Conduc’on Convec’on and Radia’on
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