negative

– If the volume remains constant, the work done

is zero

• The total work done is:

VW P dV

PV Diagrams

• Used when the pressure and

volume are known at each step

of the process

• The state of the gas at each step

can be plotted on a graph called

a PV diagram

– This allows us to visualize the

process through which the gas is

progressing

• The curve is called the path

VW P dV

Problem

Work Done By Various Paths

( )f f iW P V V

VW P dV

( )i f iW P V V ( )W P V dV

The work done depends on the path taken!

Not necessarily

an isotherm!

Isothermal Process • At right is a PV diagram of an isothermal

expansion

• The curve is a hyperbola

• The curve is called an isotherm

• The curve of the PV diagram

indicates PV = constant

– The equation of a hyperbola

• Because it is an ideal gas and the

process is quasi-static,

PV = nRT and

nRT dVW P dV dV nRT

VW nRT

Isobaric Processes

• An isobaric process is one that occurs at a

constant pressure

• The values of the heat and the work are

generally both nonzero

• The work done is W = -P (Vf – Vi) where P

is the constant pressure

intE Q W f

VW P dV PV nRT

Isovolumetric Processes

• An isovolumetric process is one in which there is

no change in the volume

• Since the volume does not change, W = 0

• From the first law, Eint = Q

• If energy is added by heat to a system kept at

constant volume, all of the transferred energy

remains in the system as an increase in its internal

energy

intE Q W f

VW P dV PV nRT

Isothermal Process

• An isothermal process is one that occurs at

a constant temperature

• Since there is no change in temperature,

Eint = 0

• Therefore, Q = - W

• Any energy that enters the system by heat

must leave the system by work

intE Q W f

VW P dV PV nRT

Adiabatic Process

• An adiabatic process is one during which no energy enters or leaves the system by heat: Q = 0

– This is achieved by: • Thermally insulating the walls of the system

• Having the process proceed so quickly that no heat can be exchanged

• Since Q = 0, Eint = W

• If the gas is compressed adiabatically, W is positive so Eint is positive and the temperature of the gas increases

• If the gas expands adiabatically, the temperature of the gas decreases

• Examples of adiabatic processes related to engineering are: – The expansion of hot gases in an internal combustion engine

– The liquefaction of gases in a cooling system

– The compression stroke in a diesel engine

– Adiabatic free expansion of a gas

• The gas expands into a vacuum, no piston: W = 0

• Since Q = 0 and W = 0, Eint = 0 : initial and final states are the same, no change in temperature is expected.

intE W

intE Q W f

VW P dV

Thermo Processes • Adiabatic

– No heat exchanged

– Q = 0 and Eint = W

• Isobaric

– Constant pressure

– W = P (Vf – Vi) and Eint = Q + W

• Isovolumetric

– Constant Volume

– W = 0 and Eint = Q

• Isothermal

– Constant temperature

Eint = 0 and Q = -W

intE Q W

VW nRT

Was ist das?

Fig. 20-9, p. 569

The First Law of Thermodynamics

• The First Law of Thermodynamics is a special case of the Law of Conservation of Energy

– It takes into account changes in internal energy and energy transfers by heat and work

• Although Q and W each are dependent on the path, Q + W is independent of the path

intE Q W

Cyclic Processes

• A cyclic process is one that starts and ends in the same state

– On a PV diagram, a cyclic process appears as a closed curve

• If Eint = 0, Q = -W

• In a cyclic process, the net work done on the system per cycle equals the area enclosed by the path representing the process on a PV diagram

intE Q W

A gas is taken through the cyclic process as shown.

(a) Find the net energy transferred to the system by heat during one complete cycle. (b) What If? If the cycle is reversed—that is, the process follows the path ACBA—what is the net energy input per cycle by heat?

intE Q W f

VW P dV

A sample of an ideal gas goes through the process as shown. From A to B, the process is adiabatic; from B to C, it is isobaric with 100 kJ of energy entering the system by heat. From C to D, the process is isothermal; from D to A, it is isobaric with 150 kJ of energy leaving the system by heat. Determine the difference in internal energy E(B) – E(A).

intE Q W f

VW P dV PV nRT

Problem

•Heat flows from HOT to COLD

•Conduction (solids)

•Convection (liquids & gases)

•Radiation (solids, gases, plasma)

Energy transferred via molecular collisions

•Good Conductors: Most Metals (free electrons!)

•Bad Conductors: Organic & Inert Materials

•Good Insulators: Air, Water, Wood

•Good Conductors are BAD Insulators

•& Visa Versa

Heat energy is transferred in solids

by collisions between free electrons

and vibrating atoms.

The heat Q conducted during a time t through a material with

a thermal conductivity k. dT/dx is the Temperature Gradient.

dTP kA

Some Thermal Conductivities

Temperature Gradient

h cdT T T

The quantity |dT / dx| is called the temperature gradient

Q dTkA

Conduction Problem

A bar of gold is in thermal contact with a bar of silver of the

same length and area as shown. One end of the compound

bar is maintained at 80.0°C while the opposite end is at

30.0°C. When the energy transfer reaches steady state, what

is the temperature at the junction? Ignore thermal

expansion of the metals.

h cT TkA

In the same room, at the same

temperature, the tile floor feels

cooler than wood floor.

How can they be the same

temperature?

Hot Air rises, expands and cools, and then sinks back down

causing convection currents that transport heat energy.

Hot air rises because fast moving molecules tend to migrate toward

regions of least obstruction - UP - into regions of lesser density!

