U3g – L2 Get out last night’s homework. Answer the following in your notebook: 1.Why do hot objects often turn red? 2.A wall adapter for your cell phone.

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Get out last night’s homework.

Answer the following in your notebook:

1. Why do hot objects often turn red?

2. A wall adapter for your cell phone may heat up when your phone is plugged in. Why isn’t this probably considered a thermal technology?

3. Putting energy into a system ________ it.

4. Taking energy away from a system ________ it.

March 19, 2010

DRILL

Heats Cools

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TEST FRIDAY – Unit 3F and Unit 3G(Fluid and Thermal Technology)

1. Identify 5 thermal technologies not mentioned in class today

• (Remember, a thermal technology produces, stores, controls, transmits or gets work from heat energy)

2. Identify the problem that the technology system solves

3. List the technological subsystems that are used in the technology

4. Identify the technology that preceded it (i.e. what was used before the technology was invented?)

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• The technology of producing, storing, controlling, transmitting and getting work from heat energy.

Example applications: • Furnace, • Hot water heater, • Toaster, • Insulation, • Heat exchanger, • Refrigerator, • Hot air balloon,• Fan.

Thermal Technology

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• Thermal energy always travels from a region of higher temperature to a region of lower temperature.

• Therefore, the driving force for heat transfer isDifference in Temperature

• Three methods of heat transfer:1. Conduction2. Convection 3. Radiation

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1. ConductionThe transfer of heat through a substance by

collision of molecules.

• Conduction happens through solids.• Metals are usually best conductors.• Free electrons greatly aid in the transfer of

thermal energy.• Fluids are less conductive of heat due to

larger distances between atoms.

1. Conduction

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2. ConvectionThe transfer of heat by movement of a fluid.

• Natural Convection– Fluid surrounding a heat source receives heat

and rises (due to lower density)– Surrounding cooler fluid moves in to replace it.– Cooler fluid is heated, rises, and cycle

continues.

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2. Convection

• Forced Convection– Fans or other means are used to propel a fluid.– An convection current is induced.– Car engine and A/C are examples

3. RadiationThe transfer of heat by electromagnetic waves

• All objects, hot or cold, emit heat by radiation• No medium is necessary – radiation works in a

perfect vacuum (the sun’s heat through space)• Light contains heat radiation• The visible spectrum carries less heat than the

infrared spectrum

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3. Radiation

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HEAT TRANSFER DEMONSTRATIONS

1. With a partner next to you, identify 3 examples of Conduction, Convection, and Radiation (total 9 examples) not discussed in class.

2. Describe your observations of the heat transfer in each example.

3. Describe what is causing the heat transfer in each example.

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