The weight of the petcock

Using the basic force-equilibrium equation on the base of the petcock:

     F - ( G + F_a ) = 0

F is the force given by the pressure inside the cooker and G is the weight of the petcock. F_a is the force acting on the base by the atmosphere.

Then the mass can be expressed as

     PA - ( mg + P_aA ) = 0
     m = ( P - P_a )(A) / g
         = P_gA / g

Put the given values into previous equation yields

     m = 0.0408 kg

The result shows that a small mass of petcock can handle a high pressure inside the cooker.

Pressure Cooker

• Before the pressure in the cooker reaches the operation pressure, no mass is transferred between the cooker and the surroundings. So the cooker is a closed system. When the pressure approaches the operation pressure, the petcock moves upward and stream can leak out the cooker. Then the system becomes an open system.
• The atmospheric pressure always exists for objects which are explored in the air. Be sure the pressure given is absolute, gage or vacuum.The concept about pressure will be introduced later.