Thermodynamic Systems and Thermal Equilibrium
Class 11 Physics notes for thermodynamic systems, boundaries, types of systems, thermal equilibrium and Zeroth Law of Thermodynamics.
Thermodynamics studies heat, work, temperature, energy and the macroscopic behaviour of matter. Before applying laws of thermodynamics, we must first identify what part of the universe is being studied, what lies outside it, and what kind of exchange is possible. This page builds the complete foundation: system, surroundings, boundary, open system, closed system, isolated system, thermal equilibrium and the Zeroth Law.
Thermodynamic System
A thermodynamic system is the selected part of the universe whose thermodynamic properties are being studied.
In thermodynamics, we do not usually track each molecule separately. Instead, we describe the system using macroscopic variables such as pressure, volume, temperature, internal energy, heat and work. A system can be as small as gas trapped in a syringe or as large as the atmosphere around Earth, depending on the problem.
Correct identification of the system is the first step in every thermodynamics question. If the selected system changes, the meaning of heat exchange, work done and matter transfer also changes.
Macroscopic Description
A system is described by measurable quantities such as pressure, temperature, volume and mass rather than by individual molecular positions.
Chosen by Observer
The system is not always a naturally separate object. It is often chosen by the student or scientist according to the question.
Exam Importance
Most mistakes in thermodynamics begin with a wrong choice of system. Always mark the system before writing equations.
Surroundings
The surroundings include everything outside the thermodynamic system that can interact with it. If gas inside a cylinder is the system, then the piston, cylinder wall, air outside, flame, table and room form the surroundings according to the level of interaction considered.
The system and surroundings together form the universe for that thermodynamic analysis.
Heat may flow from surroundings to system or from system to surroundings. Work may also be done by the system on the surroundings or by the surroundings on the system. Matter exchange depends on whether the system is open, closed or isolated.
Boundary
A boundary is the real or imaginary surface that separates the system from the surroundings. It decides what can cross between the system and surroundings.
Real Boundary
A real boundary is a physical wall or surface, such as the walls of a cylinder, the metal surface of a vessel or the glass wall of a flask.
Imaginary Boundary
An imaginary boundary is selected for convenience, such as a chosen region of air in a room or a control volume around a turbine.
Fixed Boundary
A rigid vessel has a fixed boundary. Its volume does not change even if heat is supplied.
Movable Boundary
A gas in a cylinder with a frictionless piston has a movable boundary. Its volume can change and the system can do work.
Core Diagrams
These diagrams use simple black outlines and red arrows, matching the NCERT-style idea that clarity is more important than decoration.
System, Surroundings and Boundary
Open System
Closed System
Isolated System
Thermal Equilibrium
Zeroth Law Diagram
Types of Thermodynamic Systems
Thermodynamic systems are classified according to exchange of matter and energy across the boundary. Energy can cross as heat or work. Matter crossing means actual mass entering or leaving the system.
Open System
An open system exchanges both matter and energy with the surroundings. Boiling water in an open vessel is a common example: steam escapes and heat is supplied.
- Matter exchange: Yes
- Energy exchange: Yes
- Boundary: Often imaginary or permeable
Closed System
A closed system exchanges energy but not matter. Gas sealed in a piston-cylinder arrangement is a closed system if no gas leaks.
- Matter exchange: No
- Energy exchange: Yes
- Boundary: Impermeable to mass
Isolated System
An isolated system exchanges neither matter nor energy with the surroundings. A perfect isolated system is ideal; a well-insulated thermos is an approximate example.
- Matter exchange: No
- Energy exchange: No
- Boundary: Rigid and perfectly insulated in ideal case
Comparison of Open, Closed and Isolated Systems
| Feature | Open System | Closed System | Isolated System |
|---|---|---|---|
| Matter exchange | Allowed. Mass can enter and leave. | Not allowed. Fixed amount of matter remains inside. | Not allowed. No mass exchange with surroundings. |
| Energy exchange | Allowed as heat, work or flow energy. | Allowed as heat and/or work. | Not allowed in the ideal case. |
| Boundary type | Permeable boundary or control volume surface. | Impermeable to mass but may allow heat/work. | Impermeable and adiabatic; usually considered rigid. |
| Real-life example | Open cup of tea, boiling water in open vessel, turbine. | Sealed pressure cooker before steam release, gas in piston-cylinder. | Ideal thermos flask, universe considered as a whole. |
| Exam note | Look for mass crossing: steam, fluid flow, inlet/outlet. | Look for fixed mass with heat/work exchange possible. | Look for no exchange of both heat and matter; real examples are approximate. |
Real-Life Examples
Boiling Water in an Open Vessel
The water and steam above it may be treated as the system. Heat enters from the burner and steam escapes into the surroundings, so both energy and matter cross the boundary. Therefore it is an open system.
