teaching-notes — Physics 763 PH34 (Basic Concepts of Thermodynamics)

PHYSICS 763 PH34 – COLLEGE TOPIC: BASIC CONCEPTS OF THERMODYNAMICS SUBTOPIC: THERMODYNAMIC SYSTEM AND STATE YEAR: 2026 SPECIFIC OUTCOME: 1. Define a thermodynamic system, surroundings, boundary and universe as used in heat and thermodynamics. INTRODUCTION Thermodynamics is a branch of physics that deals with heat and its relation to other forms of energy and work. It is fundamental to understanding how engines operate, how refrigerators cool, and even how living organisms sustain themselves. To study thermodynamic processes, it is essential to clearly define the specific region of interest, which we call a system, and its interaction with everything else around it. This subtopic introduces these basic concepts, providing the framework for deeper thermodynamic analysis. CORE CONCEPTS 1. THERMODYNAMIC SYSTEM A thermodynamic system is defined as a specific quantity of matter or a region in space chosen for study. It is the part of the universe that we are interested in analyzing when considering energy and mass transfers. For example, the water boiling in a kettle, the gas inside an engine cylinder, or a human body can all be considered thermodynamic systems. Thermodynamic systems are classified based on how they interact with their surroundings in terms of mass and energy exchange. There are three main types: • Open System: An open system is one that can exchange both mass and energy with its surroundings. * Example: A cup of hot tea (mass, e.g., steam, and heat energy can escape). A boiling pot of nshima in a typical Zambian kitchen without a lid is an open system, exchanging both steam (mass) and heat (energy) with the kitchen air. A car engine while running is an open system because it takes in fuel (mass) and air (mass), expels exhaust gases (mass), and releases heat (energy) to the surroundings. • Closed System: A closed system can exchange energy (usually in the form of heat or work) but not mass with its surroundings. * Example: A sealed soft drink bottle that is cooling down (heat can be exchanged with the environment, but no drink or air leaves the bottle). A pressure cooker with its lid tightly sealed is a closed system; heat can enter or leave, but no mass escapes. A balloon filled with air is a closed system; the air mass inside is constant, but the balloon can expand (work) or heat up/cool down (heat exchange). • Isolated System: An isolated system is one that exchanges neither mass nor energy with its surroundings. These systems are ideal and do not truly exist in practice, but they are useful approximations in some cases. * Example: A perfectly insulated thermos flask containing hot water, ideally, would be an isolated system (no heat or water escapes). The entire universe is sometimes considered an isolated system.

TYPES OF THERMODYNAMIC SYSTEMS

✅ Check Your Understanding

Pause here. Let learners attempt these before moving on.

1.	Define an isolated thermodynamic system.
2.	A metal can of cold Mosi lager is left on a table in a warm room. Is the Mosi lager in the can an open, closed, or isolated system? Justify your answer.
3.	True or False: A steaming plate of hot nshima (mealie meal porridge) left on a dining table is an example of a closed system because the nshima itself doesn't leave the plate. Justify your answer.

Answers

1. An isolated thermodynamic system is one that exchanges neither mass nor energy with its surroundings.
2. The Mosi lager in the can is a closed system. It can exchange heat (energy) with the warm room, causing the lager to warm up, but no mass (lager or gas) can leave or enter the sealed can.
3. False. A steaming plate of hot nshima is an open system. While the bulk of the nshima does not leave the plate, it loses steam (mass) to the surroundings as well as heat (energy) through convection and radiation. The common misconception is overlooking the mass transfer through evaporation.

2. SURROUNDINGS The surroundings (or environment) refer to everything external to the thermodynamic system. It is the region with which the system can interact and exchange mass and energy. The effects of the system's interactions are generally felt in the immediate surroundings. • Example: If a boiling kettle is our system, the air in the kitchen, the walls, and the tabletop are part of the surroundings. If the engine of a Fuso truck is the system, the air outside the engine, the engine block, and the cabin are its surroundings. 3. BOUNDARY The boundary is the real or imaginary surface that separates the system from its surroundings. It defines the extent of the system. The interaction (exchange of mass and/or energy) between the system and the surroundings takes place across this boundary. Boundaries can be: • Real or Imaginary: The wall of a container is a real boundary, while a hypothetical plane drawn around a column of air could be an imaginary boundary. • Fixed or Movable: The walls of a rigid tank are fixed boundaries. The piston in an engine cylinder is a movable boundary as it moves up and down. • Permeable, Impermeable, or Adiabatic: * Permeable: Allows both mass and energy to cross. * Impermeable: Prevents mass from crossing but allows energy. * Adiabatic: Prevents energy (heat) from crossing. A system with an adiabatic boundary is thermally insulated. * Diathermal: Allows heat to cross. Most real-world boundaries are diathermal to some extent. • Example: For a gas contained in a cylinder with a piston, the inner surfaces of the cylinder walls and the piston form the boundary. This boundary is real, and the piston part is movable. 4. UNIVERSE In thermodynamics, the universe refers to the combination of the system and its surroundings. Universe = System + Surroundings This implies that everything that is not part of the system is automatically considered part of the surroundings. • Example: If we consider a cooking pot as our system, and the kitchen as its surroundings, then the universe, in this thermodynamic context, would be the combination of the cooking pot and the kitchen. 5. STATE OF A SYSTEM The state of a system refers to the condition of the system as described by a set of its measurable properties. When all the properties of a system have definite values, the system is said to be in a definite state. These properties are often called state variables or state functions. State Variables are macroscopic properties that describe the condition of the system. Common state variables include: • *Pressure (P)*: The force exerted per unit area by the system. • *Volume (V)*: The space occupied by the system. • *Temperature (T)*: A measure of the average kinetic energy of the particles within the system. • *Mass (m)*: The amount of matter in the system. • Composition: The chemical makeup of the system (e.g., percentage of different gases in a mixture). • Density (ρ): Mass per unit volume. • *Internal Energy (U)*: The total energy contained within the system. • *Enthalpy (H)*: A measure of the total heat content of a system. When any of these properties change, the state of the system changes. For instance, if the temperature of a gas increases, its state has changed.

