Thermodynamics involved in a pressure cooker
Pressure cooker is a very common device used in cooking food by consuming lesser amount of energy.
A pressure cooker is a closed device that locks in large amount of heat inside it and never let it escape. Also the volume of the cooker remains same throughout the cooking process. Such process in which the volume remains constant with change in other physical quantities is called as isochoric process.
Equation of state for an ideal gas is given by, PV= nRT. This leads us to the conclusion that for constant volume and number of moles of a gas (steam in this case), the pressure varies directly with the temperature by Gay-Lussac's Law
As pressure increases in the cooker, temperature also increases as per the law. This increase in temperature in the cooker heats up the cooker much faster or to a higher extent, thus saving energy & time of cooking
Also,according to first law of thermodynamics,
Q= ΔU+ W
As, the volume is constant, no work is done on the system i.e W = 0. Therefore, Q= ΔU ie., heat supplied is completely utilised in increasing the internal energy of the food.
As the cooker heats up itself, it transfers heat from base metal to water filled in cooker and the food through conduction. As the high pressure steam moves around the food, the food is also cooked through convection.
Flow of steam can also be explained on the basis of thermodynamics. As volume of steam built inside reaches certain amount, it creates high pressure. It is released via whistling of cooker. The process repeats till water is present in the cooker.
Once food is cooked and the lid of the cooker is opened, large amounts of steam rapidly burst throughout the room. This happens as the air moves from an area of high pressure(inside pressure cooker) to low pressure(atmosphere).
