# Homework Help: Determining Absolute Zero

1. Feb 24, 2014

### tj19926

I just performed a lab in my physics class, and there are a few conceptual questions that I am having trouble with. The lab was simple. We took a constant volume sphere made of Stainless steel and filled with air. We changed the temperature of the water it was in to record the corresponding change in pressure.

1.The sphere used was filled with air, which is not an ideal gas. Does this play a role in increasing percent error? Why or why not?
I think it would have an effect since air is not uniform throughout, but im not sure and not sure of why.

2. The sphere is made of stainless Steel. Is this a good choice? If so why? If not, why not and what would be a good alternative? Explain why it would be better.

I'm pretty sure that this has to do with specific heats. Stainless steel's is 490 J/kg. Im not sure if you would prefer a metal with a higher (such as Aluminum) or lower(Lead) specific heat in this experiment.

2. Feb 24, 2014

### Staff: Mentor

What is the definition of an ideal gas? In which respect does air not fit exactly this definition? (There are two principle differences.)

3. Feb 24, 2014

### tj19926

An ideal gas is supposed to be composed of randomly moving non interacting particles. I read that air can be treated as an ideal gas at standard temp and pressure. Would it ruin the percent error due its deviations from ideal gas behavior when we lower the temperature and pressure?

4. Feb 24, 2014

### Staff: Mentor

Non-interacting is one characteristic of an ideal gas, but not "randomly moving". The second characteristic might not be obvious, so I'll give it: it is made of point particles.

Does this correspond to air? If not, can this affect the measurements you are taking?

Yes, that's a pretty good approximation.

"Ruin" is a strong word here. The question said "play a role".

By the way, I don't know the answer to the question, as I don't know the details of the experiment. You have to figure out not only the answer to my question above, but also if it plays a role in your particular experiment. (In other words: how did you make use of the ideal gas law in the experiment and its analysis?)

5. Feb 24, 2014

### hilbert2

Ideally, the experiment would be done at several values of density of the gas, and then the results would be extrapolated to zero density.

6. Feb 24, 2014

### tj19926

Yeah, thats what we did. We used a variety of temps from -17c to about 80c and just estimated what 0K would be

7. Feb 24, 2014

### hilbert2

I changed the words "temperature and pressure" to "density" in my post. In a single experiment, you measure several P-T data points and then extrapolate to zero pressure. What I mean is that you should do several series of measurements with different densities of the gas in the container and then extrapolate again to find out what the result would be if the gas had zero density.

Actually, when I was a first year physics student some years ago, I did an identical lab experiment.

8. Feb 24, 2014

### BvU

I pick up on the second part: your concern is about the specific heat. Why ?
For your experiment you need a constant volume. What property influences the volume of the steel sphere under temperature change ?

And if you worry about ideal gas law deviations, check out the van der Waals equation