Energy and Heat Transfer

AOS 101 Discussion Sections 302 and 303

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Energy Energy is the capacity to do

work Work is done on something

when it is either pushed, pulled, or lifted over some distance

Kinds of energy Kinetic energy

KE = ½mv2

Potential energy PE = mgh

Mechanical energy Chemical energy Thermal energy Radiant energy

Laws of Thermodynamics 1st Law of Thermodynamics

Energy cannot be created or destroyed Energy lost during one process must equal the

energy gained during another 2nd Law of Thermodynamics

Heat can spontaneously flow from a hotter object to a cooler object, but not the other way around

The amount of heat lost by the warm object is equivalent to the heat gained by the cooler object

Conservation of Energy

Amount of heat added

Internalenergy

Work done

Heat Heat is a form of energy and is the total

internal energy of a substance Revisiting the 1st law

States that heat is really energy in the process of being transferred from 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

Heat Capacity and 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)

Specific heat The heat capacity of a substance per unit mass Can be thought of a measure of the heat energy

needed to heat 1 g of an object by 1ºC Different objects have different specific heat

values

Specific Heat 1 g of water must absorb about 4 times as

much heat as the same quantity of air to raise its temperature by 1º C

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

Because of this, water has a strong effect on weather and climateSubstance Specific Heat (J/gK)

Water (liquid) 4.183

Ice 2.050

Wood 0.420

Sand 0.835

Air 1.012

Latent Heat Latent heat is the amount of energy released 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

Lowest energy Highest energy

SOLID

SOLID

LIQUID

LIQUID

GAS

GAS

FOR WATER:

Orange Example Farmers spray their

oranges with water when a frost event is about to occur Why?

When the temperature drops below 32oF, liquid water freezes into ice.

This liquid to solid phase change causes energy to be released to the fruit.

Thus, the temperature of the orange remains warm enough to prevent ruin.

Swimming Pool Example

Why do you feel cool when you get out of the pool? Drops of liquid 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

Cumulus Cloud Example Formation of 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

Release of latent heat during cloud formation drives many atmospheric processes

Conduction Conduction is the transfer of heat from

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

other

Thermal Conductivity Thermal conductivity is the measure of how

well a substance can conduct heat Depends on its molecular structure

If it is a bad conductor, it is a good insulator

Substance Thermal Conductivity (W/Km)

Air 0.024

Soil 0.2

Asphalt 0.75

Glass 1.05

Stainless Steel 16

Copper 401

Silver 429

Convection Convection is the transfer of heat by the mass

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

physics.arizona.edu

Convection

Moist Convection As the temperature of an air parcel cools, it

may reach a point where it reaches saturation (the air temperature and dew point are the close to the same)

Air parcels condense and form a cloud

Advection Advection is the transfer of heat in the

horizontal direction The wind transfers heat by advection Occurs frequently Why is advection important?

Important for the formation of precipitation and fog

arcticstudies.pbworks.com

Types of Advection Two types

Warm air advection (WAA) Wind blows warm air toward a region of colder air

Cold air advection (CAA) Wind blows cold air toward a region of warmer air

www.aviationweather.ws

Radiation Radiation is the travel of energetic particles or

waves traveling through space or another kind of medium to heat it up For Example:

The suns rays traveling through space and reaching the Earth

The warmth from a fire pit Radiation back into space from a warm Earth

Black Body A perfect absorber and emitter of radiation

Radiation, Convection, and Conduction

Solar Radiation The sun’s rays do not hit all areas of the Earth

the same Factors that determine the amount of solar

radiation hitting the Earth Position on Earth (latitude, longitude, and

elevation) Time of day (shown below in UTC) Composition of the atmosphere Amount and thickness of clouds, if any Position of Earth in orbit around the sun (i.e. time

of year)

Solar Radiation

Solar Radiation Equinoxes

Where day and night are of equal length Vernal Equinox – March 20 Autumnal Equinox – September 23

Solstices Summer Solstice – June 21

Longest day of the year in the Northern Hemisphere Where the sun is at it’s northernmost point from the

equator Winter Solstice – December 21

Shortest day of the year in the Northern Hemisphere Where the sun is at it’s southernmost point from the

equator

How radiation changes with latitude and date

Solar Radiation Budget

Energy Budget

Radiation All things with a temperature above absolute

zero emit radiation Radiation allows heat to be transferred

through wave energy These waves are called electromagnetic waves

Wavelengths of the radiation emitted by an object depends on the temperature of that object i.e., the sun mainly emits radiative energy in the

visible spectrum, and the earth emits radiative energy in the infrared spectrum

Shorter wavelengths carry more energy than longer wavelengths

Radiation

A photon of ultra-violet radiation carries more energy than a photon of infrared radiation

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

Types of Radiation Energy can be:

Absorbed Increasing the internal energy of the gas molecules

Reflected Albedo is the percentage of the light reflected off an

object Scattered

Light deflected in all directions: forward, backward or sideways

Also called diffused light Transmitted

Absorb

e

dRefl

ecte

dTran

smitt

ed

Scattere

d

Kirchoff’s Law Good absorbers of a particular wavelength are

good emitters at that wavelength and vice versa

Our atmosphere has many selective absorbers Carbon dioxide, water vapor, etc… These gases are good at absorbing IR radiation

but not solar radiation These gases are called greenhouse gases

Due to the fact they help to absorb and reemit IR radiation back toward the Earth’s surface thus keeping us warmer then we would otherwise be

More Examples Energy