February 20, 2008

• Objective: To understand heat transfer and the causes of phase changes

• Check-in Frayer Models• CW - Chapter 10 Notes• HW - Study for a super short Conduction

vs. Convection vs. Radiation Quiz Monday!

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Ch 10 Heat Transfer & Change of Phase

These notes are available on the Moodle!

10.1 Conduction

Conduction- transfer of heat E by the collisions between particles in a substance. ( especially solids!)

Conductor- solids whose electrons can move freely.

Insulator- molecules that do not have mobile electrons. POOR CONDUCTOR.

EXAMPLES

GOOD CONDUCTORS

(Poor insulator)

Silver

Copper

Aluminum

Iron

INSULATORS

(Poor conductor)

Wood

Wool

Paper

Cork

Plastic

Air

10.2 Convection

Convection- transfer of heat E by the motion of fluids.

Fluid – any substance with molecules/atoms that are free to flow past each other.

Both gases and liquids are fluids!

Convection

Liquid- • molecules at bottom move faster,• spread apart & less dense.• Buoyed up, denser liquid moves to bottom.

Gases-• Warm air expands, rises, and then cools.• Stirs the atmosphere = windAIR heats up as compressed

10.3 Radiation

Transfer of Heat by means of electromagnetic radiation.

*** DO NOT CONFUSE W/ RADIATION INVOLVING ATOMIC NUCLEI***

Similar to when you drop a stone in a pond, or shake the end of a rope Electromagnetic radiation is caused by a vibrating charge (electron(s) in atoms on the sun).

Ex:radio wavesInfrared waves light waves (light bulb)

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Wavelenth (λ) of radiation related to frequency &

Temperature

LONG WAVE = low frequency = cool

SHORT WAVE = high frequency = hot

Emission of Radiant EAll objects greater than 0 K emit some

radiant energy.• Frequency relative to temperature f ~ T

Ex: sun ,fireplace, lamp ↑ frequency, ↓ λ , ↑ temp

you feel high freq infrared waves as heat

Absorption of radiant energy

Objects emit & absorb radiant energy• Good emitters are good absorbers• Poor emitters are poor absorbers

Dark objects absorb & emit more radiant Energy

Ex: black vs white container of water in sun

Black warms faster & cools faster.

Temperature vs. Intermolecular Forces

Intermolecular Forces – attractive forces that exist between atoms/molecules of a substance.

Different strengths for different substances.

Determines what phase the substance will be at certain temperatures.Example: solids at 25°C have stronger IMF

that liquids, than gases do.http://www.pbs.org/wgbh/nova/zero/matter.ht

ml

10.5 Evaporation

A change of phase from liquid to gas.• Liquid – molecules constantly in

random motion & gain KE. as high E molecules break free of

surface, lower KE left in liquid (TEMP ↓).EVAPORATION IS A COOLING

PROCESS.Ex: sweat. evaporation cools skin.

Boiling Point: Part 1

Boiling is evaporation within a liquid• Evaporation occurs beneath surface of

liquid when boiling.• Bubbles (vapor) forms in liquid closest

to the heat source• Bubbles only form when pressure

equal to the pressure of the surroundings (atmosphere).

Boiling Point: Part 2

Boiling is evaporation at the surface of a liquid• Evaporation is always occurring at the

surface of a liquid, regardless of temperature.

• The vapor phase that exists above a liquid exerts a pressure called Vapor Pressure.

• When vapor molecules have enough KE to exert a pressure equal to that of the atmosphere then the substance is said to be boiling.

BP vs. Atmospheric Pressure

• ↑ atmospheric pressure requires a ↑ temp to boil.

Vapor molecules require more energy so that they can exert a pressure = to that of the atmosphere

• ↑altitudes = ↓ pressure

So in Denver, CO WATER BOILS AT LOWER TEMP. (95C) … takes longer to cook.

10.6 Condensation

A change of phase from a gas to a liquid.• Molecules at liquid’s surface do not

have enough KE to overcome IMF and are “attracted to liquid”

• Collisions with the surface transfer KE.• KE is absorbed by the liquid.

CONDENSATION IS A WARMING PROCESS

9.8 Thermal Expansion

• Molecules jiggle and move farther apart when heated.

• Most substances expand when heated & contract when cooled.

• i.e. solids are more dense then liquids, then gases for most substances.

• Exception: Water contracts until 4 C , then forms crystalline structure w/ open spaces and expands.

Expansion of H2O

• Ice :Hexagonal , open structure

• Ex: Melt ice cube, H2O less volume

10.8 Melting & freezing- opposite directions in phases changes.

Melting – substance changes from solid → liquid.

When heated, molecules gain KE, vibrate more and attractive force between molecules is overcome.

SOLID MELTS

Freezing - liquid changes to solid

As E is removed from liquid, molecular motion slows. IMF take over & molecules bind together and vibrate in fixed positions.

H2O w/o impurities - freezes at 0C.

w/ impurities - < 0C• foreign molecules change the vapor

pressure to less than that of pure water.

SUBLIMATION

Molecules of a solid are transformed directly into gas phase.

BP for a substance describes when the vapor molecules exert a pressure equal to that of the atmosphere.

Sublimation occurs when vapor molecules at the surface of a solid exert a pressure greater than that of the atmosphere at room temperature.

Phase Diagrams

• Under different conditions of temperature and pressure substances will exist at either solid, liquid or a gas.

Phase changes

• TRANSFER OF HEAT occurs whenever matter changes phases.

solid→liquid→gas→plasmaGaining thermal energy

Plasma→gas→liquid→solidLosing thermal energy

10.9 Energy & Phase Change

1st Law defines parameters for heat transfer under conditions of constant pressure (our atmosphere).

– Specifically, the gain or loss of thermal energy equals the amount of heat transferred.

Enthalpy (H) – the term used to describe heat absorbed or released under constant pressure.

Energy is needed for a change of phase

ΔH system = ΔH surrounding or…

Energy lost by the system must equal energy gained by the surroundings.

Energy may cause an increase in Temp OR a phase change!

Changing Phase

Melting PointBoiling Point

Changes in state require energy!Add KE and TemprisesAt BP/MP temp stopsrising and E used to break IMF

Heat of fusion

• Amount of E needed to change

Solid ↔ liquid.

Heat of fusion of H2O = 335 J/ g H2O

Heat of vaporization

• Amount of E required to change a substance from liquid ↔ gas

heat of vaporization of H2O = 2255 J/g H2O

• End of chapters 9 & 10