Thermodynamic Processes

Nonquasi-static process

(i)

Quasi-static process

(ii)

pin

(1)

System

(2)

pout

(3)

Heat

(4)

Constant T heat bath

(5)

An isothermal expression from a state labeled A to another state labeled B on a pV diagram. The curve represents the relation between pressure and volume in a ideal gas at constant temperature.

Thermodynamic System

The part of the universe selected for study, whose properties (pressure, volume, temperature, etc.) are of interest

Surroundings

Everything outside the system that can exchange heat or work with it.

Thermodynamic Variables

Macroscopic quantities that describe a system’s state (e.g., p, V, T, n for an ideal gas).

State of a System

A unique condition defined by its thermodynamic variables (e.g., specific p, V, T)

Thermodynamic Process

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

Quasi-static Process

An idealized, infinitely slow process during which the system remains in internal equilibrium at all times.

Non-quasi-static Process

A real, finite-speed change through states that are not in equilibrium; intermediate states are not well defined.

Only quasi-static cells processes have a well-defined paths on a pV diagram and allow use of W = ∫pdV.

Why quasi-static Matters?

Isothermal Process

A process that occurs at constant temperature; requires continuous thermal contact with a heat bath.

Isobaric Process

A thermodynamic process in which the pressure remains constant.

Isochoric (Isovolumetric) Process

A process in which the volume remains constant; thus, no work is done (W = 0)

Adiabatic Process

A process during which no heat enters or leaves the system (Q = 0); temperature changes due to work.

Adiabatic Expansion

Gas expands without heat input; internal energy decreases → temperature falls

Adiabatic Compression

Gas is compressed without heat loss; internal energy increases → temperature rises.

Cyclic Process

A process in which the system returns to its initial state; all state variables return to original values

ΔEint = 0

Mathematical representation of the internal energy in a cycle

First Law for a Cycle

Net heat absorbed equals net work done over the cycle.

Q = W

Mathematical representation of the First Law of Cycle

Reversible Process

A quasi-static process that can be reversed by infinitesimal changes in the surroundings; no dissipation

Irreversible Process

Any process involving finite changes, friction, turbulence, or dissipation; cannot retrace original states.

Path on a pV Diagram

Graphical representation of a quasi-static process, and volume change.

Isothermal Curve on pV Diagram

A hyperbola showing p ∝ 1/V for an ideal gas at constant T.

Because intermediate states aren’t equilibrium states, no unique pV curve can be drawn.

Why is the path undefined for non-quasi-static Process

Heat Bath

An idealized system with constant temperature, capable of absorbing or releasing heat without changing T.

The system must remain in continuous thermal equilibrium with the heat bath.

What is the requirement for isothermal quasi-static change?

Thermodynamic Equilibrium

The state in which macroscopic properties are uniform and unchanging throughout the system.