Pressure Cooker
A pressure cooker can behave approximately as a closed system while the whistle is not releasing steam. Heat enters but matter is nearly fixed. When steam escapes through the valve, it behaves more like an open system. It is not isolated because heat is supplied from outside.
Thermos Flask
A thermos flask is designed to reduce heat transfer and prevent matter exchange. It is an approximate isolated system, not a perfect one, because some heat leakage is always possible in real life.
Gas in a Cylinder with Piston
If the piston is sealed, gas cannot escape, so matter does not cross the boundary. But the piston may move and heat may be supplied, so energy exchange is possible. This is a standard closed system used in thermodynamics problems.
Formula and Definition Boxes
Heat is energy transferred because of temperature difference. It flows naturally from higher temperature to lower temperature.
Work is energy transfer due to force acting through displacement. In gases, boundary work is commonly associated with volume change.
Internal energy is the total microscopic kinetic and potential energy of molecules of the system.
Thermal equilibrium condition: no net heat flows between bodies A and B.
Thermal Equilibrium
Two systems are said to be in thermal equilibrium when there is no net flow of heat between them while they are in thermal contact. This does not mean that both bodies contain the same heat. It means they have the same temperature.
If body A and body B are placed in contact and heat flows from A to B, then they are not initially in thermal equilibrium. Heat flow continues until their temperatures become equal. At that stage, the condition is:
Thermal equilibrium is a condition of equality of temperature, not equality of mass, volume, internal energy or heat content. A hot iron nail and a bucket of water may have very different amounts of internal energy, but if both are at 30 °C, they are in thermal equilibrium with each other.
Key Features of Thermal Equilibrium
- No net heat flow occurs between bodies in contact.
- Temperature becomes the same for the bodies considered.
- It is independent of size and material of the bodies.
- It allows us to compare temperatures using a thermometer.
Zeroth Law of Thermodynamics
The Zeroth Law states that if two systems are separately in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. It sounds simple, but it is one of the foundations of thermodynamics because it makes temperature a measurable and comparable property.
A thermometer works because of the Zeroth Law. When a thermometer is placed in contact with a body, heat exchange occurs until both reach thermal equilibrium. The thermometer reading then represents the temperature of the body.
Why is it called Zeroth Law?
The law was recognized after the First, Second and Third Laws had already been named, but it was more fundamental than them. Therefore, it was placed before the First Law and named the Zeroth Law.
Important Exam Notes
- A system is the part of the universe selected for study.
- Surroundings are everything outside the system that may interact with it.
- Boundary separates system and surroundings.
- Open system allows both mass and energy exchange.
- Closed system allows energy exchange but no mass exchange.
- Isolated system allows neither mass nor energy exchange.
- Thermal equilibrium means equality of temperature.
- Thermal equilibrium does not mean equality of heat.
- Zeroth Law provides the basis for temperature measurement.
- A thermometer measures temperature only after reaching thermal equilibrium with the body.
- The universe as a whole is often treated as an isolated system.
- Real isolated systems are approximations; perfect insulation is ideal.
Common Mistakes
Confusing Closed and Isolated Systems
A closed system can exchange energy. An isolated system cannot exchange energy or matter. A sealed hot-water bottle losing heat is closed, not isolated.
Thinking Same Temperature Means Same Heat
Thermal equilibrium means same temperature, not same heat. Heat is energy in transit, while temperature indicates thermal state.
Forgetting Zeroth Law Defines Temperature
Zeroth Law justifies the use of a thermometer. Without it, temperature comparison would not be logically defined.
Assuming Pressure Cooker is Isolated
A pressure cooker receives heat from outside. If steam escapes, even matter leaves. It is not isolated in normal cooking.
Numericals with Step-by-Step Solutions
Use these numericals to strengthen definitions, classification and temperature-equilibrium logic. Solutions are hidden in toggles for active recall.
Show solution
When no heat flows, the bodies are in thermal equilibrium.
Therefore, final condition is TA = TB.
Final answer: Their final temperatures are equal.
Show solution
The gas is sealed, so matter does not leave or enter.
Heat enters and work is done through piston motion, so energy is exchanged.
Final answer: Closed system.
Show solution
Heat supplied to system is represented by Q.
Work done by the system is represented by W.
Final answer: Q = 120 J, W = 45 J done by the system.
Show solution
The thermometer comes into thermal equilibrium with the liquid.
The validity of measuring liquid temperature through thermometer equilibrium follows from Zeroth Law.
Final answer: Zeroth Law of Thermodynamics.
Show solution
Water enters and leaves, so matter exchange occurs.
Heat enters, so energy exchange occurs.
Final answer: Open system.
Show solution
Matter exchange is absent because no gas leaks.
Energy exchange as heat is absent due to perfect insulation. Rigid wall also prevents boundary work.
Final answer: Isolated system.
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A in equilibrium with C means TA = TC.
B in equilibrium with C means TB = TC.
Since TC = 310 K, both A and B have temperature 310 K.
Final answer: TA = 310 K and TB = 310 K.
