Absolute Zero and the Conquest of Cold  
   
 
 

 

 

 

Topic 2: Understanding Heat and Energy

To study the extremes of cold temperatures, one first has to know why anything becomes cold -- or hot. Step outside on a freezing day and it feels like the cold seeps right in to your bones, but that's not actually what happens. "Cold" can't move; it's heat that travels. Any two objects when touching each other will try to reach the same temperature. Around your body, this means your body heat rushes out to warm the cold air around it. You feel cold because you've lost heat, not because the cold air has transferred any "cold" to your body.

In the late 1700s, heat was thought to be an actual material, called caloric, that could seep from one object to the next and make it hot. The more caloric in a substance, the hotter it was. Because liquids were seen to expand when they warmed up, it made sense to early scientists that heated liquids swelled because a big dose of caloric had just been added. Since they thought caloric was a physical substance, they also thought it couldn't be created or destroyed, and the amount of heat in any system would always stay the same, or that heat is always conserved.

The caloric theory isn't true. In the late 1800s, scientists realized that heat isn't some kind of material you can touch. Instead, heat is a property of matter that depends on how fast the atoms inside it are moving -- the faster they move, the more energy they have, and that is what makes matter hot.

Even before scientists knew there were atoms inside matter, they realized that there was a standard relationship between heat and motion: movement can create heat and heat can create movement. This relationship is what makes a train’s steam engine work. Hot steam is moved into a cylinder, where its pressure pushes a piston, which in turn forces the train’s engine parts to turn. In the 1840s, scientists like James Joule discovered that a given amount of work would generate the same amount of heat every time. Joule experimented on the amount of mechanical work needed to raise the temperature of a pound of water by one degree and this number became known as the mechanical equivalent of heat .

Remember that when scientists thought that heat was created by something called caloric, they thought heat was always conserved. The discovery that heat could change into motion and vice versa taught everyone that heat isn't conserved at all -- but energy is. That insight turned out to be crucial in the quest to make things colder and colder: if you can turn heat into another kind of energy like mechanical energy -- then you’ve gotten rid of the heat! Heat is lost the same way your body loses heat in the wintry outdoors, and the material gets colder.

This is also what makes a refrigerator work. Inside a refrigerator, a liquid refrigerant is turned into a gas. Turning a liquid into a gas requires energy. Where does that energy come from? It comes from heat energy that was inside the refrigerator -- and stealing heat from the refrigerator makes its inside colder.

Additional Resources
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More Topics

  • Topic 1: Measuring the Cold - Thermometers
  • Topic 2: Understanding Heat and Energy
  • Topic 3: States of Matter
  • Topic 4: Refrigeration
  • Topic 5: Cryogenics
  • Topic 6: The Quest for Absolute Zero
  • Topic 7: How Animals Survive the Cold
  • Topic 8: Superconductivity
  • Topic 9: Astronomy
  • Topic 10: Spaceflight
  • Topic 11: Agriculture
  • Topic 12: Cold Medicine
 
 
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