Chapter 22

Heat Transfer

The Big Idea• Heat can be transferred by conduction, by

convection, and by radiation

• Heat transfer from warmer to cooler objects

• Will reach same temperature– Thermal equilibrium

• Three ways to equalize temperatures– Conduction– Convection– Radiation

22.1 Conduction• Conduction: transfer of energy with in materials

and between different materials that are in direct contact

• Conductors: materials that conduct heat well

• Metals are the best conductors– Silver is the most conductive

• In conduction, collisions between particles transfer thermal energy, without any over transfer of matter

Conductors

• Materials that have loose outer electrons are good conductors of heat

• Metals best conductors of heat and electricity

• If materials are in the same vicinity, they will have the same temperature (room temperature)

Insulators

• Liquids and gases make good insulators

• Insulator: any material that is a poor conductor of heat and that delays the transfer of heat

• Heat is energy and is tangible

22.2 Convection

• Conduction involves transfer of energy from molecule to molecule– Energy moves but molecules do not

• Convection: heat transfer by movement of the heated substance itself

• In convection, heat is transferred by movement of the hotter substance from one place to another

Convection Continued• Convection occurs in all fluids

• Convection works:– Fluid is heated– Expands of fluid– Fluid becomes less dense and rises

• Application of Archimedes's principle

• Convection currents have influence on air

Moving Air

• Convection currents stirring produce winds

• Uneven absorption of heat causes uneven heating near the surface and creates convection currents

• During the day, land is warmer than the air and produces a breeze. At night, the land is cooler than the water so air flows in the opposite direction

Cooling Air

• Rising warm air expands– Less atmospheric pressure squeezes on higher

altitudes– When air expands it cools

• Molecules get speed from other molecules

• With expanding air the average speed of molecules decreases and the air cools

22.3 Radiation

• Sun’s heat is transmitted by radiation

• Radiation: energy transmitted by electromagnetic waves

• Radiant energy: any energy transmitted by radiation

Radiation Continued

• In radiation, heat is transmitted in the form of radiant energy or electromagnetic waves

• Radiant energy– Radio waves - Visible light– Micro waves - X-rays

• Radiant energy is listed in wavelengths– Longest to shortest

22.4 Emission of Radiant Energy

• All substances continuously emit radiant energy in a mixture of wavelengths

• Low temperature objects emit long waves

• High temperature objects emit short waves

• An infrared thermometer measure the infrared radiant energy emitted by a body and converts it to temperature

Emission of Radiant Energy Continued

• People emit low frequency of infrared

• Types of light:– Hot enough, visible light– 500 oC, red light– Higher then 500 oC, yellow light

• Stellar radiation: radiant energy emitted by stars– White hot, blue hot, red hot

Emission of Radiant Energy Continued

• Sun’s temperature (5500 oC) emits radiant energy, visible on electromagnetic spectrum

• Terrestrial radiation: radiant energy that is emitted by the Earth– Infrared waves, not visible to us

• Radiant energy encounters objects, some absorbed some reflected– Absorbed increases internal energy

22.5 Absorption of Radiant Energy

• Everything emits energy

• Everything absorbs energy from the environment

Absorption and Emission

• Objects absorb and radiate energy at the same rate– Goes to thermal equilibrium with its environment

• When an object radiates more energy it has a new thermal equilibrium

• Good emitters of radiant energy are also good absorbers; poor emitters are poor absorbers

Absorption and Emission Continued

• Dark objects remain hotter than their surroundings on a hot day and cool faster at night

• All objects in thermal contact reach thermal equilibrium

Absorption and Reflection

• Absorption and reflection are opposite processes

• Good absorber reflects little radiant energy– Appear dark

• Radiant energy that enters an opening has little chance of leaving before it is completely absorbed

Absorption and Reflection Continued

• Good reflectors are poor absorbers

• Light colored objects reflect more light and heat and dark colored objects absorb more light and energy

• Sun during the day is a net absorber, at night it is a net emitter

22.6 Newton’s Law of Cooling

• An object hotter then its surroundings eventually cools to match the surrounding temperature

• Rate of cooling: how many degrees change per unit of time

• The rate of cooling of an object depends on how much hotter the object is than the surroundings

Newton’s Law of Cooling Continued

• The colder the object’s surroundings, the faster the object will be cool

• The temperature difference is small, the rate of cooling is low and vice versa

• Newton’s law of cooling: states the rate of cooling is approximately proportional to the temperature difference between the object and its surroundings

22.7 Global Warming and the Greenhouse Effect

• Greenhouse effect: warming of the planet’s surface due to the trapping of radiation by the planet’s atmosphere

Causes of Greenhouse Effect

• Two concepts– All things radiate heat and wavelength depends

on temperature• High temperatures have short waves• Low temperatures have long waves

– Transparency of things depends on the wavelength of radiation

Causes of the Greenhouse Effect Continued

• Same effect in Earth’s atmosphere– Surface of Earth absorbs energy – Reradiates part of the energy in longer wavelengths– Long wavelengths cannot escape the Earth’s

atmosphere, warms Earth

• Earth’s temperature depends on the energy balanced between incoming solar radiation and outgoing terrestrial radiation

Consequences of the Greenhouse Effect

• Over years, solar radiation that comes to Earth equals terrestrial radiation Earth emits

• Materials such as fossil fuels changes absorption and reflection of solar radiation– Not if energy is solar, wind, water– Can change Earth’s temperature

Consequences of the Greenhouse Effect Continued

• The near unanimous view of climate scientists is that human activity is a main driver of global warming and climate change

• Water vapor is the main greenhouse gas

• CO2 is the most rapidly increasing

THE END!