Temperature of resistor in latent heat experiment.

[peripatein]

Hi,

I have conducted an experiment for finding the latent heat of liquid nitrogen using a 5W power supply and a 200 ohms resistor. As part of my conclusions I am asked to discuss/refer to the resistor's temperature during the experiment. I am not sure whether I am expected to provide a quantitative answer, though.
How may I answer that? Isn't its temperature essentially the same as that of the liquid nitrogen, provided it is inside the dewar?

 

[gneill]

The resistor will be made of some materials with physical properties like thermal mass, thermal resistance, and so on. As such it will behave as a physical system with time constants and so on. Will the core of the resistor always have the same temperature as its skin? How will heat move from the resistor to the nitrogen? Radiation? Convection? Is there a coefficient that describes heat transfer at this interface? Is heat conducted in or out of the dewar via the resistor leads?

Do the time constants associated with heat transfer in the resistor matter in the context of the experiment, or does the experiment proceed so slowly that such time constants are insignificant?

These are the sorts of things you need to ask yourself about your equipment so that you can properly evaluate the "meaning" of your observations.

 

[peripatein]

That doesn't really answer the question. I am not expected to discuss it in great length or analyse it rigorously (I think). I am expected to provide a relatively succinct, general answer.

 

[gneill]

That doesn't really answer the question. I am not expected to discuss it in great length or analyse it rigorously (I think). I am expected to provide a relatively succinct, general answer.

Fine, but it's up to you to distill an appropriate answer from what you know or can find out about the setup. Have you made the assumption that the temperature difference between the resistor and the surrounding bath of liquid nitrogen is insignificant? Is the assumption supported in any way (either by direct measurement or by experimental methodology)?

 

[peripatein]

I haven't checked these aspects and I am not sure I am expected to. During the experiment we didn't delve into anything related to the physical properties of the resistor itself. We merely measured dm/dt for various powers and then plotted dm/dt(P) in order to find 1/L, hence L (where L denotes latent heat).

 

[gneill]

I haven't checked these aspects and I am not sure I am expected to. During the experiment we didn't delve into anything related to the physical properties of the resistor itself. We merely measured dm/dt for various powers and then plotted dm/dt(P) in order to find 1/L, hence L (where L denotes latent heat).

If you there were no 'corrections' to be applied for the resistor's thermal properties, then you can perhaps state the implied assumptions.

 

[peripatein]

Meaning? That its temperature was throughout, aside momentarily being altered by the power, essentially the same as that of its ambiance?

 

[gneill]

Meaning? That its temperature was throughout, aside momentarily being altered by the power, essentially the same as that of its ambiance?

That is a possible conclusion, yes. Was the power applied all in one short burst or was it applied evenly over time during a "run"? If the latter, then there was probably not even a "momentary" difference.

 

[peripatein]

The resistor was inserted into the dewar and the power was on throughout the experiment (the voltage was altered, for measuring dm/dt for different powers), apart from at the beginning and end (for evaluating natural vaporization purposes).
Do you reckon that assumption valid then?

 

[gneill]

The resistor was inserted into the dewar and the power was on throughout the experiment (the voltage was altered, for measuring dm/dt for different powers), apart from at the beginning and end (for evaluating natural vaporization purposes).
Do you reckon that assumption valid then?

Very likely I would, given the information presented. But then, I wasn't there when the lab was being done

 

[peripatein]

Thanks. By the way, as the experiment progressed the latent heat which was found in the second part (through the process as described above) turned out to be smaller than that found in the first part (through measuring the rate of change of the mass for one certain voltage). Is it merely because the mass/volume/pressure of the liquid nitrogen had meanwhile decreased?

 

[gneill]

Thanks. By the way, as the experiment progressed the latent heat which was found in the second part (through the process as described above) turned out to be smaller than that found in the first part (through measuring the rate of change of the mass for one certain voltage). Is it merely because the mass/volume/pressure of the liquid nitrogen had meanwhile decreased?

Quite possibly. There's an assumption or two being made about the thermal equilibrium of the contents of the dewar. The ratio of thermal masses in the dewar is changing over time (more "resistor" and less "nitrogen").