Show solution
Steam escaping means matter leaves the selected system.
Heat supplied means energy enters.
Final answer: Open system.
Show solution
Heat flows naturally from higher temperature to lower temperature.
Since heat flows from X to Y, X was hotter than Y initially.
Final answer: TX > TY initially.
Show solution
The bottle is sealed, so matter exchange is absent.
It cools, so heat leaves the system. Energy exchange occurs.
Final answer: Closed system.
PYQs and Exam Questions
The following CBSE-style, NEET, JEE Main, assertion-reason and case-study questions cover the full conceptual range of this topic.
CBSE-Style Questions
Answer and explanation
A thermodynamic system is the selected part of the universe under study. Surroundings are everything outside the system. Boundary is the real or imaginary surface separating the two.
Answer: System + surroundings form the thermodynamic universe for the problem.
Answer and explanation
No gas crosses the sealed piston-cylinder boundary, so matter exchange is absent. Heat and work exchange may occur.
Answer: It is closed because mass is fixed but energy exchange is possible.
Answer and explanation
If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
Answer: If A is in equilibrium with C and B is in equilibrium with C, then A is in equilibrium with B.
Answer and explanation
Thermal equilibrium means equal temperature and no net heat flow. Heat content depends on mass, material and internal energy, so two bodies at the same temperature need not contain the same energy.
Answer: Thermal equilibrium means equal temperature only.
NEET MCQs
Answer and explanation
An open system permits exchange of both mass and energy.
Answer: (C) Open.
Answer and explanation
A thermos reduces heat and matter exchange but cannot make them exactly zero in real life.
Answer: (C) Approximate isolated system.
Answer and explanation
Net heat flow stops at thermal equilibrium, where temperatures are equal.
Answer: (C) Temperature.
Answer and explanation
The law establishes temperature comparison using thermal equilibrium.
Answer: (B) Temperature.
Answer and explanation
Steam escaping means mass crosses the boundary; heat is also supplied.
Answer: (B) Open.
JEE Main Conceptual and Numerical Questions
Answer and explanation
An isolated system cannot exchange energy. Since heat is absorbed, energy crosses boundary.
Answer: No, it is not isolated. It may be a closed system.
Answer and explanation
By Zeroth Law, A and B are in thermal equilibrium with each other.
Answer: TA = TB = 27 °C.
Answer and explanation
Boundary work due to expansion requires boundary movement. A rigid container has fixed volume.
Answer: No boundary work occurs due to volume change.
Answer and explanation
Steam crosses the control surface at inlet and outlet, so mass exchange occurs.
Answer: Open system.
Answer and explanation
Heat flows from higher temperature to lower temperature.
Answer: From block 1 to block 2.
Assertion-Reason Questions
Answer and explanation
Both assertion and reason are true. The reason correctly explains the assertion.
Answer: Both true, and Reason is the correct explanation.
Answer and explanation
The reason is true because thermal equilibrium implies equal temperature. The assertion is false because equal temperature does not mean equal heat content.
Answer: Assertion false, Reason true.
Answer and explanation
Both statements are true. A thermometer reads correctly after reaching thermal equilibrium with the body.
Answer: Both true, and Reason is the correct explanation.
Answer and explanation
In an isolated system, no energy transfer occurs. Work is a mode of energy transfer, so it cannot cross the boundary.
Answer: Both true, and Reason is the correct explanation.
Case Study Questions
Answer and explanation
Heat enters the water and steam escapes. Thus both energy and matter exchange occur.
Answer: Open system.
Answer and explanation
Heat flows from milk to thermometer until both reach the same temperature. The constant reading shows thermal equilibrium. Zeroth Law allows the thermometer temperature to represent the milk temperature.
Answer: Thermal equilibrium is reached; Zeroth Law explains measurement.
Answer and explanation
No matter crosses the sealed can. Heat enters from hot water, so energy crosses.
Answer: Closed system.
Answer and explanation
Heat flows internally from the hotter block to the colder block until thermal equilibrium is reached. If the entire insulated box is the system, no energy or matter crosses its outer boundary.
Answer: The selected system is isolated; internal thermal equilibrium will be reached.
Quick Revision Notes
- Thermodynamic system is the chosen part of the universe under study.
- Surroundings include everything outside the system.
- Boundary separates system from surroundings and controls exchange.
- Open system exchanges matter and energy.
- Closed system exchanges energy but not matter.
- Isolated system exchanges neither matter nor energy.
- Heat is represented by Q.
- Work is represented by W.
- Internal energy is represented by U.
- Thermal equilibrium means no net heat flow.
- Thermal equilibrium condition is TA = TB.
- Zeroth Law: if A and B are each in equilibrium with C, then A and B are in equilibrium with each other.
- Zeroth Law is the foundation of thermometer-based temperature measurement.
- Same temperature does not mean same heat or same internal energy.
- Always identify system, surroundings and boundary before solving thermodynamics questions.
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