Conduc'on  Convec'on  and  Radia'on  

Heat  Transfer  

•  The  science  of  how  heat  flows  is  called  heat  transfer.    •  There  are  three  ways  heat  transfer  works:  conduc'on,  convec'on,  and  radia'on.    

•  Heat  flow  depends  on  the  temperature  difference.  

 Thermal  Equilibrium  

•  Two  bodies  are  in  thermal  equilibrium  with  each  other  when  they  have  the  same  temperature.  

•  In  nature,  heat  always  flows  from  hot  to  cold  un'l  thermal  equilibrium  is  reached.  

 Heat  Conduc'on  

•  Conduc'on  is  the  transfer  of  heat  through  materials  by  the  direct  contact  of  maEer.  

•  Dense  metals  like  copper  and  aluminum  are  very  good  thermal  conductors.  

 Heat  Conduc'on  

•  A  thermal  insulator  is  a  material  that  conducts  heat  poorly.  

•  Heat  flows  very  slowly  through  the  plas'c  so  that  the  temperature  of  your  hand  does  not  rise  very  much.    

 Heat  Conduc'on  

•  Styrofoam  gets  its  insula'ng  ability  by  trapping  spaces  of  air  in  bubbles.  

•  Solids  usually  are  beEer  heat  conductors  than  liquids,  and  liquids  are  beEer  conductors  than  gases.    

 Heat  Conduc'on  

•  The  ability  to  conduct  heat  oIen  depends  more  on  the  structure  of  a  material  than  on  the  material  itself.  –  Solid  glass  is  a  thermal  conductor  when  it  is  formed  into  a  beaker  or  cup.  

– When  glass  is  spun  into  fine  fibers,  the  trapped  air  makes  a  thermal  insulator.  

Thermal  Conduc'vity  

•  The  thermal  conduc'vity  of  a  material  describes  how  well  the  material  conducts  heat.  

Why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor, wood is an insulator. The metal conducts the heat away from your hands, the wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Convec'on  

•  Convec'on  is  the  transfer  of  heat  by  the  mo'on  of  liquids  and  gases.  –  Convec'on  in  a  gas  occurs  because  gas  expands  when  heated.  

–  Convec'on  occurs  because  currents  flow  when  hot  gas  rises  and  cool  gas  sink.  

–  Convec'on  in  liquids  also  occurs  because  of  differences  in  density.  

Why is it windy at the seaside?

Freezer compartment

It is put at the top, because cool air

sinks, so it cools the food on the way

down.

It is warmer at the bottom, so this warmer air

rises and a convection

current is set up.

Where is the freezer

compartment put in a fridge and why?

Convection questions

Why are boilers placed beneath hot water tanks in people’s homes?

Hot water rises.

So when the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Why does hot air rise and cold air sink?

Cool air is more dense than warm air, so the cool air ‘falls through’ the warm air.

 Radia'on  

•  Radia'on  is  heat  transfer  by  electromagne'c  waves.  

•  Thermal  radia'on  is  electromagne'c  waves  (including  light)  produced  by  objects  because  of  their  temperature.  

•  The  higher  the  temperature  of  an  object,  the  more  thermal  radia'on  it  gives  off.  

Radiation

Radiation travels in straight lines True/False

Radiation can travel through a vacuum

True/False

Radiation requires particles to travel

True/False

Radiation travels at the speed of light

True/False

The __________ container would be the warmest after ten minutes because its shiny surface reflects heat _______ back into the container so less is lost. The ________ container would be the coolest because it is the best at _______ heat radiation.

shiny metal radiation

dull black emitting

Shiny metal

Dull metal

Dull black

Shiny black

Four containers were filled with warm water. Which container would have the warmest water after ten minutes?

Emission experiment

Absorption experiment

Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation.

dull black radiation

shiny metal

absorbing

Shiny metal

Dull metal

Dull black

Shiny black

Radiation questions

Why are houses painted white in hot countries?

White reflects heat radiation and keeps the house cooler.

Why are shiny foil blankets wrapped around marathon runners at the end of a race?

The shiny metal reflects the heat radiation from the runner back in, this stops the runner getting cold.

Explain  the  roles  of  conduc'on  convec'on  and  radia'on    in:  

•  Laptops  •  Car  engines  •  A  wood  stove  •  Clothing  •  Bees  cooling  their  hives  •  A  squirrel  holding  its  tail  over  its  back  •  Desert  animals  having  tall  narrow  ears  •  Elephants  large  flapping  ears