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?

On 2014-11-03 17:06:02 +0000, Lostgallifreyan said:

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?

That's energy to keep all the heat from the surrounding environment
out. In a system completely separated from hot material or radiation,
such as space, the energy is exactly the same, because of the way
temperature is defined.

--

Percy Picacity

Percy Picacity wrote in
:

That's energy to keep all the heat from the surrounding environment
out. In a system completely separated from hot material or radiation,
such as space, the energy is exactly the same, because of the way
temperature is defined.


Great, so there's a justification for researching superconductors in space,
no? Just when someone here assiduously claimed there was not.

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

rickman wrote in :

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.


Ok, that helps. It's close to what I had in mind, though my reasoning may
still be bad. For what it's worth... if a superconductor is very cold,
needing to be so, then because there is no way to go below zero K, there are
more things hotter, than colder, so they have more effect than the shaded
space conditions. That balance might favour a need for forced cooling just to
play safe in many cases, but I accept that isolation might be fairly easy to
do, and I also accept that 77K is likely far enough above shaded space
conditions that it gives a wide margin to prevent small leaks from nearby
heat sources causing failure.

On 11/3/2014 3:43 PM, Lostgallifreyan wrote:
rickman wrote in :

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.


Ok, that helps. It's close to what I had in mind, though my reasoning may
still be bad. For what it's worth... if a superconductor is very cold,
needing to be so, then because there is no way to go below zero K, there are
more things hotter, than colder, so they have more effect than the shaded
space conditions. That balance might favour a need for forced cooling just to
play safe in many cases, but I accept that isolation might be fairly easy to
do, and I also accept that 77K is likely far enough above shaded space
conditions that it gives a wide margin to prevent small leaks from nearby
heat sources causing failure.

There is nothing at all in your posts that is based on any facts.
Science and engineering don't work on "might favor". Get the equations
and do the math. Otherwise it is just waving hands and flapping gums.

--

Rick

"rickman" wrote in message
...
Get the equations and do the math.

Physician, heal thyself.