Heat Part 2

Heat

•Phase change - the process of a substance changing from a solid, liquid, or gas to a solid, liquid, or gas

•Examples include: melting, vaporizing, solidification (freezing), condensing, deposition, and sublimation

Heat

• Deposition - when a substance goes from a vapor to a solid without the liquid phase in between

• Frost on your windshield

• Sublimation - when a substance goes from a solid to a vapor without the liquid phase in between

• Dry ice

Heat

•While a substance is going through a phase change the temperature remains constant

•The substance still absorbs energy even though the temperature is not changing

Heat

•Melting point - the temperature where a solid changes into a liquid.

•The temperature stays constant until all the solid has turned to liquid

•Heat of fusion - the amount of heat (J)required to change one kg of solid into liquid

Heat

•Boiling point - the temperature where a liquid changes into a gas

•The temperature stays constant until all the liquid has changed into a gas

•Heat of vaporization - the amount of heat (J) required to change one kg of a substance from a liquid to a solid

Heat•Latent heat - the heat of fusion or

heat of vaporization required for phase changes

•Temperatures of substances do not change during latent heating

•They just undergo a phase change

•Values can be looked up or experimentally determined

Heat

•Example 2:

•How much heat energy is required to change a 5 kg block of ice (-20°C) to a vapor (120°C)? The latent heats of fusion and vaporization are 3.33 x 105 J/kg and 22.6 x 105 J/kg.

Heat

•Methods of heat transfer

•Conduction

•Convection

•Radiation

Heat• Conduction - Heat transfer when materials are in

direct contact

• Conductors - materials that transfer heat very well (metals)

• Insulators - material that do not transfer heat well (plastic)

• Conduction usually occurs between two solids

•Energy moves but the molecules do not

Heat•Convection - heat transfer where the

molecules move

•Usually occurs in fluids (liquids and gases)

•Warmer molecules become less dense and buoy upward to spread out

•They are replaced by cooler molecules that can then be heated

Heat

•Convection is responsible for our weather

•Convection is also what causes plate tectonics (the movement of the Earth’s crust over the mantle)

Heat

•Radiation - a means of heat transfer that does not involve molecules (the energy can travel through a vacuum)

•Radiation is energy traveling on waves

•Does not require direct contact like conduction

•Molecules do not move like conduction

Heat

•Ultimately the energy we have here on Earth came to us through radiation from the sun

•Throughout the day the Earth absorbs energy from the sun

•At night the Earth radiates the energy outward

Heat

•Objects that are darker in color tend to absorb more radiant energy

•Objects that are lighter in color tend to reflect more radiant energy

•A good absorber (take in) of radiant energy is also a good emitter (give away) of radiant energy

Heat•Radiation is the culprit in “global

warming”

•Radiation from the sun is absorbed, but due to green house gases the radiation cannot escape, thus the average kinetic energy (temperature) rises

•The reduction in the polar ice caps also reduces Earth’s ability to reflect the sun’s radiation