> Well, with no actual data to estimate with, it's kinda hard. :-P

The point is you are meant to have a rough understanding of density of 
gases.

> Heh. Well, I have no idea what the volume of this room is.

Usually best to simplify it to a shape that it is easy to calculate the 
volume of, ie a cuboid.

> I don't even "really" know how big a meter is. It used to be the length of 
> my arms, but they're a tad bigger now. ;-)

You can estimate though, I suspect you can tell if your room is 2 metres 
long or 10 metres long, or 50 metres long.  Remember you are about 2 metres 
tall...

> (E.g., off the top of my head Oxygen is supposed to be something silly 
> like 0.000004 g per cubic meter or something.)

Wikipedia tells me it's 1.429 kg per cubic metre...

> Now, see, I was under the impression that lagging traps air, and hence the 
> "effective" surface area would still be the same.

No, because the lagging has thickness, and the outer surface of the lagging 
is the effective surface area for radiating the heat.  Even though the 
surface temperature will be less with the lagging, for small radii the 
surface area increase is enough to overcome the effectiveness of the 
lagging.  Obviously the exact radius depends on the situation, but there 
will always be a minimum value where lagging works (below that it has the 
opposite effect than you probably imagined).

> Aww, c'mon! I thought that was a really neat drawing! :-D

Yes I have to admit it was pretty cool.

> ...OK, I am now completely bemused. Apparently e^3 is roughly 20. I cannot 
> work out how that can possibly be. 2^3 is clearly 8. My head hurts just 
> thinking about it.

Think about 2^3 and 3^3, then e^3 doesn't seem so bad.

> As an aside, I notice the difference turns out to be especially tiny. :-P

Would have been cool if e^pi = pi^e

> I think you need... THERMITE!! 8^D

Unfortunately this was one of the few lecture courses with no fun examples 
shown off by the lecturer - we had boomerangs, magnetic guns, launched 
ping-pong balls etc, but sadly no model jet engine :-(  The whole course was 
spent studying the thermodynamic cycles of various types of heat engines, 
combined cycle power stations, pressures, velocities, energies argghh my 
head hurts just thinking about it.

Post a reply to this message