Question

What are the types of boundaries in Thermodynamics?

Answer 1

1. In thermodynamics, a boundary is a closed, 3D surface that encloses the system and separates it from its surroundings. Usually, the boundary coincides with real, physical surfaces such as the walls of a flask (or some other vessel), the interface between two phases etc. Sometimes, some parts of or all of the boundaries can be imaginary surfaces in 3D space.
2. Boundaries may be fixed (such as the walls of a vessel) or moveable (such as a piston within a cylinder).
3. Transfer of quantities such as heat and energy can occur into or out of the system across the boundary. The properties of the boundary determine what transfers can occur.
4. Based on this, boundaries can be of the following types.
5. Adiabatic boundary: This is a boundary that does not allow the transfer of heat and mass into or out of the system. They are walls that have perfect thermal insulation and a perfect radiation shield. Example: The walls of a thermos flask.
6. Diathermal boundary: This is a boundary that allows only heat transfer and not mass transfer between the system and surroundings. A system enclosed by a diathermal wall becomes a closed system. Example: The walls of a steel container.
7. Impermeable boundary: This type of boundary prevents the transfer of mass into or out of the system. A system enclosed by an impermeable boundary becomes an isolated system. Adiabatic boundaries and diathermal boundaries are examples of impermeable boundaries.
8. Final Statement: A thermodynamic boundary is a closed 3d surface that separates the system from its surroundings. Boundaries can be fixed or movable. They can be either adiabatic, diathermal or impermeable.