Chapter 8 Heat Transfer & Change of Phase

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 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/meter2 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

Phases of Matter Four States Solid Liquid Gas Plasma

Solids Atoms or molecules are well ordered into a crystalline lattice for most materials Exceptions are things like glass which is really a very slow flowing liquid Metallic Solids don’t need all their electrons for bonding Insulators need all their electrons for bonding

Liquids Pretty incompressible Atoms or molecules loosely bonded to neighbors, but in close proximity Allows fluid flow

Gases Compressible Atoms or molecules are widely separated and have little interaction with each other aside from elastic collisions

Plasmas Temperature is so high that outer electrons are separated from their underlying atoms No molecules exist Particles are electrically charged Sun, stars, inside a fluorescent bulb

Changing States For a given substance, all four states are possible Increase temperature and a PHASE CHANGE occurs Solid > Liquid > Gas > Plasma

Phase Changes Evaporation Condensation Boiling Melting Freezing

Water Molecule Two Hydrogen Atoms, One Oxygen Atom - - + +

Evaporation

Evaporation Liquid cools because it loses higher energy molecules or atoms Air cools because the molecules leaving give up a lot of energy to break free, so they have much lower energy after they gain their freedom Solid > Gas directly is sublimation

Condensation

Condensation The opposite of evaporation Evaporation and Condensation are a SURFACE phenomenon Interaction of molecules at the liquid-air interface

Boiling Molecules break free of the liquid bonds and form little bubbles If the vapor pressure in the bubble is large enough to resist the pressure of the liquid, and the air pressure above the interface they rise and escape to the outside Remember, liquid is cooling and air is cooling

Boiling

Boiling

Melting and Freezing Change between liquid and solid Bonds between molecules break when solid melts Takes energy to break the bonds Freezing is the opposite of melting Molecules move slowly enough that bonds can form

Melting and Freezing Why does salt melt ice?

Energy and Phase Change Adding heat at a constant rate T Time

Energy and Phase Change Adding heat at a constant rate T Boiling Melting Time

Energy and Phase Change

Energy and Phase Change For Pure Water 334 joules/gram 2256 joules/gram

Boiling Potatoes So, when you want garlic mashed potatoes and you take potatoes and cut them up and put them in boiling water, do they cook faster when the water is really bubbling compared to when the water is just barely bubbling???

Boiling Potatoes