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HEAT & HEAT TRANSFER

HEAT & HEAT TRANSFER. Recall: heat is energy transferred from one object to another Energy in transit Once thought that objects contained “heat fluid” – the caloric Exp showed heat could be continuously produced as long as work was done. Heat – Cont.

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HEAT & HEAT TRANSFER

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  1. HEAT & HEAT TRANSFER • Recall: heat is energy transferred from one object to another • Energy in transit • Once thought that objects contained “heat fluid” – the caloric • Exp showed heat could be continuously produced as long as work was done

  2. Heat – Cont. • Further confirmation that energy is conserved – proved by James Joule • Mechanical work = heat, Q • Originally measured as the calorie • 1 cal = amount of heat needed to raise the temperature of 1 g of water by 1 C • 1 Cal = 1 kcal (the standard food calorie) • It turns out: 1 Cal = 4186 J

  3. Conduction • Direct physical contact • One end of a rod placed in a fire causes the molecules to vibrate with an increasing amplitude • These jostle their neighbors causing them to vibrate more • This effect travels molecule by molecule down the rod

  4. Conduction – Cont. • A wooden rod seems to resist heat transfer, one end remaining cool • Thus, conduction depends on type of material

  5. Conduction – Cont. • The amount of heat, Q, flowing through rod: • Proportional to cross-sectional area • Proportional to temperature difference • Increases steadily with time • Decreases with the length of the rod

  6. Conduction – Cont. • That is: Q α A ΔT t / L • the constant that creates an equality: k = thermal conductivity • Material dependent, looked up in table

  7. Conduction – Cont. • Metals – thermal conductors – transfer heat well because of excess electrons (e) • Feel cool to touch – good at taking heat away • Nonmetals – thermal insulators – few free e • Feel warm to touch • Heat conduction generally depends of phase • Closer molecules – better conductors • Solid > liquid > gas

  8. Conduction – Cont. • Since air is a terrible conductor, its used as an insulator • Double paned windows • Fiberglass insulation • Snow

  9. Radiation • Electromagnetic (EM) radiation • Needs no medium • A distribution of different wavelengths • Long wavelengths  cool objects, short  hot • Room temperature objects  infrared radiation • As objects warm, wavelength decreases (frequency increases)

  10. Radiation – Cont. • Color (wavelength) can be used to measure temperature • How we know the temperature of stars • Objects absorb & radiate energy at the same time • Absorb > emit = warms • Absorb < emit = cools • Good emitters are also good absorbers • Objects that are dark (black) • Global Warming

  11. Convection • Due to density differences • Warmer fluid warms, expands, becomes less dense & rises • Creates currents are cooler fluid moves in to replace rising fluid – convection currents • Uneven heating of earth’s surface causes convection currents near large bodies of water

  12. Convection – Cont. • Global convection • Cool air near poles replaces warm air at equator • Ocean currents • Inside earth – liquid rock rises to surface causing the “plates” to move • The Sun

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