Heat Transfer and Phase Change

Heat is the transfer of thermal energy. We learned that in the last chapter!

There are three processes which enable this transfer. They are:

• Conduction

• Convection

• Radiation

NASA web site reference

Conduction is the molecule to molecule transfer of thermal energy. One molecule gains energy, moves faster, and “bangs against” nearby molecules giving them some of this energy.

Conduction:

•Requires a material where molecules/electrons have some freedom of movement (a conductor)

•Is a fairly slow process

•Always moves thermal energy from hot to less hot (or “cold”…)

A nail stuck into the ice will conduct thermal energy from your hand to the ice.

If you step barefoot on wood it will usually not feel cold – because it is a poor conductor and will not conduct heat away from your foot. A tile (or a quarter on the floor!) will conduct heat away and feel cold!

Long-stem Hawaiian-punch glasses are designed to minimize conduction of heat from the person’s hand…

Patterns of conduction are evident in snow melt-off

A thermal infrared image of a coffee cup filled with a hot liquid. Notice the rings of color showing heat traveling

from the hot liquid through the metal cup. You can see this in the metal spoon as well. This is a good example of

conduction.

Convection is the movement of heat due to actual movement of the material which is

carrying the heat.

Heater element submerged in water shows the convection current of heat

Convection is gas and liquid

“Land-Sea” breezes at the beach are caused by convection resulting from different temperatures of the

land and air.

In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continuous circulation pattern and heat is transferred to cooler areas.

Pan of hot oil on burner

viewed from above.

Using convection to produce energy…

The third, and perhaps the most common in the overall Universe, is heat transfer by Radiation.

This is not “radiation” as in nuclear weapons. It is the movement of energy by Electromagnetic Waves.

No material is required for radiation. The waves can pass through a vacuum (like space!)

Examples of radiation we’re used to are:

• Light

• Microwaves

• X-rays

• Radio signals (cell phones!)

Examples of radiation

Electromagnetic radiation has different wavelengths. They range from the size of atoms to the size of

mountains.

Generally shorter wavelengths mean more energy in the type of radiation being moved by the waves.

A thermal infrared image of the center of our galaxy. This heat from numerous stars and interstellar clouds traveled about 24,000 light years (about 150,000,000,000,000,000 miles!) through space by

radiation to reach our infrared telescopes.

Web link

Hotter objects emit radiation at shorter wavelengths (higher frequencies) and they emit more energy than cool objects

This means the Sun emits a lot of EM (Electromagnetic!) radiation at a short wavelength…..while the cooler Earth emits much less energy at longer wavelengths

Absorption of Radiant Energy

When EM radiation hits an object, it is either absorbed or reflected.

If absorbed, the energy it carries is transferred to the object.

Lots of paths in, but few out..

Evaporation – liquid becoming gas! The evaporating molecules require heat to “break loose”, and they get this heat from the object

they are on this is how your body cools itself on a hot day…

Dogs have sweat glands only between

their toes (??), so must pant in order to cool

off.

Pigs use a different approach…!

Condensation is the opposite of Evaporation – it is a warming process!

When you step out of the shower, the water on your skin evaporates and you feel cool. If you step back into the shower (even if it’s off now!) you’ll feel warmer because in there moisture will condense on your cooler skin..

Condensation is a warming process…

The movement between liquid and solid phase is called melting and freezing.

Energy removed from a liquid will slow down the molecules and they will become “trapped” in a crystal or grid structure (amorphous solids are different).

Energy added to a solid will break down this structure, and allow the molecules to move freely as a liquid.

Boiling occurs when water within the fluid (not at the surface) gains enough energy to “evaporate” and can push away the liquid water to form a “bubble”.. This bubble is the gas water vapor.

Heating and cooling can reach

equilibrium.

Energy and Change of Phase

You can stick your finger in

molten lead …. As long as your finger is wet!

This is because the heat makes

the water evaporate instead of

cooking your finger!