Heat Transfer How does the energy move from

Heat Transfer • How does the energy move from a hotter to a colder object? • Three mechanisms • Conduction • Convection • Radiation

Conduction • Stir your hot soup with a metal spoon • Pretty soon you need a pot holder because the end of the spoon you are holding gets hot • This is heat transfer by conduction • Energy travels up the spoon from the end in the hot soup to the end in your hand

Conduction • We sense the movement of energy by the increasing temperature • This means the atoms and molecules have higher average kinetic energy • Primarily occurs by the movement of electrons in the material • The more easily the electrons can move, the better the conduction

Conduction • Metals have some electrons that are very loosely bound to the atoms in the material • These electrons can move easily and can rapidly pick up additional kinetic energy • Metals are good conductors • Wood and plastic don’t have loosely bound electrons, so they are poor conductors

Conduction

Conduction • Air is a poor thermal conductor • If you stand in the sun on a cold winter day and are shielded from the wind, you stay pretty warm • Snow is a poor conductor, while water is better • Makes igloos a useful as a house

Convection • A phenomenon in fluids • Instead of having energy moved by successive collisions of electrons, atoms and molecules, the fluid itself is set into motion called a current • These moving fluid currents are convection

Convection

Convection • When the radiator heats the air, it becomes less dense and rises • Cool air moves in to replace the air that rose • This generates the air flow • So radiators don’t need a fan to stir the air and to distribute heat throughout a room • The rising air cools until its density matches that of the surrounding air

Convection • We take advantage of the cooling that occurs during an expansion • We make refrigerators and air conditioners operate by forcing gas under pressure through a small hole and expanding it into an empty space

Convection • Explains why breezes come from the ocean in the day and from the land at night

Radiation • • Energy carried by electromagnetic waves Study waves later in detail Light, microwaves, radio waves, x-rays Wavelength is related to vibration frequency

Radiation

Radiation • Every object is emitting electromagnetic waves regardless of temperature • Things we can see from their own radiation are very hot to have energy emitted in the visible region of the spectrum • Most things emit primarily in the infrared • Night vision goggles, etc.

Radiation • Things also absorb radiation • If they didn’t, they would run out of energy to emit • Good emitters are also good absorbers • Equilibrium established between emission and absorption • When something can’t equilibrate, it gets hotter or colder

Radiation • Interior of a car on a sunny day • Sunlight comes in as visible light • Seats and interior are much cooler so they radiate in the infrared instead of visible • Glass in the windows blocks infrared so energy can’t get out • Car interior heats up!

Radiation • • A good absorber reflects very little energy Think about dark pavement A poor absorber reflects a lot of energy Think about snow that doesn’t melt in sunshine even though 1400 watts/meter 2 are hitting it

Radiation • At night, objects receive no input energy from the sun • But, they are warmer than outer space, so they continue to radiate energy • Thus, they cool off • Can we make ice in the desert without a refrigerator?

Newton’s Law of Cooling • Rate of cooling of an object is proportional to the temperature difference between an object and its surroundings • Works both ways, cooling and heating • Rate of heating also depends on the temperature difference