1. Chapter 22
Heat Transfer

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 22.3 Radiation Sun’s heat is transmitted by radiation
Radiation: energy transmitted by electromagnetic waves
Radiant energy: any energy transmitted by radiation

11. 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

12. 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

13. 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

14. 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

15. 22.5 Absorption of Radiant Energy
Everything emits energy
Everything absorbs energy from the environment

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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. 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

23. 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

24. 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

25. 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

26. 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

27. THE END! 