Chapter 22: Heat Transfer All about thermal energy moving – heat flow – how does it work?

Too Hot to Handle Thermal Energy Transferred in 3 ways Conduction Convection Radiation Ask them if they know the 3 ways first…

Conducting Business Conduction Heat transferred through CONTACT – atoms of one material crash into atoms of other material Conductors are good at it Insulators are bad at it What is heat? What is temperature? Conduction is atoms crashing – momentum being transferred Do desk example – touch metal, touch top Show Hewitt videos – air is insulator and walking on hot coals Styrofoam, down jackets, house insulation, double pane windows, metal vs. plastic windows, vacuum bottles

Convection Convection Transfer of energy through fluids Not so much energy transfer as matter transfer Archimedes’ principle applied Convection currents produce winds Show Eureka Video – Show example on the board – lake wind examples – boiling water example – Sun example Works in all fluids!

Full of Cold Air Air is cooler at higher altitudes Less pressure so: gases are able to expand Lower density = less kinetic energy = lower temperature Air is very hot @ low altitudes This is why it is cold at higher altitudes – cold in Denver, on Everest, warm in Death valley & Jordan Valley Death Valley

You’re So Radiant Forms of radiant energy Microwaves UV IR Radio Visible Gamma Heat transfer from sun is radiation Energy transmitted via radiation is radiant energy Doesn’t have to be touching, doesn’t need air or fluid – only way energy can enter from outside the planet

Riding the Waves Good absorbers = Good Emitters Low temperature emit long wavelengths High temperature emit short wavelengths We absorb IR waves (heat lamp) Remember Movie at start of semester – this is black body radiation!!! Only depends on temperature Heat lamp – absorb & emit IR waves

Radiant Energy Absorbing and Reflecting are opposites Light colors reflect: radiant energy All wavelengths Dark colors absorb: Silver coffee & drink pitchers, black clothing, white clothing Coffee/Cream NTQ

Newton’s Cool R~∆T Newton’s law of cooling Rate of cooling for an object is proportional to the difference in temperature True for heating also Very hot things cool faster than just warm things – More energy is transferred per bigger difference

Keeping Warm Greenhouse effect Radiant energy entering is greater than Radiant energy leaving Works like this…

How a Green House Works Greenhouse effect Short waves from sun pass through glass & are absorbed inside the green house Long waves emitted by greenhouse & plants are absorbed & remitted inward by the glass You may never have been in a green house – but same thing happens with cars and houses. – keeps green house hot, eventually enough heat rise to allow escape by conduction, radiation

The Earth as a Greenhouse Sun is very hot; sends short waves of energy to Earth Sun waves pass through atmosphere Earth is cooler; sends out longer waves Water vapor and CO2 prevent terrestrial radiation from leaving