10.4 Controlling Heat pp Mr. Richter

Agenda  Warm Up  Review HW  Business:  Science Fair Posters  What if there’s a snow day?  Introduction to Heat Transfer  Notes:  Conduction  Conductors and Insulators  Sweating  Convection  Radiation

Objectives: We Will Be Able To…  Explain how heat is transferred through conduction, convection and radiation.  Explain and identify conductors and insulators.

“Warm”-Up:  When it is a cold night, sometimes we use an extra blanket on the bed. Why does this make us feel warmer? Does our body just turn up the thermostat? What happens?  Discuss at your table. Then write a 2-3 sentence explanation in your notes about what you think is happening.

Conduction

 Heat transfer by conduction is the transfer of thermal energy by direct contact between particles of matter.  Basically, by touching atoms together.  Warming your hands on a hot coffee mug.  Touching a cold car door handle.  Energetic molecules excite their neighbors, who excite their neighbors, and so on…  It’s like spreading the news of a party.

Conductors and Insulators  Different materials allow the flow of thermal energy better than others.  Most materials can be divided up into conductors and insulators.

Conductors  Thermal conductors are materials that allow atoms and molecules to easily transmit their kinetic energy to other atoms and molecules.  Allow thermal energy to flow easily.  These tend to be flexible solids like metals:  gold, silver, copper  atoms packed closely together  can bend and vibrate easily  Good party people!

Insulators  Thermal insulators do not allow atoms and molecules to easily transmit vibrations.  Inhibit (prevents or partially prevents) the flow of thermal energy.  They tend to be either  very rigid solids  no flexibility, no vibrations  hard rubber, wood, concrete  gases  molecules are too far apart to transmit vibrations  air, helium, etc.  vacuums (no atoms at all)

Air: The Perfect(ish) Insulation  Humans use air as an insulator because it’s cheap and easy to use. All you have to do is trap it.  Puffy jackets and down comforters keep air trapped in the layers, making it difficult for heat to pass.  Thermoses keep a layer of air between the inner and outer layers, trapping the heat (or cold) inside.

Sweating and Evaporation  When we sweat and produce moisture, we obviously do not boil off the water we exude. Sweating ≠ boiling!  The average temperature of each particle of water is the same as the temperature on our skin (because of conduction!), but:  Some particles have a much higher level of energy. High enough to fly away!  The remaining particles have a lower average kinetic energy than they did before, cooling us off. Neat!

Convection

 Heat transfer by convection is the transfer of thermal energy through a fluid (liquid or gas).  Convection happens when warm fluids switch places with colder fluids somewhere else.

Convection  There are two types of convection: natural and forced.  Natural convection occurs when the hotter fluid expands and becomes less dense than the colder fluid, allowing them to change places.  Forced convection occurs when a hotter fluid is forced (pumped, fanned, etc.) to a colder location.

Radiation

 Thermal radiation is the heat transfer in the form of electromagnetic waves, including light.  x-rays, microwaves, the sun, etc.  Thermal radiation is the only form of heat transfer that can travel through a vacuum.  No molecules need to touch or change places.

Radiation  All objects emit (give off) and absorb thermal radiation.  Because they have moving electrons.  The more reflective the object (lighter color, shinier), the less radiation it will absorb.  The less reflective (darker) the object, the more thermal radiation it will absorb.  This is why you feel warmer in dark clothes on a sunny day.

Wrap-Up: Did we meet our objectives?  Explain how heat is transferred through conduction, convection and radiation.  Explain and identify conductors and insulators.

Homework  p. 389 #