• Is it possible to transfer heat to a substance and it not increase in temperature?

• Yes, during a phase change

CHANGES OF STATES OF CHANGES OF STATES OF MATTERMATTER

Cooling SystemCooling SystemA device that transfers A device that transfers

energy as heat out of an energy as heat out of an object to lower its object to lower its

temperature.temperature.

Definition of a Cooling Definition of a Cooling SystemSystem

• A device that transfers energy as heat out of an object to lower its temperature.

Purpose of a cooling systemPurpose of a cooling system• To remove unwanted thermal energy from a

space• Examples: air conditioner, refrigerator• Analogy: house or refrigerator is like a leaky

boat… – thermal energy is constantly being transferred inside

(like a leak in the boat) – Thermal energy must be removed (like bailing the water

out of the boat)– Thermal “bailing” involves absorption of excess thermal

energy inside by evaporation of a refrigerant (at the evaporation coil) and the release of this thermal energy outside by condensation of the refrigerant (at the condensation coil).

Ice Boxes

Ice Boxes

Modern Air conditioners and Modern Air conditioners and RefrigeratorsRefrigerators

Parts of a cooling system/refrigeratorParts of a cooling system/refrigerator

Main parts

•Refrigerant

•Expansion valve

•Evaporator coil

•Compressor pump

•Condensation coil

Work is done by the compressor to force the gas into liquid (condensation releases heat)

Expansion valve releases pressure, allowing liquid to evaporate (evaporation absorbs heat)

Applications of Phase Changes

23.8 Energy and Changes of Phase

A refrigerator’s cooling cycle uses the changes of phase of the refrigeration fluid (not water).

Applications of Phase Changes

23.8 Energy and Changes of Phase

• Liquid is pumped into the cooling unit, where it is forced through a tiny opening to evaporate.

Applications of Phase Changes

23.8 Energy and Changes of Phase

• It draws heat from the things stored in the food compartment.

Applications of Phase Changes

23.8 Energy and Changes of Phase

• The gas then goes to coils located outside the cooling unit.

Applications of Phase Changes

23.8 Energy and Changes of Phase

• As the gas condenses in the coils, heat is given off.

Applications of Phase Changes

23.8 Energy and Changes of Phase

• The liquid returns to the cooling unit, and the cycle continues.

Parts of a cooling system/ACParts of a cooling system/AC

Evaporation and Evaporation and CondensationCondensation

•Evaporation is a cooling process: takes heat from surroundings

•Condensation is a warming process: releases heat into surroundings

FUNCTION OF A FUNCTION OF A REFRIGERATORREFRIGERATOR

                              

Remove unwanted Thermal Energy from inside

Deposit unwanted Thermal Energy on the outside

By evaporation

By condensation

HOW DO AIR CONDITIONERS HOW DO AIR CONDITIONERS AND REFRIGERATORS WORK?AND REFRIGERATORS WORK?

• Evaporation of refrigerant removes thermal energy from inside.– Evaporation occurs when liquid refrigerant is

allowed to expand (pressure release—lowers vaporization point—heat drawn from food causes evaporation)

• Condensation of refrigerant releases thermal energy to the outside – Work must be done by the compressor to

compress the refrigerant gas so that the thermal energy drawn from food is released outside when the gas is forced to condense under pressure)

The evaporation-The evaporation-condensation processcondensation process

Blue is evaporation,

Red is condensation.

Leaky Boat AnalogyLeaky Boat Analogy• How does this video clip relate to a

air conditioner or refrigerator cooling system?

http://video.google.com/videoplay?docid=7180930982205868945&ei=QIPLSdnhH4bWqALJk6zGBw&q=bailing+water+&hl=en

REVERSE HEAT ENGINEREVERSE HEAT ENGINE

MECHANICAL ENERGY IN : THERMAL ENERGY OUT

Cooler gas becomes warmer when compressed

The Function of an air The Function of an air conditionerconditioner

Nature of GasesNature of Gases• Gases cool as they expand.• Gases warm as they are compressed.• Why?• Gas particles have to do work to expand (farther

to travel). This reduces the average KE and therefore the gas cools.

• Gases heat up when they are compressed because the work done to compress the gas is transferred to the gas particles, increasing the average KE.

HEAT ENGINEHEAT ENGINE

THERMAL ENERGY IN : MECHANICAL ENERGY OUT

• Heat can be made to flow the other way only if work is done to the system. External effort!

• Example: air conditioner or refrigerator

• What if we do mechanical work on a system?

• We should expect an increase in thermal energy.

• Example: Pumping up a bicycle tire.