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5.19: Cooling Systems

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A refrigerator door makes a great message center. Its smooth metal surface is perfect for sticky notes and magnets. In most homes, a refrigerator is one of the hardest working appliances, but not just because it holds messages. Unlike most other home appliances, a refrigerator generally runs nonstop every day of the year. Can you think of another home appliance that gets such constant use?

Purpose of a Cooling System

A refrigerator is an example of a cooling system. Another example is an air conditioner. The purpose of any cooling system is to transfer thermal energy in order to keep things cool. A refrigerator, for example, transfers thermal energy from the cool air inside the refrigerator to the warm air in the kitchen. If you’ve ever noticed how warm the back of a running refrigerator gets, then you know that it releases a lot of thermal energy into the room.

Q: Thermal energy always moves from a warmer area to a cooler area. How can thermal energy move from the cooler air inside a refrigerator to the warmer air in a room?

A: The answer is work.

How a Refrigerator Works

A refrigerator must do work to reverse the normal direction of thermal energy flow. Work involves the use of force to move something, and doing work takes energy. In a refrigerator, the energy is usually provided by electricity. You can read in detail in the Figure below how a refrigerator does its work. For an animation of how a refrigerator works, go to this URL: http://www.chemistry.wustl.edu/~courses/genchem/LabTutorials/Thermochem/fridge_movie.html

The Refrigerant

The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance such as Freon™ that has a low boiling point and changes between liquid and gaseous states as it passes through the refrigerator. As a liquid, the refrigerant absorbs thermal energy from the cool air inside the refrigerator and changes to a gas. As a gas, it transfers thermal energy to the warm air outside the refrigerator and changes back to a liquid. Work is done by a refrigerator to move the refrigerant through the different components of the refrigerator.

Summary

  • The purpose of a cooling system such as a refrigerator or air conditioner is to transfer thermal energy in order to keep things cool.
  • A refrigerator transfers thermal energy from the cool air inside the refrigerator to the warm air in the kitchen. Thermal energy normally moves from a warmer area to a cooler area, so a refrigerator must do work to reverse the normal direction of heat flow.
  • The key to how a refrigerator or other cooling system works is the refrigerant. A refrigerant is a substance with a low boiling point that changes between liquid and gaseous states as it passes through the refrigerator.

Vocabulary

  • refrigerant : Substance with a low boiling point that is used to transfer thermal energy in a cooling system such as a refrigerator.

Practice

At the following URL, watch the video “How Air Conditioners Work.” Then fill in the blanks in the statements below.

http://www.physics.org/explorelink.asp?id=537&q=air&currentpage=1&age=0&knowledge=0&item=5

  1. The basic concept behind an air conditioner is __________.
  2. The liquid in an air conditioner __________ at a very low temperature.
  3. Evaporation occurs inside metal __________.
  4. Air is cooled by being blown over the __________.
  5. The __________ turns the gas back into a liquid.

Review

  1. What is the purpose of a cooling system? What are examples of cooling systems?
  2. Outline how a refrigerator works.
  3. What is a refrigerant? Why is it the key to how a cooling system works?

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Description

Difficulty Level:

Basic

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Aug 22, 2014
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