heat energy difference in temperature. objects do not have heat. …lwillia2/p10/p10ch9.pdf ·...
TRANSCRIPT
•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 = mcΔT or a Phase Change: Q = mL
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.
• The change of internal energy of a system due to a temperature or phase change is given by:
Temperature Change: Q = mcΔTPhase Change: Q = mL
• Q is positive when the system GAINS heat and negative when it LOSES heat.
Specific Heat: Thermal InertiaThe 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= Δ 0
Q Jcm T kg C
= =Δ ⋅
•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)
Specific Heat
Why does water have such a highspecific heat?
Heat goes into other modes of energy so that temperature changes slowly.
0
0
0
4186
2410
452
water
glycerin
iron
Jckg C
Jckg C
Jckg C
=⋅
=⋅
=⋅
More Specific Heat Values
Q mc T= ΔHow 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= Δ + Δ
.75 (900 / ) 2.5 (4186 / ) (70 )kg J kg C kg J kg C C⎡ ⎤= +⎣ ⎦o o o
( )Al Al w wm c m c T= + Δ
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!
Warming or Cooling Process?
Phase Change: Melting•Solid to Liquid @ the melting temperature•Melting is a cooling process
Phase Change: Freezing•Liquid to Solid @ the melting temperature•Freezing is a warming process.
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
Phase Change: Melting & Freezing
Phase ChangeLatent Heats of: Fusion & Evaporation Lf, Lv
Q mL=
( )( )
334 / solid-liquid
2256 / liquid-gasf
v
L kJ kg
L kJ kg
=
=Water:
Sample Latent Heat Values
Phase Change: WaterHow much steam @ 100 °C does it take to melt 1kg of ice?
Q mL=
4
5
5 0
0
33.5 x 10 /
22.6 x 10 /
2.00 x 10 /
4186 /
f
v
ice
water
L J kg
L J kg
c J kg C
c J kg C
=
=
= ⋅
= ⋅
•How much energy is needed to melt 1kg of ice?•How much energy is needed to raise the ice to 100 °C ?•How much energy is given up by the steam? •What happens to the steam that is melting the ice?
Why do farmers spray peaches with water to save them from frost?
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 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 lower.•Boiling is a cooling process.•At low pressures, liquids are boiled (‘freeze-dried’) into solids.
Increasing the Pressure
Does increasing the external pressure increase or decrease the boiling
temperature of water?
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.
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 rain.
Windward: WetLeeward: Dry
WarmHumidAir PushedUp
Cools and condenses at Top
WarmDryAirFallsDown
The Water Cycle
Phase Change: Evaporation•Takes place at the surface of a liquid due to escaping molecules.•Cooling process : escaping molecules take energy with them!•Occurs at all temperatures•Boiling is evaporation under the surface of the liquid.•Why does blowing on hot coffee cool it?
Phase Change: Vaporization•Change of state from liquid to gas at the boiling point of the liquid•Cooling process : escaping molecules take energy with them!•Increasing the pressure increases the boiling temperature.•Decreased pressure decreases the boiling temperature.•Why do cookbooks say add pasta to rapidly boiling water?
Is it hotter?
Phase Change: Condensation•Gas molecules condense to form a liquid @ boiling temperature.•Warming process: gas molecules transfer KE to the liquid!•Why is a rainy day warmer than a cloudy or clear day in winter?•Why do we feel uncomfortable on a muggy day?
Phase Change: Freezing
Phase Change: Melting•Solid to Liquid @ the melting temperature•Cooling process: solid absorbs energy from the environment
•Liquid to Solid @ the melting temperature•Warming process: liquid gives up energy when it freezes!•Why do farmers spray peaches with water to save them from frost?
Phase Change:Sublimation
The conversion of a solid directly to a gas & visa versaExamples: snowflakes, Moth Balls, dry ice
•Heat flows from HOT to COLD•Energy transferred via molecular collisions•Conduction (solids)•Convection (liquids & gases)•Radiation (solids, gases, plasma)
•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 collisions between free electrons and
vibrating atoms.
Hot Air rises, expands and cools, and then sinks back downcausing convection currents that transport heat energy.
Hot air rises because fast moving molecules tend to migrate towardregions of least obstruction - UP !
Rising air cools because decrease in densityreduces number of collisions & speeds decrease.
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 causesthe WEATHER.
Convection between water and land causes the Winds.
Fur is filled with air. Convection currents are slow because the convection loops are so small.
How do fur coats keep you warm?
Electromagnetic Radiation is emitted and absorbed via atomic excitations.
No Medium is required to transfer energy or transmit EM waves!
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 andhigher frequency EM waves.
f ~ T : 500 C (red) - 1200 C (hot white light)
A Good absorber reflects little and appears Black
A good absorber is a good emitter.
Stefan-Boltzmann Law:
4Q e T Atσ=
The heating effect of a medium such as glass or the Earth’s atmosphere that is transparent to short wavelengths but opaqueto longer wavelengths: Short get in, longer are trapped!
The heating effect of a medium such as glass or the Earth’s atmosphere that is transparent to short wavelengths but opaqueto longer wavelengths: Short get in, longer are trapped!
Global Climate Models (GCM)
AGU
Tipping Point of No Return?Greenland Ice Sheet Melts!
Global average temperatures must not rise more than 3.6oF above pre-industrial temperatures, 2.3oF above current temperatures, or just 1.3oF above what the temperature will be after the warming in the pipeline takes place.
Preindustrial level: 280 ppm
Scientists estimate that if global temperatures rise 3.6oF above pre-industrial levels (2.3 oF above what they are now), the melting will become unstoppable. Within 100 years or so, the Greenland ice sheet could be gone, causing a 20 foot increase in sea levels.
EcoCool
Coldplay Mango Offset
Greenwashing
British Petroleum oil refinery in Whiting, Indiana
Carbon Trading
Carbon OffsetsPromoting Business as Usual
‘For every problem there is a solution which is simple, clean and wrong.’
--- HL Mencken, social critic (1880-1956)
‘Promoting a more systemic approach to climate change would not seek toreduce the problem to marketing gimmicks, celebrity endorsements,technological quick-fixes,or neo-colonial exploitation…. real solutions to climate change require socialchange and ...spending time and energy towards achieving such change.’
The Biofuel of the Future NOW
Cannabis sativa L. Marijuana
Benefits of Hemp
• Hemp as food• Hemp for body care• Paper from hemp• Hemp as ethanol fuel• Hemp as paint & plastic• Hemp as textile fiber• Concrete from hemp• Hemp replacing wood• Hemp as rotation crop and soil rejuvenator
"plant hemp seed, not tobacco”---Thomas Jefferson
Drug Harm StudyClassifications were based on a drugs' physical damage to the user; how likely the drug was to induce dependency; and the effect of its use on families, communities and society.
-Study published in The Lancet, a British medical journal
H.R. 1009: Industrial Hemp Farming Act of 2007
CO2 : Greenhouse Gas
Unless we change our direction, we are likely to end up where we are headed.
“It is much too late for sustainable development; what we need is a sustainable retreat.”
-James Lovelock, The Revenge of Gaia