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7.4 Heat Transfer

7.4 Heat Transfer. pp. 181 - 185 Mr. Richter. Agenda. Warm-Up Check HW from 7.3 Lab Questions? Chapter 7 Test: Heat Tuesday Jan. 15 Introduction to Heat Transfer Notes: Heat Transfer and Equilibrium Types of Heat Transfer Conduction Conductors and Insulators Convection Radiation.

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7.4 Heat Transfer

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  1. 7.4 Heat Transfer pp. 181 - 185 Mr. Richter

  2. Agenda • Warm-Up • Check HW from 7.3 • Lab Questions? • Chapter 7 Test: Heat • Tuesday Jan. 15 • Introduction to Heat Transfer • Notes: • Heat Transferand Equilibrium • Types of Heat Transfer • Conduction • Conductors and Insulators • Convection • Radiation

  3. Objectives: We Will Be Able To… • Explain the three methods of heat transfer and name examples of each. • Give examples of thermal conductors and thermal insulators. • Describe the direction of heat transfer between two objects.

  4. “Warm”-Up: • When it is a cold night, sometimes we use an extra blanket on the bed.Why does this make us feel warmer? Does our body just turn up the thermostat? What happens? • Discuss at your table. Then write a 2-3 sentence explanation in your notes about what you think is happening.

  5. Heat Transfer and Thermal Equilibrium

  6. Heat Transfer and Thermal Equilibrium • Nature is always trying to get things to level out. • Remember, the ball resting at the top of a hill is unstable, and nature wants to level it out with the rest of the ground.

  7. Heat Transfer and Thermal Equilibrium • Similarly, if two objects are different temperatures, nature wants them to be the same temperature. • Objects will transfer heat from one to another by exciting nearby atoms and molecules until they are the same temperature (vibrate the same amount). • They have reached THERMAL EQUILIBRIUM. • Why don’t we get cold when it’s 75 °F outside?

  8. Heat Transfer and Thermal Equilibrium • The greater the difference in temperatures, the greater the rate of heat transfer. • Remember, our perception of temperature is of how fast thermal energy is transferring. • A 60 °F shower in the summer feels refreshingly cold. It is a moderate amount of thermal energy we give away. • A 60 °F shower in the winter can feel very hot. We are taking thermal energy FROM the water.

  9. Conduction

  10. Conduction • Heat transfer by conduction is the transfer of thermal energy by direct contact between particles of matter. • Basically, by touching atoms together. • Warming your hands on a hot coffee mug. • Touching a cold car door handle. • Energetic molecules excite their neighbors, who excite their neighbors, and so on… • It’s like spreading the news of a party.

  11. Conductors and Insulators • Different materials allow the flow of thermal energy better than others. • Most materials can be divided up into conductors and insulators.

  12. Conductors • Thermal conductors are materials that allow atoms and molecules to easily transmit their kinetic energy to other atoms and molecules. • Allow thermal energy to flow easily. • These tend to be flexible solids like metals: • gold, silver, copper • atoms packed closely together • can bend and vibrate easily • Good party people!

  13. Insulators • Thermal insulators do not allow atoms and molecules to easily transmit vibrations. • Inhibit (prevents or partially prevents) the flow of thermal energy. • They tend to be either • very rigid solids • no flexibility, no vibrations • hard rubber, wood, concrete • gases • molecules are too far apart to transmit vibrations • air, helium, etc. • vacuums (no atoms at all)

  14. Air: The Perfect(ish) Insulation • Humans use air as an insulator because it’s cheap and easy to use. All you have to do is trap it. • Puffy jackets and down comforters keep air trapped in the layers, making it difficult for heat to pass. • Thermoses keep a layer of air between the inner and outer layers, trapping the heat (or cold) inside.

  15. Convection

  16. Convection • Heat transfer by convection is the transfer of thermal energy through a fluid (liquid or gas). • Convection happens when warm fluids switch places with colder fluids somewhere else.

  17. Convection • There are two types of convection: natural and forced. • Natural convection occurs when the hotter fluid expands and becomes less dense than the colder fluid, allowing them to change places. • Forced convection occurs when a hotter fluid is forced (pumped, fanned, etc.) to a colder location.

  18. Radiation

  19. Radiation • Thermal radiation is the heat transfer in the form of electromagnetic waves, including light. • x-rays, microwaves, the sun, etc. • Thermal radiation is the only form of heat transfer that can travel through a vacuum. • No molecules need to touch or change places.

  20. Radiation • All objects emit (give off) and absorb thermal radiation. • Because they have moving electrons. • The more reflective the object (lighter color, shinier), the less radiation it will absorb. • The less reflective (darker) the object, the more thermal radiation it will absorb. • This is why you feel warmer in dark clothes on a sunny day.

  21. Wrap-Up: Did we meet our objectives? • Explain the three methods of heat transfer and name examples of each. • Give examples of thermal conductors and thermal insulators. • Describe the direction of heat transfer between two objects

  22. Homework • p 189 #22-31 Due Monday

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