Chapter 10 Heat Transfer and Change of Phase

Conduction Heat transfer through material by atoms & electrons colliding inside the object being heated Solids whose atoms or molecules have loosely held electrons are good conductors of heat (ie. Metals) Poor conductors of heat are called insulators Wood is a good insulator even when its red hot Air is a very poor conductor of heat.

Conduction cont. Good insulating properties are largely due to air spaces between molecules Snow is a poor conductor Insulation does not stop heat transfer, only slows it down

Convection Heat transfer by motion of a fluid (liquid or gas) by currents Figure 10.4 As the fluid is heated the molecules move faster, spread out and become less dense. The denser, cooler fluid migrates to the bottom.

Convection currents stir the atmosphere and produce winds

Radiation Heat transfer through radiant energy (electromagnetic waves) For example, from the sun to the earth through mostly empty space Good absorbers of radiant energy are also good emitters A dark object that absorbs plenty of radiant energy must emit a lot as well Figure 10.11 When the containers are filled with hot (or cold) water, the darker one cools (or warms) faster Every surface, hot or cold, both absorbs and emits radiant energy.

Heat transfer occurs whenever matter changes phase – adding thermal energy changes phase from solids to liquids to gasses to plasma

Melting Substance changing from a solid to a liquid When a solid is heated the heat is absorbed and the molecules vibrate more violently and the attractive forces between the molecules can no longer hold them together During melting added thermal energy increases the potential energy of the molecules breaking the bonds that hold them together, not the kinetic energy – temp. does not increase

Evaporation A change of phase from liquid to gas at the surface of the liquid Molecules at the surface gain kinetic energy by being bumped As water is heated, some molecules gain more energy than others; ones with most energy break free leaving molecules with less energy then dropping the temperature of the liquid Perspiration to cool our body temperature

Vaporization or Boiling Evaporation within a liquid Bubbles of vapor form in the liquid and rise to the surface, where they escape At the boiling point, any added thermal energy is used to increase the potential energy of the molecules, not the kinetic energy. Therefore, the temperature does not change during boiling

Sublimation A change of phase from solid to gas Ex. Snow, ice cubes, etc.

Condensation Change of phase from gas to liquid When gas molecules near the surface of a liquid are attracted to the liquid, they strike the surface with increased kinetic energy and become part of the liquid resulting in a rise in temperature Steam burns

Why do you feel warmer in the moist shower area than outside the shower?

Why do they say “its not the heat, it’s the humidity” to compare the climate in Arizona to the climate in Wisconsin?

Freezing Liquid changes to solid Liquid freezes when its molecules vibrate about fixed positions and form a solid

Energy needed for changes in phase Whenever a substance changes phase, a transfer of energy occurs Heat of fusion – the amount of energy needed to change any substance from solid to liquid (and vice versa) Heat of vaporization – the amount of energy required to change any substance from liquid to gas (and vice versa)

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