# Gas in windows?

1. Mar 7, 2005

### Physics is Phun

We have to to a report in our chemisty class on heating houses. One aspect is the window. Our teacher says that good windows are filled with argon becuase they have a high specific heat. but looking on the internet I seem to find the specific heat of argon to be about 520 J/kgC which is relatively low, but my teacher says it is very close to Helium which is 5250 J/KgC this seems to be off by a factor of ten. Which is right? it seems that helium and argon would be close to the same as they are both noble gases and are very close on the Table, so they should share very similar properties.

2. Mar 7, 2005

### Physics is Phun

Another question that may help me with this project. Why is it that our attics are not entirely filled with insulation. Since there is a layer on the floor of the attic(this is assuming the type of attic that is not meant to be lived in or have stuff stored in) why not fill the whole thing with insulation?

3. Mar 8, 2005

### Physics is Phun

anyone any ideas?

4. Mar 8, 2005

### GCT

I would imagine the two noble gases to be fairly different in there properties, here's a site you can go to investigate.

http://www.webelements.com

5. Mar 8, 2005

### Gokul43201

Staff Emeritus
I'm pretty sure He and Ar have nearly identical Cv of about 12.5 J/K-mol (ideal monoatomic gas), so I think your number for He may be wrong (I can doublecheck later to make sure). However, gases like oxygen or nitrogen will have higher specific heats (they are diatomic). So, doesn't that suggest that perhaps specific heat is not the only important property ? Can you think of any other thermal/thermodynamic properties that are important for insulation ?

6. Mar 8, 2005

### brewnog

So that you can have a room there, obviously.

7. Mar 8, 2005

### DaveC426913

Consider the cost-benefit analysis. What would it cost, and how much would you gain over the perfectly free insulating substance filling the room already?

8. Mar 8, 2005

### Speed

SHC of He: 5.19kJ/kgK
SHC of Ar: 0.523kJ/kgK

Atomic mass of He: 4.00
Atomic mass of Ar: 39.95

Number of He atoms in 1 kg: $$1505 \times 10^{23}$$
Number of Ar atoms in 1 kg: $$150.7 \times 10^{23}$$

There are 10 times more He atoms than Ar atoms.

Using the relationship:
Average kinetic energy of molecules = $$\frac{3}{2}kT$$
Total KE of molecules = $$\frac{3}{2}NkT$$

Where N = number of atoms/molecules. 10 times more He means 10 times more energy to heat them by 1K.

9. Mar 8, 2005

### Speed

And another thing: gas insulated inside a window needs specific heat capacities at constant volume. These heat capacities will be lower than the ones at constant pressure.

10. Mar 8, 2005

### Gokul43201

Staff Emeritus
Oops ! Didn't pay attention to the units in the OP.

The relevant units are J/K-mol, and not J/K-kg, since it's the volume of gas used that's important, not the weight.

11. Mar 8, 2005

### Gokul43201

Staff Emeritus
Taking this one step further, to its natural conclusion :

$$E = \frac{3}{2}NkT$$

$$C = \frac {\partial E}{\partial T} = \frac{3}{2}Nk = \frac{3}{2} R = 1.5 * 8.315 \approx 12.5~J/K-mol$$

Of course, this is for a monoatomic ideal gas.

12. Mar 17, 2005

### rachmaninoff

Also, using argon is much more practical because 1) it's relatively common (compared to helium) and 2) helium leaks very easily (small atoms).

13. Mar 18, 2005

### Bystander

C'mon folks --- pay attention! Does Gokul have to tie the hint to a brick and drop it on your toes? Heat capacities of gases at one atmosphere are trivial compared to the heat capacities of the panes confining them. Think insulation, think heat flow, think thermal conductivity. Think inversely proportional to molecular weight (sq. rt.), rms speed --- kapiche?

14. Mar 19, 2005

### pack_rat2

I think what he really MEANT was that they should have low thermal conductivity. A high sp. heat will result in something of a thermal "flywheel" effect, which can smooth out temperature variations (log cabins are really good in this respect). But a window isn't massive enough to make much difference. On the other hand, windows are the major cause of heat loss (or gain in the summer) in a room, and the greater the insulating value of a window (low thermal conduction), the better.