Rising air cools because a decrease in density

reduces number of collisions & speeds decrease.

As the air cools, it becomes denser, sinking down,

producing a convection current.

Uneven heating on the earth and over water cause convection

currents in the atmosphere, resulting in WINDS.

Global wind patterns (Trade Winds, Jet Streams) are due to

convection current from warmer regions (equator) to cooler

regions (poles) plus rotation of Earth.

Convection Currents in the Ocean (Gulf Stream)

transport energy throughout the oceans.

Air & Ocean Convection causes

the WEATHER.

Convection between water and land causes the Winds.

Sea Breeze

High Pressure

Dry Warm Weather

Low Pressure

Stormy Weather

Electromagnetic Radiation is emitted and absorbed via atomic

excitations. All objects absorb and emit EM waves.

Electromagnetic Radiation is emitted and absorbed via atomic

excitations. All objects absorb and emit EM waves.

When an object it heated it will

glow first in the infrared, then the

visible. Most solid materials break

down before they emit UV and

higher frequency EM waves.

Frequency ~ Temperature

Stefan’s Law

• P = σAeT 4

– P is the rate of energy transfer, in Watts

– σ = 5.6696 x 10-8 W/m2 . K4

– A is the surface area of the object

– e is a constant called the emissivity

• e varies from 0 to 1

• The emissivity is also equal to the absorptivity

– T is the temperature in Kelvins

A good absorber reflects little and appears Black

A good absorber is also a good emitter.

4P e T A

Radiant heat makes it impossible to stand close to a hot

lava flow. Calculate the rate of heat loss by radiation

from 1.00 m2 of 1200C fresh lava into 30.0C

surroundings, assuming lava’s emissivity is 1.

The net heat transfer by radiation is: 4 4

2 1( )P e A T T

8 4 2 4 41(5.67 10 / )1 ((303.15 ) (1473.15 ) )x J smK m K K

266P kW

Fur is filled with air. Convection currents are slow

because the convection loops are so small.

How do fur coats keep you warm?

Any two systems placed in thermal

contact will have an exchange of heat

energy until they reach the same

temperature.

If the systems are in thermal

equilibrium then no net changes take

place.

Why is winter cold and summer

Intensity: The Radiation Power, P, passing through an area,

Why are cloudy nights warmer

than cold nights?

The heating effect of a medium such as glass or the Earth’s

atmosphere that is transparent to short wavelengths but opaque

to longer wavelengths: Short get in, longer are trapped!

CO2 & Temperature Change

Impacts of a Warming Arctic

The Arctic Climate Impact Assessment, a study

commissioned by the United States and the seven

other countries with Arctic territory, projects that

rising global concentrations of heat-trapping

emissions will drive up temperatures particularly

quickly at high latitudes.

RISING SEAS One of the most important

consequences of Arctic warming will be increased

flows of meltwater and icebergs from glaciers and

ice sheets, and thus an accelerated rise in sea

levels.

Forrest vs Tundra

Caught between rising seas on one side and expanding

shrub-filled zones to the south, tundra ecosystems around

the Arctic will likely shrink to their smallest extent in at

least 100 years, the scientists concluded. This could

reduce breeding areas for many tundra-dwelling bird

species and grazing lands for caribou and other mammals.

1 Meter Rise In Florida

ZEPO:A Melting Glacier in Tibet

"Thirty years ago, there was no river here.

If you come back here in another 30 years, one thing is for sure:

There will definitely be no more ice here."

-Dr. Yao Tandong,

Institute of Tibetan Plateau Research

Global Glacial Ice Melting

On Kilimanjaro in Kenya, an 11,700-year-old ice cap that

measured 4.3 square miles in 1912 had shrunk to 0.94 square

miles in 2000, and is projected to disappear altogether in

about 15 years. Melting of glaciers in Patagonia has doubled

in recent years.

Ice Caps Melting in Peru

In Peru, the Quelccaya ice cap retreated a rate of more than

600 feet a year from 2000 to 2002 - up from just 15 feet a year

in the 1960's and 70's - leaving a vast 80-foot-deep lake where

none had existed when his studies began.

Unless we change our

direction, we are likely to end

up where we are headed.

Happy Earth Day!

“The organic and inorganic components of Planet Earth have

evolved together as a single living, self-regulating system

Life maintains conditions suitable for its own survival.”

- James Lovelock

“It is much too late for sustainable

development; what we need is a sustainable

retreat.”

-James Lovelock, The Revenge of Gaia

“...we’re all astronauts aboard a little spaceship called Earth”

- Bucky Fuller

One island in one ocean...from space

Our Spaceship Earth

"We are on a spaceship; a beautiful one.

It took billions of years to develop.

We're not going to get another.”

- Bucky Fuller,

Operating Manual for Spaceship Earth

500,000 miles/hr 67,000 miles/hr

Space Ecology

Pressure Acts ONLY

Perpendicularly to the Surface

Pressure depends on depth.

Pressure IN a Fluid

•Is due to the weight of the fluid above you

•Depends on Depth and Density Only

•Does NOT depend on how much water is present

•Acts perpendicular to surfaces (no shearing)

•Pressure’s add

•At a particular depth, pressure is exerted equally in ALL directions

including sideways (empirical fact)

Temperature Thermal Expansion Ideal Gas Law Heat Transfer ...Lwillia2/41/41Ch17_s14.pdfPhase Change:...

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