How does a pressure cooker work ?
Explain why the time to cook something is so much shorter

Increasing the pressure while keeping a volume constant a provides an additional increase in temperature, according to

\frac{V_1P_1}{T_1} = \frac{V_2P_2}{T_2}

Since volume is constant,

\frac{P_1}{T_1} = \frac{P_2}{T_2}

Increasing the pressure while keeping a volume constant a provides an additional increase in temperature

I think the cause and effect here is backwards, and it ignores the effect of water turning to steam in a closed container. You put the pressure cooker on the stove and that heats up the air & water inside. There is an increase in air pressure as the temp goes up (because the lid is sealed V is constant, so as T rises P rises). The big deal though is when the water heats up enough to boil - now you get steam which is confined within the cooker and the pressure really increases. Using the ideal gas law PV=nRT, you have just increased n, which is the number of moles of gas in the container. So P increases dramatically. And the food cooks quicker because the water vapor in the steam has a higher heat content than dry air - that's why you can instantly be scalded by hot water at 200 degrees F but not by hot air at the same temperature.

I forgot that formulae (PV=nRT):o. What's 'R'?:o

I forgot that formulae (PV=nRT):o. What's 'R'?:o

R is the ideal gas constant, which makes the units work out. See:
Gas constant - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Gas_constant)

Darn, if I understood well, another irrational number...

Hi, king-of-science!

In addition to the fine answers that you've already received here, have you tried the following search?

How a Pressure Cooker Works - Google Search (http://www.google.com/search?hl=en&safe=off&q=How+a+Pressure+Cooker+Works)

Thanks!

Darn, if I understood well, another irrational number...

Why do you think R is an irrational number? It's just a constant that has been measured to some level of accuracy. Without getting too far off track from the original topic - for all practical purposes we treat all measurements and constants in nature as rational numbers in so far as there's a limit to how accurate anything can be measured. You can determine the value for a constant like R to perhaps 8 or maybe 10 decimal places, but even though the digits may not repeat that doesn't mean that R is fundamentally an irrational number. Nor does it mean that it's a rational number - it's impossible to tell. Even if you could measure R to a million decimal places that still wouldn't determine it. There is always a level of uncertainty in all measurements, and within the bounds of uncertainty in the measurement there are an infinite number of possibilities, both rational and irrational.

Why do you think R is an irrational number? It's just a constant that has been measured to some level of accuracy. Without getting too far off track from the original topic - for all practical purposes we treat all measurements and constants in nature as rational numbers in so far as there's a limit to how accurate anything can be measured. You can determine the value for a constant like R to perhaps 8 or maybe 10 decimal places, but even though the digits may not repeat that doesn't mean that R is fundamentally an irrational number. Nor does it mean that it's a rational number - it's impossible to tell. Even if you could measure R to a million decimal places that still wouldn't determine it. There is always a level of uncertainty in all measurements, and within the bounds of uncertainty in the measurement there are an infinite number of possibilities, both rational and irrational.


Ok, I was just saying it like that...