The refrigerators that we use in our kitchens cool continuously. No matter when we put anything warm inside, the refrigerator will immediately start cooling it down. All the heat that the refrigerator absorbs from the object it's cooling is dumped straight into the room. Likewise, any heat that leaks in through the insulation goes right back out. (Normally we don't think about the heat being dumped into the room, because there's not that much of it. But into the room it goes, because energy cannot be destroyed.)
The ADR does not run continuously. It stores the heat that it absorbs, both heat from cooling warm objects and heat that leaks in. The part of the ADR that stores the heat is called the "salt pill". This name sounds like something you'd take to avoid heat exhaustion, but in fact it doesn't necessarily look like a pill and is not made of table salt. It's a block of a paramagnetic (i.e. weakly magnetic) substance. Often, the material is one of the general class of materials called "salts", which includes table salt as well as many other chemicals. The salt pill for the XRS ADR, for example, is made of the salt ferric ammonium sulfate. The salt pill may be shaped like a pill, but it doesn't have to be. The salt pill for the XRS ADR is a long, narrow cylinder.
The low temperature ADR's that we use cannot dump the heat into the room. They need a much colder heat sink to dump the heat. For example, the XRS ADR dumps its heat into a liquid helium bath at 1.3 Kelvin (1.3 degrees above absolute zero.)
In a paramagnetic substance, each molecule acts like a tiny electromagnet, with the electrons playing the part of tiny electic currents. In nonparamagnetic substances, the fields of the various electrons all cancel each other out, leaving the molecule with no overall field. In paramagnetic molecules, however, the fields don't quite cancel, so the molecule produces a small field. An ADR salt pill, then, is like a group of microscopic magnets all packed in together.
To get a mental image, you might try picturing a tiny compass needle attached to each molecule. The microscopic compass would point in the direction of the molecule's magnetic moment. (The magnetic moment of a magnet is a measure of the direction and strength of the magnet's field. The magnetic moment of a compass needle, for instance, is along the direction of the needle.)
A salt pill would thus be like an array of tiny compass needles. Here's a schematic diagram of a section of a salt pill. In this diagram, we imagine that a weak magnetic field in the vertical directon has been applied to the salt pill. If the field is weak, some of the microscopic compasses will line up with it, and some won't.
A real compass needle and the magnetic moment of a paramagnetic molecule are similar in some ways and different in others.
Here are two similarities:
Now some differences:
Here, for example, are the directions that one such molecule's magnetic moment
could point if the applied field were straight up.
D
This diagram is a bit oversimplified. The magnetic moments wouldn't stay in the plane of the screen. They'd be spinning, pointing now out of the screen, now in, but always tilting at the same angle to the applied field.
Here's a diagram of a small section of the paramagnetic salt pill, as it would be with a strong magnetic field forcing all the molecular magnetic moments to line up.
The behavior of the molecular magnetic moments seems really strange to those of us used to everyday things like compass needles. There's a whole field of physics, called Quantum Mechanics, that deals with the strange ways that microscopic things behave. A full explanation of quantum mechanics is beyond the scope of this website (and, let's face it, beyond the scope of this website writer!) If you'd like an interesting account of how physicists came up with the quantum theory, try Questioners: Physicists and the Quantum Theory by Barbara Cline.
The salt pill can absorb heat because of the strange properties of the molecular magnetic moments. On the microscopic scale, heat energy consists of random vibrations of molecules. When the applied magnetic field is weak, there is enough energy in the random thermal vibrations to knock a molecular magnetic moment out of alignment with the field. Thus, the energy that was heat energy gets changed into magnetic energy of the molecules. As the salt pill absorbs more and more heat energy, more and more of the molecular magnetic moments get knocked out of alignment with the applied magnetic field. Eventually, the salt pill can't absorb any more heat. Instead of being all lined up, as in the diagram above, the spins are pointing every which way, like this:
At this point, the heat must be dumped. To dump the heat, the ADR operator does two things. One step is increasing the applied magnetic field, the other is turning on the heat switch that connects the salt pill with the helium coolant bath.
When the magnetic field is turned up, that increases the amount of energy the molecular magnetic moments must have to stay out of alignment with the field. When the field becomes high enough, the molecular magnetic moments give up their energy and flip back in line with the magnetic field. As the energy gets dumped by the molecular magnetic moments, it converts back into random molecular motion, i.e. into heat. All this dumping of heat causes the salt pill's temperature to rise.
As the salt pill's temperature rises above that of the liquid helium coolant, the operator turns on the heat switch. A heat switch does for heat what an electrical switch does for electricity. When you want the heat to be able to flow, you turn the heat switch on. When you want to block the flow of heat, you turn the heat switch off.
When enough heat has flowed to the coolant bath, the operator turns off the heat switch, then reduces the magnetic field. Once again, the amount of energy needed to knock a molecular magnetic moment out of alignment is small enough that random thermal vibrations have enough energy. Thus, the molecular moments begin absorbing heat, and the salt pill cools, starting another cycle.
For more information on the Goddard ADR, including pictures, see our ADR Page.
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