On 11/3/2014 12:06 PM, Lostgallifreyan wrote:
rickman wrote in :

It takes the same amount of heat to
raise a substance 1 degree at 77 °K as it does at room temperature.

Ok, but when I read (or hear on BBC radio science programs) that it takes FAR
more effort (energy) to pump from 2K to 1K than it does from 300K to 299K,
what am I supposed to make of that given what you just said?

Ok, I'll grant that few who have not had thermodynamics really
understand heat. Thermo was not an easy part of the curriculum in
school. The reason why cooling something gets harder as it approaches
absolute zero is because the heat flow is proportional to the difference
in temperature. Even if your pump is perfect and acts as if you put the
thing being cooled in contact with a heat sink at 0 °K, the rate of heat
flow decreases as that temperature delta diminishes.

The reality is that thinking 77 °K is especially cold is a bit of an
exaggeration. Yes, it is cold by human experience, but in the world of
cryogenics it is just a step stool to board the rocket. Thinking that
any little heating effect would warm a high temperature superconductor
is thinking with your feelings and not your brain. Not that we don't
all do that. But you need more experience with this stuff to let your
instinct guide you.

--

Rick