# Chapter 22 Heat Transfer.

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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

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

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

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

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

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! 