Thermodyanmics Terms

0th Law

If two systems are both in thermal equilibrium with a third system, they are also in thermal equilibrium with each other

1st Law

Heat cannot be created or destroyed, only transferred. The total energy remains the same.

2nd Law

Energy has a quality and a quantity; energy tends to go towards decreasing quality of energy. The entropy of the universe is always increasing.

3rd Law

The entropy of a pure, perfect crystal approaches zero as its temperature approaches absolute zero (0 Kelvin).

System

A quantity of matter or a region of space chosen for study.

Closed System

A system where mass cannot cross its boundary in either direction, but energy can cross the boundary.

Isolated System

Neither mass nor energy can cross the system’s boundary.

Open System

Both mass and Energy can freely cross the system’s boundary.

Extensive Properties

Units that change with the size of the system (mass, volume, ..)

Intensive Properties

Units that do not change with the system’s size (temperature, density, …)

Equilibrium

A state of balance where a system has no net flows of mass or energy.

Thermal Equilibrium

Same temperature throughout the system.

Mechanical Equilibrium

No unbalanced forces; relates to pressure

Phase equilibrium

Mass of each phase reaches equilibrium and stays there

Chemical equilbrium

No chemical reactions occur

Process

Any change that happens on a system from one equilibrium state to another.

Cycle (of a system)

A system that returns to its initial state at the end of the process

Isobaric Process

A process in which the pressure ‘P’ remains constant.

Isothermal Process

A process in which temperature ‘T’ remains constant

Isochoric process

A process in which the volume ‘V’ remains constant.

Quasi-Equilibrium Process

A process in which the system remains infinitesimally close to an equilibrium state at all times. This is due to a very slow process that allows the system to adjust internally.

Incompressible Substance

A substance whose density (or specific volume) is essentially constant. Most solids and liquids in this class are modeled in this way.

Pure Substance

A substance that has a uniform chemical composition throughout, which doesn’t have to be just one element (i.e, water).

Compressed (or subcooled) Liquid

A substance that is not about to vaporize

Saturated Liquid

A liquid that is about to vaporize

Saturated liquid-vapor mixture

The state at which the liquid and vapor phases coexist at equilibrium.

Saturated Vapor

A vapor that is about to condense.

Superheated Vapor

A vapor that is not about to condense.

Saturation Temperature ‘T_sat’

The temperature at which pure substance changes phase

Saturation Pressure ‘P_sat’

The pressure at which a pure substance changes phase.

Critical Point

The point at which the saturated liquid and saturated vapor states are identical.

Quality ‘q’

The proportions of the liquid and vapor phases in the mixture. ‘q’ varies from 0-1, q=0 being a saturated liquid, q=1 being a saturated vapor.