Martin Brown writes
On Jul 30, 1:06 pm, "Uncle Marvo"
wrote:

Essentially because air has next to no thermal inertia and so the heat
pump in the fridge cycles much faster when there is nothing at all
except the plastic and air inside the freezer. Many freezers have a
couple of cold blocks for picnic baskets in them by default which
helps. Basically a freezer chock full of stuff has a large thermal
inertia and the compressor for the heat pump only needs to come on
very rarely.

What do you mean by 'thermal inertia'? Sorry, my lack of interest in A
level physics is showing up here.

From below, I think what you're saying is that the small amount of heat
getting though the insulation is able to raise the temperature of air by
more than it is able to raise the temperature of something solid, so the
deepfreeze has to do more work to keep the temperature down when there
is more air inside compared with solids.

Why is that? Is it simply because air is a mixture of gases, ie it's the
physical gaseous state?


In this case it gives the freezer a larger heat capacity so that the
tiny leak of heat getting in through the insulation can only change
the interior temperature very slowly. When it is empty the pump will
come on more often.

But if having lots of air in means the leak of heat can change the
temperature more quickly, doesn't the reverse also happen - that
lowering the temperature of a solid mass takes more energy than lowering
the temperature of a gaseous mass. And if not, why not?

Does exchange of air when you open the door have an effect? Cold air
billows out, so must be replaced by warmer air, that then needs to be
cooled.
--
Kay