Is it possible to transfer heat to a substance and it not increase in temperature? Yes, during a phase change

CHANGES OF STATES OF MATTER

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

Definition of a Cooling System A device that transfers energy as heat out of an object to lower its temperature.

Purpose 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

Modern Air conditioners and Refrigerators

Parts 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/AC

Evaporation and Condensation Evaporation is a cooling process: takes heat from surroundings Condensation is a warming process: releases heat into surroundings

FUNCTION OF A REFRIGERATOR Remove unwanted Thermal Energy from inside Deposit unwanted Thermal Energy on the outside By evaporation By condensation

HOW DO AIR CONDITIONERS 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- condensation process Blue is evaporation, Red is condensation.

Leaky Boat Analogy How does this video clip relate to a air conditioner or refrigerator cooling system? = &ei=QIPLSdnhH 4bWqALJk6zGBw&q=bailing+water+&hl =en

REVERSE HEAT ENGINE MECHANICAL ENERGY IN : THERMAL ENERGY OUT Cooler gas becomes warmer when compressed

The Function of an air conditioner

Nature 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 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.