Starter Question If you touch two objects that are the same temperature, why would one feel colder than the other?

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Starter Question If you touch two objects that are the same temperature, why would one feel colder than the other?

Starter Question If you touch two objects that are the same temperature, why would one feel colder than the other? One is a better conductor of thermal energy than the other. Example: sauce pan

Insulators vs. Conductors
Conductors transfer thermal energy rapidly (metals) Insulators reduce the transfer of thermal energy (wood, foam) Insulation reduces unwanted transfer of thermal energy

R-value of insulating materials
Which materials are the best insulators? What is insulating glass and how does it differ from flat glass? How does changing the thickness of building materials affect insulating effectiveness?

Trapped air makes a good insulator

Cooling System A device that transfers energy as heat out of an object to lower its temperature. Work must be done against heat flow to transfer heat from inside air to outside air

2nd law of thermodynamics
Heat flows from hot to cold Heat flows until thermal equilibrium is reached Temp inside = Temp outside During the summer… Heat flows from outside your home to inside and does not stop unless thermal equilibrium is reached Refrigerator… Heat flows from outside the refrigerator to inside

Air conditioners and Refrigerators

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

The Function of an air conditioner

HOW DO AIR CONDITIONERS AND REFRIGERATORS WORK?
These appliances produce a thermal energy flow by evaporation and condensation. Evaporation removes thermal energy. Condensation releases thermal energy.

23.8 Energy and Changes of Phase
Applications of Phase Changes A refrigerator’s cooling cycle uses the changes of phase of the refrigeration fluid (not water).

23.8 Energy and Changes of Phase
Applications of Phase Changes Liquid is pumped into the cooling unit, where it is forced through a tiny opening to evaporate.

23.8 Energy and Changes of Phase
Applications of Phase Changes It draws heat from the things stored in the food compartment.

23.8 Energy and Changes of Phase
Applications of Phase Changes The gas then goes to coils located outside the cooling unit.

23.8 Energy and Changes of Phase
Applications of Phase Changes As the gas condenses in the coils, heat is given off.

23.8 Energy and Changes of Phase
Applications of Phase Changes The liquid returns to the cooling unit, and the cycle continues.

Air Conditioning and Refrigeration
Work is done to remove heat that is entering a house, car or refrigerator Evaporation removes heat and condensation releases heat (evaporation inside, condensation outside) Analogy: Leaky boat Water represents heat flow Bailing represents removal of thermal energy Collecting water in bucket (evaporation of refrigerant) Dumping water outside boat (condensation of refrigerant)

Evaporation and Condensation
Evaporation is a cooling process: takes heat from surroundings Condensation is a warming process: releases heat into surroundings Uses the property of gases cooling during expansion and warming during compression Uses the concept of reverse heat engine to compress a gas (mechanical energy in – thermal energy out)

The evaporation-condensation process
Blue is evaporation of refrigerant (draws heat from inside) Red is condensation of refrigerant (releases heat to the outside)

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.

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

HEAT ENGINE THERMAL ENERGY IN : MECHANICAL ENERGY OUT

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

CHANGES OF STATES OF MATTER

Heat can be made to flow the other way only if work is done to the system. External effort!
Example: air conditioner or refrigerator

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