Cryogenics for English Majors

The State of the Matter

And now there came both mist and snow,
And it grew wondrous cold:
And ice, mast-high, came floating by,
As green as emerald.

Most of us remember from science class that there are three basic states (or phases) of matter: gas, liquid and solid. As our Ancient Mariner illustrates, water is the most familiar example of this: You can evaporate it into a gas, drink it as a liquid, or freeze it into a solid cube of ice. And to go from one state to another, you have to change the temperature.

We could get fancy here and talk about other exotic states of matter that scientists have created in laboratories, such as Bose-Einstein condensates and fermionic condensates. But for our purposes we’ll stick with gas, liquid and solid.

The important thing to remember is that when you move from one state of matter to another, you’re either adding or subtracting heat (i.e., energy). That means, in part, that atoms and molecules behave differently, depending on what state they’re in. Gas atoms, which exist at the highest temperatures, are the athletes of the group, solids are the couch potatoes at the low-end of the temperature scale for any particular material, and liquid atoms are in between. The athletes require more space to run around in, while the couch potatoes don’t mind sitting neatly in a row, crammed together on the sofa. It’s the whole heat expands, cold contracts thing.

Of course, not all atoms of a particular solid – let’s take for example a piece of aluminum – are vibrating at the same speed. If the solid is warm, they’re vibrating faster. If the solid is very cold, they’re slower. In many substances, when the atoms reach a specific critical temperature, they become quite sedate. This is what makes superconducting current possible: the electrons flow through the material with no resistance from the atoms (though the atoms continue to vibrate a little bit). That means there’s no heat generated in the process, as occurs in normal electricity. That’s the current that fuels the superconducting magnets.

The threshold between solid and liquid, of course, is the freezing point; between liquid and gas, it’s the boiling point. Different elements and compounds change states at different temperatures. Some elements don’t boil – i.e., go from liquid to gas – until well over 5,000 degrees Celsius ( 9,000 degrees Fahrenheit), hotter than some parts of the sun.

Helium is at the other extreme. It has the lowest known boiling point of any element, just a tad above the absolute coldest that anything can be.

Isn’t that special? We think so. Helium is special for a lot of other reasons as well. To talk about that, let’s make a dramatic temperature shift (again courtesy of Coleridge) from the nether regions of the thermometer to a much hotter spot.

Next Page Something Special about Helium

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