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Q & A: energy in liquid nitrogen

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Most recent answer: 10/22/2007
Q:
It has been said that when objects get very cold they are said to be at a low energy state. If energy can neither be created nor destroyed, then why doesn’t a very cold object have the high potential energy? After all it took a great deal of energy to convert to this state. Example: It takes a great deal of electrical energy to convert gas nitrogen into liquid nitrogen (LN2). A quantity of LN2 should be the same energy equivalent as the energy it took to manufacture except for losses in the conversion process. Then what is the Energy Density of LN2 at 3 Degrees Kelvin?
- Eddie
A:
It does take a lot of energy to cool something far below room temperature. However, that energy doesn't get stored in the cold material. Instead, it leaves the refrigeration unit in the form of heat. (Try feeling the coils on your kitchen refrigerator.)

At ordinary pressures, nitrogen is a solid, not a liquid, at 3K. Helium is liquid at 3K, and used when a coolant is needed for something that cold.

The 'energy density' is a little arbitrary. The total energy density is given by the mass density times the square of the speed of light, but that consists almost entirely of rest energy. The thermal energy (the extra energy needed to heat from near 0 K) at 3K is low, beacause the heat capacity (how much heat is needed per unit temperature increase) goes toward zero near absolute temperature of zero.

This isn't a precise number, but a glance at a graph of the heat capacity of liquid helium indicates that at 3K the thermal energy would be around one or two calories per gram, i.e. 4 to 8 Joules per gram.

Mike W.

LN2 has less thermal energy than the room-temperature gaseous nitrogen it is made from. You have to add energy to it (thermal energy, which is really kinetic energy), to warm it up to its boiling temperature (usually it's delivered at this temperature and keeps liquid by constantly boiling off some), to boil it, and then to warm the resulting gaseous N2 up to room temperature. In so doing, LN2 removes energy from its surroundings, cooling them off, which is why it's such a great refrigerant. Of course the liquification process releases more heat than the LN2 can extract from whatever it's refrigerating because of the necessity of doing work on it to refrigerate it (LN2 liquifiers typically run on electricity).

Tom

(published on 10/22/2007)

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