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Heating and Cooling Curves

Graphs used to describe changes of state

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A Useful Wet Blanket

A Useful Wet Blanket

Credit: Lepsius
Source: https://commons.wikimedia.org/wiki/File:Mutbenret-dwarfs.jpg
License: CC BY-NC 3.0

Life in the desert was difficult for the ancient Egyptians. One of the major problems was the heat. The common folk just had to live with it, but royalty carried their cooling system wherever they went. Fan bearers were available to wave fans, make a breeze, and cool the sweaty brow of the aristocrats.

News You Can Use

  • “Don’t be a wet blanket” is a phrase that means, “You’re ruining the party.” The term possibly comes from the wet fire blankets that were used decades ago to put out fires. In that context, a wet blanket was a useful item. In terms of social relationships, a wet blanket is no fun at all.
  • In hot, dry areas of the world, the inhabitants take advantage of a fundamental principle of thermodynamics. When water makes the transition from the liquid to the vapor state, energy is required to break the hydrogen bond holding the liquid molecules together and to provide sufficient energy for the individual water molecules to move freely (the same principle explains why melting ice will cool material around it.) The ancient Egyptians are credited with hanging wet blankets across the doors of their homes to provide cooling. The hot air would blow against the blanket, absorbing the moisture and cooling the air.
  • Modern evaporative cooling devices operate on the same principle. Water moistens a set of absorbent pads that have air blowing across them. As the air passes over the water, it is cooled while the water absorbs some of that heat energy and evaporates. The cooler air is then blown out of the cooler into the room. This system is only effective in dry climates (low humidity). Areas of the western United States are prime users of evaporative cooling.
  • Regular air conditioning operates on a different principle. Coolant is compressed by a pump. When it expands, the coolant absorbs heat from the surroundings. Air conditioners need a closed area for proper functioning, whereas evaporative cooling relies on a constant flow of outside air.
  • Evaporative cooling devices are often referred to as “swamp coolers”. The origin of the term is unclear. The cooling apparatus may be so named because of the odor that eventually develops from the growth of fungus on the wet pads. The moral: keep your equipment clean at all times.
  • Credit: Chris Johnston
    Source: http://www.flickr.com/photos/31387329@N07/7820804288/
    License: CC BY-NC 3.0

    Swamp cooler designed for cars without air conditioners [Figure2]


  • Watch a video about evaporative cooling at the link below:


Show What You Know

Use the links below to answer the questions that follow.

  1. What is evaporation?
  2. What three things are needed for evaporation to take place?
  3. How much energy is needed to evaporate one gram of water?
  4. How do evaporative coolers affect humidity?

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Image Attributions

  1. [1]^ Credit: Lepsius; Source: https://commons.wikimedia.org/wiki/File:Mutbenret-dwarfs.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Chris Johnston; Source: http://www.flickr.com/photos/31387329@N07/7820804288/; License: CC BY-NC 3.0

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