dewar vacuum flask

Something I thought about while I couldn't sleep last night. If you create a vacuum in a dewar flask with a thermometer inside, what would the thermometer read? What would the trend be?

Initially, the thermometer would read whatever its temp was when you placed it in the assumed “hard” vacuum.

Then slowly over time, the thermometer would begin to read the same temperature as the inside walls of the dewar. It would reach equilibrium with the inside walls by exchanging radiant energy with the walls.

Suppose you could shield the thermometer from radiant energy. What then would happen?

I guess a "shielded" thermometer means "no heat in, and no heat out".

So the thermometer would forever "read" its original temperature. Perfect dewar.

Well damned said Henry. BUT, it the thing reads 44 degrees, how could it retain that temperature? Does it not require energy to maintain temperature?

It requires energy, but no “new” energy input to retain temperature. Any new energy input and a proper thermometer would have to indicate this extra energy as a temperature increase.

A mercury-in-glass thermometer actually just indicates the volume of the fixed mass of mercury captured inside the glass envelope. When the temperature of the mercury increases, the molecular motion of the mercury increases and the average distance between the mercury molecules increases LINERALY (as it turns out) with temperature. This yields a linear increase in mercury volume with temperature.

So the energy within the mercury is actually the sum of the energy of each molecule of mercury. Make sense?

Why is the increase in volume linear?

I guess it obeys the “universal gas law”

P*V = n*R*T