Chapters 23 & 24

Chapter 23: Change of Phase

Chapter 24: Thermodynamics

3 Common Phases of Matter

A fourth phase, plasma--ionized gas (not shown), is the most common phase of matter in the universe. 

2 Changes of Phase• From solid → liquid

• From liquid → gas

• Depends on temperature & pressure.

• Usually involves a transfer of energy.

Change of Phase- Energy

Evaporation• Liquid to Gas

• Takes place at the surface of a liquid.

• Molecules at the surface are being bumped from below & given enough energy to break free.

Evaporation is “COOL”• Because energy was lost

when the molecule broke free, the avg. KE left in the container is lowered.

• EVAPORATION = COOLING!!!

• Shower, dog’s tongues, swimming, etc…

Condensation is “HOT”• Gas to Liquid.

• CONDENSATION = WARMING!

• Fast gas molecules collide with slow ones and loose energy. They don’t have enough energy to stay as a gas. (Not fast enough.)

De-Foggers. How do they work?

• Prevent condensation.• Clear, thin film.• Takes the water formed & immediately spreads it out

evenly.• You can’t see the water!

Evaporation & Condensation

• Greater evaporation = COOLING

• Equal = No net change

• Greater condensation = WARMING

Condensation in the Atmosphere• At each temp, there is a limit

to the amount of water vapor in the air.

• Limit reached = saturation

• Called “humidity”

Saturation• 100% humidity

• More water vapor needed to saturate high temp. air than low temp. air.

• Condensation must be happening in the air. (water sticks to itself)

Boiling

• Liquid to gas

Question: Is boiling a cooling process or a warming process????

Answer: It is a COOLING process!!!

As faster molecules with a larger KE leave…

the avg. KE left lowers = cooling!

Pressure Cooker

Question: How does it cook so darn fast???????

Answer: Increases the boiling temp so food cooks at a higher temperature!

Boiling occurs at different temps…

At higher altitudes, pressure is less, so water needs less KE to fight against the force of the air on their way off the surface of the liquid.

Did you know…At the ocean floor (approx. 2 miles down) water won’t boil until it is ~700 ºF!!

Freezing

• When energy is withdrawn from a liquid.

• It slows down until it’s a solid.

Change of Phase Equation

•Q = mL• Heat energy = (mass)(heat of fusion or vaporization)

. “lost or gained” “latent heat”

• J = (kg)(J/kg)

Thermodynamics

• The study of heat & it’s ability to transform into mechanical energy.

• Power plants, cars, etc…

• There are 3 Laws.

Thermodynamics

• The study of heat & it’s ability to transform into mechanical energy.

• Power plants, cars, etc…

• There are 3 Laws.

1st Law of Thermodynamics

• When heat is added to a system, it transforms to an EQUAL amount of some other form of energy.

• …because energy is ALWAYS conserved

1st Law Example…• A battery powered light is

switched on and heat is produced.

• That heat energy actually already existed inside the system as chemical energy inside the battery.

• So no energy is either gained or lost as a result of switching on the light!

1st Law Equation

• Q = ΔE + W• Heat added = change of internal E + external work

• J = J + J

2nd Law of Thermodynamics

• Heat will NEVER, by itself, flow from cold to hot.

• It will flow from hot to cold until equilibrium has been reached.

2nd Law Equation

• Ideal Efficiency

IE = Thot – Tcold

Thot

• No SI unit label for ideal efficiency.

• Temperature is measured in K. (Remember your conversion equations for the test!)

It’s impossible…

• Question: To produce usable energy with ZERO impact to the environment… WHY?

• Answer: Because heat (waste) will always be given off!

(It will warm air, rivers, etc…)

3rd Law of Thermodynamics

• You can’t reach absolute zero.

The End. Any questions?