B Atmospheric Burn-up During Re-Entry

[DaveC426913]

This is not correct. If you reduce the volume but carefully avoid adding energy to the contents (draining as much in thermal energy as you are injecting by performing mechanical work) the result is an isothermal compression.

The same amount of thermal energy is in a smaller space, but the temperature is unchanged.

Did you not see the reference to gas laws? They don't involve bleeding off excess.

The OP believes that compressing a volume of gas will not raise its temperature. That is not true.

You can always complicate the experiment to obfuscate the principle being demonstrated if you want.

 

[jbriggs444]

Did you not see the reference to gas laws? They don't involve bleeding off excess.

The OP believes that compressing a volume of gas will not raise its temperature. That is not true.

You can always complicate the experiment to obfuscate the principle being demonstrated if you want.

Both your post and the point you were responding to were incorrect. It does little to help correct a misconception if you do so by promulgating a new one.

 

[DaveC426913]

Both your post and the point you were responding to were incorrect.

How is my drawing attention to the ideal gas laws incorrect? The OP seems not to be aware of them.

(OK, the first half of my post phrased it using the OP's terms, but the point is made that the ideal gas laws show that compressing a gas will raise its temperature - other factors being equal.)

 

[jbriggs444]

the point is made that the ideal gas laws show that compressing a gas will raise its temperature - other factors being equal.)

And that point is incorrect.

In fact, per the ideal gas law, PV=nRT, compressing a gas (reducing its volume) while holding pressure and amount of substance constant can only be achieved by reducing temperature. If you are planning to have all factors equal, you'd better spell out which ones you are holding constant and how.

 

[davenn]

They are, however, always in the same place in the sky so that once you point your antenna at one it will stay pointed at it.

not quite ... they actually move in an oscillation north and south of the equator by a small amount
an example ...

​

 

[Rensslin]

Temperature isn't like "cats" in this analogy though. You have a box with 1 kg of air in it. You shrink the box. It still has 1 kg of air in it, but in the process of shrinking it, you had to apply force, so now the air that is in the box has more energy, and thus is at a higher temperature. The equivalent to the cats in your box is the quantity of air.

Got it: that makes sense. Thanks

 

[DaveC426913]

And that point is incorrect.

In fact, per the ideal gas law, PV=nRT, compressing a gas (reducing its volume) while holding pressure and amount of substance constant can only be achieved by reducing temperature. If you are planning to have all factors equal, you'd better spell out which ones you are holding constant and how.

< sidebar >
OK, I take full responsibility for not grokking this - after all, as a diver, I'm supposed to know the gas laws down pat.

Yes, it is obvious (even to me) that - if you plan to keep pressure constant - you'll have to reduce the temperature to get the volume to reduce. This I know.

But if you were to take a volume of gas, in a closed container - and compress it (reduce its volume) - both temperature and pressure will go up. After all, this is how diesel engines work. (Please tell me I'm right about this, or I'm going to set my laptop on fire, and go live in a shack in the woods)

So what am I missing? Is it simply that I didn't specify that pressure doesn't have to remain constant?

< /sidebar >

 

[Herman Trivilino]

But if you were to take a volume of gas, in a closed container - and compress it (reduce its volume) - both temperature and pressure will go up. After all, this is how diesel engines work. (Please tell me I'm right about this, or I'm going to set my laptop on fire, and go live in a shack in the woods)

Well, that is certainly one of the possibilities. In fact the quantity $pT$ must go up, but there are ways of doing that that will make $p$ or $T$ go down.

 

[phyzguy]

@DaveC426913, I think what people are objecting to is the following statement you made:

"Amount of heat" is an absolute value, yes. But if you reduce the volume, the temperature will increase proportionally (because the same "amount of heat" is now in a smaller space).

When you compress a gas, the temperature does not increase because, "the same "amount of heat" is now in a smaller space". It increases because the act of compression does work on the gas and increases the "amount of heat" in the gas. The "amount of heat", also called the internal energy, goes up. If the internal energy is held constant, the temperature of the gas will not go up, even if it occupies a smaller volume

 

[DaveC426913]

Cool. I learned some new things today.

One of them is that I had a naive understanding of the source of the temperature rise when you compress a gas.