Explain the working principle of pressure cookers.
A pressure cooker is basically a pot with a really tight fitting lid and some safety valves. When you lock the lid onto the cooker, you are basically sealing the pot shut and creating a closed pressure system.
If you have had any basic Chemistry course, you might remember the Ideal Gas Law and the equation PV=nRT, where P = pressure, V= volume and T = temperature ( for completeness, n= the number of moles and R = is the gas constant, but they will not change in this case so we can ignore them, and since the volume of your metal pot will not really change, we will ignore this also). This equation applies to a closed system.
So basically we have an equation of P=T. If you remember you basic algebra, anything you do to one side of the equation, you must also do to the other. So if you increase the pressure, the temperature will also increase, and vice versa.
In an uncovered pot at sea level, water boils at 100ºC (212ºF). The steam that evaporates from this pot is also at 100ºC (212ºF). No matter how much heat is supplied to the water, it will remain at 100ºC (212ºF). If a tightly-sealing cover is put on the pot to trap the steam, the pressure inside the pot goes up. As the pressure rises, the temperature of the water and steam inside the now pressurized pot also rises above the normal 100ºC (212ºF) boiling point temperature.
So when you begin heating the cooker, the internal pressure will increase, and as the pressure increases, the internal temperature also rises. This will continue to increase until it reaches the trigger pressure of the safety valves, about double the psi at sea level.
Here is an excerpt from an answer I made to a related question:
The reason why pressure cookers can cook so much faster than regular cookware? Regular pots can only cook at the boiling temperature of water at sea level, 212 deg F (100 deg C). Generally for most chemical reactions, for every 10 deg C increase, the rate of reaction is doubled, which allows you to reduce the cook time in half. By raising the pressure by ~15 psi, you can braise your food at ~250 deg F (121 deg C) and cut your cook time to a quarter of the standard time, however you also have to factor in the time it will take to come up to pressure as well as the cool down time, so it ends up actually being about one third the original time, still a huge reduction.
A pressure cooker is basically a pot with a really tight fitting lid and some safety valves. When you lock the lid onto the cooker, you are basically sealing the pot shut and creating a closed pressure system.
If you have had any basic Chemistry course, you might remember the Ideal Gas Law and the equation PV=nRT, where P = pressure, V= volume and T = temperature ( for completeness, n= the number of moles and R = is the gas constant, but they will not change in this case so we can ignore them, and since the volume of your metal pot will not really change, we will ignore this also). This equation applies to a closed system.
So basically we have an equation of P=T. If you remember you basic algebra, anything you do to one side of the equation, you must also do to the other. So if you increase the pressure, the temperature will also increase, and vice versa.
In an uncovered pot at sea level, water boils at 100ºC (212ºF). The steam that evaporates from this pot is also at 100ºC (212ºF). No matter how much heat is supplied to the water, it will remain at 100ºC (212ºF). If a tightly-sealing cover is put on the pot to trap the steam, the pressure inside the pot goes up. As the pressure rises, the temperature of the water and steam inside the now pressurized pot also rises above the normal 100ºC (212ºF) boiling point temperature.
So when you begin heating the cooker, the internal pressure will increase, and as the pressure increases, the internal temperature also rises. This will continue to increase until it reaches the trigger pressure of the safety valves, about double the psi at sea level.
Here is an excerpt from an answer I made to a related question:
The reason why pressure cookers can cook so much faster than regular cookware? Regular pots can only cook at the boiling temperature of water at sea level, 212 deg F (100 deg C). Generally for most chemical reactions, for every 10 deg C increase, the rate of reaction is doubled, which allows you to reduce the cook time in half. By raising the pressure by ~15 psi, you can braise your food at ~250 deg F (121 deg C) and cut your cook time to a quarter of the standard time, however you also have to factor in the time it will take to come up to pressure as well as the cool down time, so it ends up actually being about one third the original time, still a huge reduction.
