lecture 1- thermodynamics

7/27/2019 Lecture 1- Thermodynamics

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1st Law of Thermodynamics

Heat Transfer

Lecture 1

August 25,2011


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Energy

Energy is the ability or capacity to do work

on some form of matter

Work is done on matter when matter is

either pushed, pulled, or lifted over some

distance

Potential energy how much work that

an object is capable of doing

PE = mgh

Kinetic energy the energy an object

possesses as a result of its motion

KE = mv2


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Laws of Thermodynamics

1st Law of Thermodynamics Energycannot be created or destroyed.

Energy lost during one process must equalthe energy gained during another

2nd Law of Thermodynamics Heat canspontaneously flow from a hotter object toa cooler object, but not the other way

around. The amount of heat lost by the warm

object is equivalent to the heat gained bythe cooler object


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First Law of Thermodynamics

Conservation of energy:q = e + w

The amount of heat (q) added to asystem is equal to the change in

internal energy (e) of the system plus

any work (w) done by the system


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Heat

Heat is a form of energy and is the totalinternal energy of a substance

Therefore the 1st law states that heat is reallyenergy in the process of being transferredfrom a high temperature object to a lower

temperature object. Heat transfer changes the internal energy of

both systems involved

Heat can be transferred by: Conduction

Convection

Advection

Radiation


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Specific Heat

Heat capacity of a substance is the ratio of

heat absorbed (or released) by that

substance to the corresponding

temperature rise (or fall)

The heat capacity of a substance per unit

mass is called specific heat.

Can be thought of a measure of the heat

energy needed to heat 1 g of an object by

1C

Different objects have different specific

heat values


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1 g of water must absorb about 4 times asmuch heat as the same quantity of air to raiseits temperature by 1 C

This is why the water temperature of a lake orocean stays fairly constant during the day,while the temperature air might change more

Because of this, water has a strong effect on

weather and climate

Substance Value (J g1K1)

Liquid Water 4.183

Ice 2.050

Wood 0.420

Sand 0.835

Air 1.012


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Latent Heat

Latent heat is the amount of energyreleased or absorbed by a substance

during a phase change

LIQUID

2260 J/g

released

334 J/g

released

2260 J/g

absorbed

334 J/g

absorbed

FOR WATER:

Lowest energy

Highest energy

SOLID

SOLID

LIQUID

LIQUID

GAS

GAS


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Example 1: Getting out of a swimming pool

In the summer, upon exiting a swimming pool

you feel cool. Why?

Drops ofliquid water are still on your skin after

getting out. These drops evaporate into water vapor. This

liquid to gas phase change causes energy to be

absorbed from your skin.


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Example 2: Citrus farmers

An orange crop is destroyed iftemperatures drop below freezing for

a few hours. To prevent this, farmers spray water

on the orange trees. Why?

When the temperature drops below32oF, liquid water freezes into ice.

This liquid to solid phase changecauses energy to be released to thefruit.

Thus, the temperature of the orangeremains warm enough to preventruin.


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Example 3: Cumulus clouds

Clouds form when water vapor condenses into tiny

liquid water drops. This gas to liquid phase change causes energy to

be released to the atmosphere.

The release of latentheat during cloudformation drivesmany atmospheric

processes.


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Types of Heat Transfer

Heat can be transferred by:

ConductionConvection

AdvectionRadiation


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Conduction Conduction is the transfer of heat from molecule

to molecule within a substance Molecules must be in direct contact with each

other

If you put one end of a metal rod over a

fire, that end will absorb the energy fromthe flame.

Molecules at this end of the road will gain

energy and begin to vibrate faster

As they do, their temperature increasesand they begin to bump into the molecules

next to them.

The heat is being transferred from the

warmer end to the colder end, and

eventually to your finger.


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Conduction

The measure of how well a substance can

conduct heat depends on its molecularstructure.

Air does not conduct heat very well

This is why, in calm weather, the hot ground

only warms the air near the surface a few

centimeters thick by conduction!

SubstanceHeat

Conductivity

Still air at 20 C 0.023

Water at 20 C 0.60

Ice 2.1

Granite 2.7

Iron 80


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Convection Convection is the transfer of heat by the

mass movement of a fluid (such as water andair) in the vertical direction (up and down)

Convection occurs naturally in the

atmosphere On a sunny day, the Earths surface is heated

by radiation from the Sun.

The warmed air expands and becomes less

dense than the surrounding cold air.

Because the warmed air is less dense

(weighs less) than cold air, the heated air

rises.


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As the warm air rises, the heavier cold air flowstoward the surface to replace the rising air.

This cooler air becomes heated in turn and rises.

The cycle is repeated.

This vertical exchange of heat is called convection

and the rising air parcels are known as thermals

Convection


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Convection The warm thermals cool as they

rise.

In fact, the cooling rate as a parcelrises can be calculated If the thermal consists of dry air, it

cools at a rate of ~10C/km as itrises. This is called the lapse rate.

Convection is one process bywhich clouds can form.

As the temperature of the thermalcools, it may reach a point where

it reaches saturation (the temp.and dewpoint are the close to thesame)

Thermals condense and form a

cloud.


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Advection Advection is the transfer of heat in the

horizontal direction. The wind transfers heat by advection

Happens frequently on Earth

Two types:

Warm air advection (WAA): wind blows warm airtoward a region of colder air

Cold air advection (CAA): wind blows cold airtoward a region of warmer air


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Cold AirAdvection Warm Air

Advection


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All things with a temperature above

absolute zero emit radiation Radiation allows heat to be transferred

through wave energy

These waves are called electromagneticwaves The wavelengths of the radiation emitted

by an object depends on the temperatureof that object (i.e., the sun mainly emitsradiative energy in the visible spectrum,and the earth emits radiative energy in theinfrared spectrum).

Shorter wavelengths carry more energy

than longer wavelengths

Radiation


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A photon of ultra-violet radiation carries moreenergy than a photon of infrared radiation.

The shortest wavelengths in the visible spectrumare purple, and the longest wavelengths are red.


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RadiationEmitted radiation can be:

Absorbed

Increasing the internal energy of the gas molecules.

Reflected

Radiation is not absorbed or emitted from an object but it

reaches the object and is reflected back. The Albedorepresents the reflectivity of an object and describes the

percentage of light that is sent back.

Scattered

Scattered light is deflected in all directions, forward,backward, sideways. It is also called diffused light.

Transmitted

Radiation not absorbed, reflected, or scattered by a gas. The

radiation passes through the gas unchanged.


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Examples of Heat Transfer

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Kirchoffs Law

Good absorbers of a particular wavelength aregood emitters at that wavelength and vice versa

Our atmosphere has many selective absorbersCarbon Dioxide, Water Vapor, etc

These gases are good at absorbing IR radiationbut not solar radiation

Thus these gases are called greenhouse gases

due to the fact they help to absorb and reemit IRradiation back toward the Earths surface thuskeeping us warmer then we would otherwise be


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Solar Radiation Budget


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Earth-Atmosphere Energy Balance


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lecture 1- thermodynamics

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