Thermodynamic System Select Definitions - Thermodynamics - Mechanical Engineering

Basic Concepts & Zeroth Law of Thermodynamics

Thermodynamic System: Select Definitions

A thermodynamic system is any part of the universe that is chosen for study and is separated from the rest by a boundary. The boundary may be real or imaginary and can be fixed or movable. The region outside the system is called the surroundings, and the combination of system plus surroundings is referred to as the universe.

Types of Thermodynamic Systems

• Isolated System: Neither mass nor energy (heat or work) crosses the boundary. Ideal isolated systems are theoretical; useful for thought experiments.
• Closed System (Control Mass): No mass transfer across the boundary, but energy in the form of heat, work, electrical energy, etc., can cross the boundary.
• Open System (Control Volume): Mass and energy both can cross the boundary; typical for devices like pumps, turbines and heat exchangers.

Piston-And-Cylinder Example

A common and important example of a thermodynamic system in engineering is a gas contained in a piston-and-cylinder assembly. The gas forms the system; the piston and cylinder walls form the system boundary. The system can exchange work (by piston movement) and heat (by thermal interaction) with its surroundings.

If internal pressure of the gas differs from the external pressure acting on the piston, a net force causes piston motion until pressures equalise. During such motion, boundary work is done by or on the system. If there is a temperature difference between the gas and the surroundings, heat will flow across the boundary.

Properties, State and State Change

Properties are measurable quantities that describe the state of a system. A state is the condition of a system as described by its properties (for example pressure, temperature, volume, mass, composition). A change of state occurs when one or more properties change.

Extensive and Intensive Properties

• Extensive Property: Depends on the amount of matter in the system (for example, total volume V, mass M, total internal energy U).
• Intensive Property: Independent of system size (for example, pressure P, temperature T, density ρ).

Specific and Molar Properties

Specific property = extensive property per unit mass. Molar property = extensive property per mole.

Specific Volume (volume per unit mass):

v = V_t / M, where V_t is total volume (m³) and M is total mass (kg). The SI unit is m³·kg⁻¹.

Molar Volume (volume per mole):

V̄ = V_t / N, where N is number of moles (mol). The SI unit is m³·mol⁻¹.

Worked Example — Specific Volume

1. Given: Total volume V_t = 0.08 m³, mass M = 0.2 kg. Find the specific volume v.
2. Formula: v = V_t / M.
3. Calculation: v = 0.08 / 0.2 = 0.40 m³·kg⁻¹.

Final answer: v = 0.40 m³·kg⁻¹

Zeroth Law Of Thermodynamics

Statement (Zeroth Law): If two systems A and B are each in thermal equilibrium with a third system C, then A and B are in thermal equilibrium with each other. This establishes temperature as a fundamental and transitive property.

Meaning: Thermal equilibrium means no net heat flow occurs between systems when they are in contact through a diathermal boundary. The Zeroth Law allows us to define a scalar quantity called temperature that orders thermal equilibrium relations.

Practical consequence: The Zeroth Law justifies the use of thermometers. A thermometer (system C) brought into contact with system A reaches equilibrium; its reading then represents the temperature of A. If the same thermometer equilibrates with system B and reads the same value, A and B have the same temperature and are in thermal equilibrium.

Temperature Scales and Measurement

• Temperature can be measured on scales such as Celsius (°C), Kelvin (K) — SI unit, and Fahrenheit (°F). For thermodynamic work, always use the Kelvin scale.
• Thermal Equilibrium is the operational test for equal temperature — no heat transfer when bodies are placed in thermal contact.

Important Points, Common Mistakes and Exam Tips

• Distinguish system types: Do not confuse closed and isolated systems — an isolated system does not exchange energy or mass; a closed system exchanges energy but not mass.
• Sign Convention for Work: For many engineering texts, work done by the system is taken as positive for boundary work during expansion; confirm the convention required in the exam or question statement.
• Units: Always express temperature in Kelvin for calculation unless asked otherwise.
• Zeroth Law Tip: Remember the transitive property: A = C and B = C ⇒ A = B. This is the simplest way to recall the law for quick answers in exams.
• Common Mistake: Assuming equal pressure implies thermal equilibrium — it does not. Pressure equality relates to mechanical equilibrium, while temperature equality relates to thermal equilibrium.
• Time Management: For quick problems, compute specific or molar properties first to reduce variables; many multi-step questions simplify after converting to per-unit-mass or per-mole basis.

Glossary