# Do Ideal Gases Absorb Heat?

1. Jul 11, 2007

### s.p.q.r

Hi,

Do IDEAL gases absorb heat when they expand? I asked a few people this question, half said yes and half said no.

Im after a simple yes or no answer with a small explanation to clarify this one.

If anyone knows, please reply.

Cheers!

2. Jul 11, 2007

### belliott4488

Geez, I sure would have thought so ... how else would their temperatures rise? What was the argument against this?

- Bruce

3. Jul 11, 2007

### s.p.q.r

The arguments against are-

-gasses expand because of the heat applied but don't actually take in any heat from around them. the heat/energy increases the activity between the atom not within the atom

-While gasses can take in heat while they are expanding, an expanding gas does not necessarily need to take in heat. For example, during an adiabatic expansion, the gas expands without exchanging heat with its surroundings. The temperature of the gas decreases because its internal energy supplies the work necessary for the gas to expand.

Sounds correct to me. But, my 1st thought was that ideal gases do absorb heat. This is a harder question then I thought.

Anyone else have an idea?

4. Jul 11, 2007

### belliott4488

What does "activity between the atom(s)" mean? It sounds like more kinetic energy, which means more heat.

Yes, during adiabatic expansion that's true, but of course, not all expansion is adiabatic. Am I not understanding the question correctly?

5. Jul 11, 2007

### Staff: Mentor

You can make a gas absorb heat or not. It is all a matter of the process. If, for example, you expand a gas through a throttling valve and the valve and pipes are insulated, the gas will expand and cool and not absorb heat. If, for example, you take a non-rigid container of a gas and apply heat to it, the gas will absorb heat and expand.

6. Jul 11, 2007

### Bystander

Free expansion, no heat; make it work to expand, and it absorbs heat --- part of the definition of an "ideal gas."

7. Jul 11, 2007

### Just some guy

ideal gases can expand isothermally so I would assume they could absorb heat.

Anyway ideal gases were meant to be a simple model of a gas that accurately reflects reality as far as it can; it's a pretty rubbish model if it forbids isothermal expansions :s

8. Jul 11, 2007

### Bystander

How does free expansion forbid isothermal expansion?

9. Jul 12, 2007

### s.p.q.r

Thank you all for your help. Much appreciated. I am interested in the reply of russ_watters.

So, they wont absorb heat through a throttling valve. (I think I know what that is)
Will the ideal gases absorb heat if they were in a high pressure container?

Thanks Again.

10. Jul 12, 2007

### Just some guy

I never said it did :/

11. Jul 12, 2007

### Staff: Mentor

Bottom of the page (you may as well read the whole page...): http://en.wikibooks.org/wiki/Engineering_Thermodynamics/First_Law
If you don't apply heat to it, it won't absorb heat.

Last edited: Jul 12, 2007
12. Jul 12, 2007

### s.p.q.r

"If you don't apply heat to it, it won't absorb heat"

What if I did apply heat to it? Will it absorb this heat? If so, to what extent?

Pls get back.

Cheers.

13. Jul 12, 2007

### alvaros

Ideal gases when they expand ( when you allow them to fill a bigger
volume ) dont absorb heat ( dont change their temperature ).

Real gases do, because their molecules are attracted between them.

In ideal gases its supposed the molecules dont feel any attraccion.

14. Jul 12, 2007

### Andrew Mason

There is no correct answer your question. It is like asking whether a car gains energy when it goes down the road.

You have to apply the first law of thermodynamics to any situation.

$$\Delta Q = \Delta U + \Delta W$$

where $\Delta W$ is the work done by the gas. If in any process, $\Delta Q > 0$ then there is a heat flow into the gas. If $\Delta Q < 0$ then there is a heat flow out of the gas.

If the gas expands, the gas does work, so $\Delta W > 0$. But that does not tell you if heat flows into the gas. You have to know the change in temperature of the gas in this process. If it does not change temperature ($\Delta U = 0$) then Q is positive. If it loses internal energy in an amount that is less than the work done, Q is positive. If it loses more internal energy than the work done, then Q is negative. etc.

AM

15. Jul 13, 2007

### Staff: Mentor

As I said in my first post, it is all a matter of the process. If you apply heat, it will absorb heat, but the particulars depend on the process.

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