10.4 Controlling Heat

10.4 Controlling Heatpp. 383 - 385

Mr. Richter

Agenda

Warm Up

Review HW

Business: Science Fair Posters What if there’s a snow

day?

Introduction to Heat Transfer

Notes: Conduction Conductors and

Insulators Sweating Convection Radiation

Objectives: We Will Be Able To… Explain how heat is transferred through conduction,

convection and radiation. Explain and identify conductors and insulators.

“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.

Conduction

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.

Conductors and Insulators

Different materials allow the flow of thermal energy better than others.

Most materials can be divided up into conductors and insulators.

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!

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)

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.

Sweating and Evaporation When we sweat and produce moisture, we

obviously do not boil off the water we exude. Sweating ≠ boiling!

The average temperature of each particle of water is the same as the temperature on our skin (because of conduction!), but: Some particles have a much higher level

of energy. High enough to fly away! The remaining particles have a lower

average kinetic energy than they did before, cooling us off. Neat!

Convection

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.

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.

Radiation

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.

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.

Wrap-Up: Did we meet our objectives?

Explain how heat is transferred through conduction, convection and radiation.

Explain and identify conductors and insulators.

Homework

p. 389 #36 - 39