Pressure Cooker Gas Law: Pressure Cooker Physics Explained

Denis Papin invented the pressure cooker in 1679, and being a physicist, it should come as no surprise that he used the laws of physics to create it. Understanding the pressure cooker and how it works means knowing the principle called the Ideal Gas Law.

This is the formula – PV = nRT.

PV = nRT Explained!

I put together this chart to make things easier to understand before hopping into the equation:

P =	Pressure
V =	Volume
n =	Amount of substance
R =	Ideal gas constant
T =	Temperature

The Ideal Gas Law takes pressure (P) times volume (V) to equal the amount of substance (n) and the ideal gas constant (R) multiplied by temperature (T). How does this equation relate to pressure cookers? Pressure cookers are semi-sealed devices, which is what makes the Ideal Gas Law work. The Ideal Gas Law says that when pressure increases, the temperature increases.

Pressure cookers maintain volume at constant levels, following the principles of the Ideal Gas Law.

How the Ideal Gas Law Relates to Pressure Cookers

Now that we looked at the law as a whole, we should understand what enters the equation as it relates to pressure cookers. In truth, volume, amount of substance, and the ideal gas constant won’t play much of a role in pressure cookers because the variables remain constant. For example, the water remains the same and the volume doesn’t change.

The pressure cooker equation looks more like this:- P = T

You should note that if either side of the equation increases, the other side increases as well. Previously, we used the example that if the pressure increases, the temperature increases. However, it works the other way as well. If the temperature increases, the pressure will increase, also. Whatever you do to one side, you have to do to the other.

The Ideal Gas Law operates in a pressure cooker because the steam cannot escape the device, increasing the pressure within the container. When the pressure increases, the temperature of the water and steam increases to the normal boiling point of 212 degrees Fahrenheit.

Temperature and pressure continue to rise in the container until it reaches the desired pressure, usually 15 PSI, which provides consistent cooking temperature across all variables.  Once it reaches 15 PSI, the steam exits the safety valves to release pressure and prevent explosions.

You want to turn down the heat at 15 PSI, or the temperature will continue to rise until too much pressure builds, and the device explodes. Even with the safety valves – which play an essential role – they can only release so much steam. Modern-day pressure cookers usually have 10 or more safety features.

How the Pressure Cooker Cooks Food Faster

Food cooks faster in a pressure cooker because of another law known as Gay-Lussac’s Law. Gay-Lussac’s Law says that as the cooker heats up, the water turns to vapor that cannot escape building pressure from within.

The pressure and temperature continue to build until it exceeds the normal boiling point of water at 212 degrees Fahrenheit. Once it exceeds that, the friction inside cooks the food faster. The air cannot expand because of the fixed volume inside.

Because of the high temperature and pressure in the cooker, food cooks 30 percent faster while using 50 to 75 percent less energy. The higher the pressure, the shorter your cooking time.

As a side note, if you wanted a faster cooking time, choose an aluminum pressure cooker, which cooks up to 20 percent faster than stainless steel because the heat absorbs through the metal faster. While aluminum cooks faster, stainless steel cleans more easily. Choose based on what you value more.

Cooking at High Altitudes: Same Phenomena

Pressure cookers follow the same phenomena as cooking at higher altitudes. The higher your elevation from sea level, the more air pressure decreases. Water boils faster at lower temperatures when closer to sea level.

What temperature water boils at hinges on the surrounding air pressure? This explains why the water will boil at 212 degrees Fahrenheit in a normal pot at a normal elevation.

Contrast to the inside of a pressure cooker, water doesn’t boil until 250 degrees Fahrenheit. While the food cooks more slowly in the mountains, it cooks faster in a pressure cooker.

The food cooks faster because of the semi-sealed container. The vapor cannot escape the vessel and heats up faster. Many people in the mountains use pressure cookers.

You may, for example, have a harder time cooking potatoes all the way through in the mountains. If you plan to go anywhere in the mountains, take one with you if possible.

Pressure Explained!

We have spoken a lot about pressure, but it may be useful to define it through science. As the temperature rises in the pressure cooker, more energy arises in the water vapor molecules. This causes the molecules to bounce off each other and off the walls of the vessel.

How much force strikes the walls of the pressure cooker gives you the definition of pressure based on the Kinetic Theory of Gases. A simple model that looks at the behavior of gases was what began the science of thermodynamics.

The Second Law of Thermodynamics explains why food cooks in a pressure cooker. The hot air inside the container flows over to the food and cooks it.

Because the pressure cooker is only partially sealed, some heat does get lost, but not as much as a regular pot. The higher pressure within also bounces off the food and accelerates the cooking process.

Food within the container cooks through a process known as convection. Convection means a transfer of heat through the movement of gas and liquids.

How the Lid and Gasket Play a Role

The lid locks the pressure cooker tightly, and the gasket seals the vapor in the cooker. As temperatures rise, the vapor cannot escape and rises with the temperatures based on the Ideal Gas Law. Without the gasket, the lid will boil water faster, as you’ve probably noticed when cooking. The gasket seals in the vapor so that it cooks faster.

You might say that the pressure in the pot is only a side effect of trapping the gases within the container. The biggest benefit comes from higher cooking temperatures due to the steam trapped inside the pressure cooker. This device literally forces heat into the food.

The Ideal Gas Law Also Applies to Regular Cooking

Gases shrink and expand all the time whether using the pressure cooker or baking in the oven. The gases have independent behavior, and this law shows you how gases behave at different pressures, temperatures, and volumes.

Why Would You Want to Know About the Ideal Gas Law?

The greatest thing that chefs can do by knowing the Ideal Gas Law compares the different situations that can arise. For example, have you ever seen a bubble in your bread dough? By understanding the Ideal Gas Law, you will know that you have gas in your bread dough. The molecules will remain constant, and the gas will remain constant.

Another thing to understand is that as the temperature rises, the bubble will continue to expand because of increased pressure – what happens on one side of the equation must happen on the other.

In knowing this, you know how to reduce bubbles. Keep the temperature as low as possible to prevent air bubbles from forming. Most chefs never would have guessed that physics could apply to their profession.

What is the principle used in a pressure cooker? Pressure cookers trap water vapor to cook foods faster. The pressure increases inside the sealed vessel because of a physics law known as the Ideal Gas Law. As the temperature increases, the pressure increases from the buildup of steam.

Does a pressure cooker use Boyle’s Law? To a lesser extent, Boyle’s Law influences the pressure cooker but not directly. This law allows liquids to heat higher than their normal boiling point because of the higher pressure in the pressure cooker.

What law explains the mechanisms of a pressure cooker? Gay-Lussac’s Law governs the mechanisms of a pressure cooker. As temperature increases within the device, pressure increases with it. Whether you add or subtract temperature, the pressure follows it or vice versa.

What is the gas pressure law? Instead of a single law, there are four main laws that dictate the gas pressure laws. They include Gay-Lussac’s Law, Charle’s Law, Boyle’s Law, and Avogadro’s Law.

Conclusion – Pressure Cooker Gas Law

The key gas law that influences the pressure cooker is the Ideal Gas Law, but multiple gas laws influence it on some level. Sometimes, the Ideal Gas Law has also been called the Perfect Gas Law.

Along with pressure cookers, some of the other areas where it applies include airbag deployment, scuba diving, and the human lungs. From a simple equation, physicists demonstrate gas behavior to us. Understanding the gas laws behind the pressure cooker and how it works makes the device more interesting.

One-Pot Cooking Rocks!

Resource: Eric.ed.gov