Common Thermodynamic State Variables

Variable	Description	SI Unit
Pressure (P)	Force per unit area exerted by the system.	Pascal (Pa)
Volume (V)	Space occupied by the system.	Cubic metre (m³)
Temperature (T)	Measure of hotness or coldness.	Kelvin (K)
Mass (m)	Quantity of matter in the system.	Kilogram (kg)

Figure: Common thermodynamic state variables and their SI units

✅ Check Your Understanding

Pause here. Let learners attempt these before moving on.

1.	What is the primary role of a boundary in a thermodynamic system?
2.	Identify three state variables that define the condition of a given mass of gas in a container.
3.	True or False: The universe, in a thermodynamic context, is simply a very large system. Justify your answer.

Answers

1. The primary role of a boundary is to separate the system from its surroundings and to define the region where mass and energy exchanges occur.
2. Three state variables for a given mass of gas are pressure (P), volume (V), and temperature (T). (Other valid answers include internal energy, enthalpy, density).
3. False. The universe in a thermodynamic context is defined as the combination of the system and its surroundings. It is not merely a large system, but rather the entirety of what is being considered, including everything outside the system's defined boundaries. Common error: confusing the universe with just a larger system.

SUMMARY Understanding the basic concepts of a thermodynamic system, surroundings, boundary, and universe is crucial for analyzing energy and mass interactions in various physical processes. A system is the region of interest, separated by a boundary from its surroundings. The universe encompasses both the system and its surroundings. Systems are classified as open (exchanges mass and energy), closed (exchanges energy only), or isolated (exchanges neither). The state of a system is defined by its state variables like pressure, volume, and temperature. These fundamental definitions provide the vocabulary for studying heat, work, and energy changes. ASSESSMENT QUESTIONS 1. Define the following terms as used in thermodynamics: a) Thermodynamic system b) Surroundings c) Boundary d) Universe 2. State the key difference in mass and energy exchange between an open system and a closed system. 3. Classify each of the following as an open, closed, or isolated system, providing a brief reason for your choice: a) A rocket launching into space. b) A perfectly sealed and insulated container of hot coffee. c) A human being exercising. d) The coolant flowing through the radiator of a car. 4. Describe what is meant by the "state of a system" and give two examples of state variables. 5. Imagine a balloon filled with air at standard temperature and pressure. a) Identify the system and its immediate surroundings. b) Describe the nature of the boundary in terms of being real/imaginary and fixed/movable. COMMON DIFFICULTIES & MISCONCEPTIONS 1. Confusing "System" and "Surroundings": Learners often struggle to clearly define the specific part chosen for study. Emphasize that the system is whatever you decide to focus on, and everything else is the surroundings. 2. Misidentifying System Types: Open vs. Closed: Many forget that steam/vapour escaping is mass transfer, leading to incorrectly classifying an open container of boiling water as closed. They may also confuse the container itself with the contents. For instance, a sealed can contains a closed system, but the can itself* is not the system; the contents are. Truly Isolated Systems: Learners often think a good thermos flask is a perfectly* isolated system. It's important to clarify that truly isolated systems are theoretical ideals; real-world systems always have some interaction, however small. 3. The Nature of the Boundary: The concept of an imaginary boundary can be challenging. Use examples like drawing a virtual box around a specific volume of air to illustrate this. 4. "Universe" as the Astronomical Universe: Learners sometimes think of the astronomical universe when the term "thermodynamic universe" is used. Clarify that in thermodynamics, the "universe" is simply the system and its relevant surroundings, which may be a very small part of the actual astronomical universe. 5. State Variables vs. Process Variables: While not explicitly covered in this outcome, a common future misconception is confusing state variables (which depend only on the current state) with process variables (like heat and work, which depend on the path taken between states). It's good to lay a strong foundation for state variables now. QUICK REFERENCE SUMMARY

Key Terms: Thermodynamic System and State

Thermodynamic System	A specific quantity of matter or region in space chosen for study.
Surroundings	Everything external to the system with which it interacts.
Boundary	The real or imaginary surface separating the system from its surroundings.
Universe	The combination of the system and its surroundings.
State of a System	The condition of the system described by its measurable properties (state variables).
State Variables	Macroscopic properties that define the state of a system, e.g., pressure, volume, temperature.

Figure: Key terms and definitions for thermodynamic systems

teaching-notes — Physics 763 PH34 (Basic Concepts of Thermodynamics)

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