Chapter 22

Heat Transfer

3 METHODS TO TRANSFER HEAT:

• ___________________• ___________________• ___________________– All methods occur

continuously and at the same time.

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CONDUCTION:

• _________________________________________________.

• _________________________________________________.

• The transfer of thermal energy is due to the collision of molecules between substances as well as intermolecular forces between molecules within a substance.

• Transfer continues until “thermal equilibrium” (same temperature) is reached for all materials.

Heat flows from hot cocoa into and up the spoon as well as the cup.

Thermal Conductors:

• Solid materials conduct heat better than liquids and gases.

• ___________________________________________.

• _________________________________________________________________.

INSULATORS:

• Poor conductors of heat.• ____________________________

________________________.• “cold” is the absence of heat.

There is no transfer of “cold” on the transfer of loss of heat.

• Why are down jackets good insulators?– ______________________________

_________________________.

What is the common insulating factor in each of these images?

WOOLPOLAR FLEECE

FIBER GLASS

VACUUMS:• Heat transfer in a vacuum

cannot occur. Why not?– _______________________

____________________________________________.

• Thermos bottles keep liquids warm because they are surrounded by a vacuum.

http://media.lonelyplanet.com/lpi/25100/25100-12/681x454.jpg

What are some possible reasons that some roofs don’t have snow on them?

CONVECTION:

• _____________________________________________.

• Transfer of energy from one place to another through moving fluids (gases and liquids).

• ____________________________________________________________________.

DENSITY – mass per unit of volume

• Density = g/ml– When mass stays the same and

volume increases due to expansion, density decreases.

• _________________________________________________.

• All materials try to reach “equilibrium” (density remains the same)

• The transfer of heat in a fluid causes a circulation (movement) of materials.

D mV

D mV

CONVECTION AROUND US:

• ________________• ________________• ________________• Sea breezes• Land breezes • ________________

https://encryptedtbn1.gstatic.com/images?q=tbn:ANd9GcQ7uUnVIn157znDcxb424wEhQTXAcayKgO3dd2N4av_yB1TPp82A

http://4.bp.blogspot.com/_vgawnhRg_KY/S43kondqOII/AAAAAAAACPY/EGsHyr42BrU/s400/convection.jpg

• Based on your knowledge of convection currents, explain what is happening in the following diagram.

• Which is the “land’ breeze and which is the “sea breeze”. • Why are the H (high pressure) versus L (low pressure)

symbols located where they are?• What is different between the daytime and nighttime

image?• What do you know about the specific heat capacity of

water versus land?• At about what time of day would the scenario on the left

take place?• What do you think the “relative” temperature of the land

versus the water is in both scenarios?

http://www.nc-climate.ncsu.edu/secc_edu/images/sea_breeze.jpg

EXPLANATION FOR THE PREVIOUS SLIDE:

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

THERMAL(heat)RADIATION:

• The transfer of heat energy in the form of electromagnetic waves, including light.

• All substances above Absolute Zero (0 Kelvin) emit (give off) heat energy.

• Substances can also absorb (take in) energy.

Types of Waves

• Infrared waves: – Heat from a fire.

• Light Waves: – Visible form of

radiant energy• Radio Waves: – Transmit signals

through the air.

http://zebu.uoregon.edu/~imamura/122/images/electromagnetic-spectrum.jpg

http://lot.astro.utoronto.ca/images/spectrum.png

http://www.makingitmagazine.net/wp-content/uploads/2010/03/Zhengrong-Shi_suntech_solar.jpg

INFRARED IMAGE OF EARTH

ABSORPTION and EMISSION:

• Substances that effectively absorb radiant energy also effectively emit (give off) radiant energy.

• ____________________________________________________.

• The object will then emit more radiant energy in order to reach a “new thermal equilibrium”.

• Dark objects absorb and emit energy effectively.– Think about the temperature of

pavement on a sunny summer day and a summer night.

http://homtvnews.files.wordpress.com/2012/07/kataro.jpg

ABSORPTION and REFLECTION:• Energy is bounced back

when reflected.• _____________________

_________________________________________.

• Think about wearing dark clothing versus wearing light colored clothing on a hot summer day.

• In which outfit would you remain cooler?

http://www.ashevillegeothermal.com/Solar_files/EarthAbsorption.jpg

http://www.global-greenhouse-warming.com/images/GreenhouseEffectDiagram.jpg

RATE OF HEAT TRANSFER:• The transfer is always from hot to

cold and is continuous until equilibrium is reached.

• The greater the difference in temperature, the faster the flow of heat. ____________________– This is an “empirical relationship” –

(based on observations)• 98.6°F – average temperature of the

inside of a human body. • At 75°F, rate of heat flow out of your

body matches the rate at which the body generates heat internally.

the end…. ©Kathleen Tringale