B.4
Thermodynamics
The first and second laws of thermodynamics, work done in thermodynamic processes, isobaric, isovolumetric, isothermal and adiabatic processes on PV diagrams, cyclic processes, thermal efficiency, the Carnot cycle, and entropy.
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Key Concepts, Thermodynamics
The First Law of Thermodynamics
The first law of thermodynamics is a statement of conservation of energy applied to a thermodynamic system. It states that the thermal energy (heat) added to a gas equals the increase in internal energy of the gas plus the work done by the gas: Q = ΔU + W. Here Q is positive when heat flows into the system, W is positive when the gas does work on its surroundings (expansion), and ΔU is positive when internal energy increases. This single equation links thermal energy, internal energy and mechanical work, and is the foundation for analysing all thermodynamic processes. The system used throughout this topic is the simple engine: an ideal monatomic gas enclosed in a piston and cylinder.
Work Done by or on a Closed System
When a gas expands, it pushes a piston and does work on the surroundings. This work is W = PΔV, where P is pressure and ΔV is the change in volume. When a gas is compressed, work is done on the gas and W is taken as negative. On a PV diagram, the work done by or on the gas is equal to the area under the process curve. For a rectangular process this is straightforward. For a curved process, you may need to count squares under the graph. For a complete cycle, the net work done is the area enclosed by the cycle on the PV diagram.
Isobaric, Isovolumetric, Isothermal and Adiabatic Processes
Each thermodynamic process holds one variable constant. An isobaric process holds pressure constant: the process is a horizontal line on a PV diagram and W = PΔV applies directly. An isovolumetric (isochoric) process holds volume constant: the line is vertical, ΔV = 0, so W = 0 and all heat input goes into changing internal energy. An isothermal process holds temperature constant: the process follows a curved isotherm, ΔU = 0, so all heat input does work. An adiabatic process has no exchange of thermal energy with the surroundings: Q = 0, so all work done by the gas comes at the expense of its internal energy. On a PV diagram, an adiabatic curve is steeper than an isotherm and crosses between isotherms.
Adiabatic Processes for Monatomic Ideal Gases
For an adiabatic process involving a monatomic ideal gas, the relationship between pressure and volume follows P₁V₁^(5/3) = P₂V₂^(5/3), where the exponent 5/3 applies specifically to monatomic gases. This allows you to find an unknown pressure or volume after an adiabatic change. Combined with the ideal gas equation PV = nRT, you can also find the temperature at any point. A common exam technique is to use nR = PV/T (which is constant for a fixed mass of gas) to simplify calculations, avoiding the need to know n and R individually.
Cyclic Processes and Heat Engines
A cyclic process is a series of thermodynamic transformations that returns the gas to its original state. On a PV diagram, the cycle forms a closed loop. The net work done by the gas per cycle equals the area enclosed by the loop. In a simple heat engine, fuel is burned to heat the gas (chemical to thermal energy), the gas expands and does work on the piston, heat is then released to the surroundings, and the gas is compressed back to its original state. Not all the heat input is converted to useful work: some is always lost to the surroundings, which is why no real engine is 100% efficient.
Thermal Efficiency and the Carnot Cycle
The thermal efficiency of a heat engine is defined as the ratio of useful work output to heat input. The Carnot cycle is a theoretical ideal cycle consisting of two isothermal and two adiabatic processes. It represents the maximum possible efficiency any heat engine can achieve when operating between a hot reservoir at temperature T_H and a cold reservoir at temperature T_C. The Carnot efficiency is η = 1 - T_C/T_H, where both temperatures must be in Kelvin. A real engine always operates at lower efficiency than the Carnot limit because of irreversible processes, friction and heat losses. The efficiency equation η = 1 - Q_out/Q_in can also be used to find efficiency from the heat transferred during each cycle.
Entropy and Microstates
Entropy is a measure of the disorder of a system, defined in terms of the number of possible microstates Ω available to the system: S = k_B ln(Ω), where k_B is the Boltzmann constant. A microstate is a specific way the molecules can be arranged and distributed in energy. More microstates means more disorder and higher entropy. When molecules in an ordered state (like air flowing into a balloon) are released, they spread out into many possible configurations, vastly increasing the number of microstates and therefore the entropy. Entropy can also be calculated from the thermal energy transferred: ΔS = Q/T, where Q is heat added and T is the temperature of the body in Kelvin. The units of entropy are J/K.
The Second Law of Thermodynamics
The second law of thermodynamics has two equivalent forms. The Kelvin form states that it is not possible for a heat engine working in a cycle to absorb thermal energy and convert all of it to work: some must always be released to the surroundings, because otherwise the net entropy of the universe would decrease. The Clausius form states that heat cannot spontaneously flow from a cold body to a warmer one without work being done. Both forms reflect the same underlying truth: in any real process, the total entropy of the universe either stays the same or increases. It never decreases. This is why 100% efficient engines are impossible and why energy naturally becomes more spread out and disordered over time.
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Frequently Asked Questions, IB Physics Thermodynamics
What is Thermodynamics in IB Physics? ↓
The first and second laws of thermodynamics, work done in thermodynamic processes, isobaric, isovolumetric, isothermal and adiabatic processes on PV diagrams, cyclic processes, thermal efficiency, the Carnot cycle, and entropy. This topic is part of Theme B (The Particulate Nature of Matter) in the current IB Physics syllabus.
Is Thermodynamics SL or HL in IB Physics? ↓
Thermodynamics is an HL-only topic. It is not assessed in the SL IB Physics exam.
What equations do I need for IB Physics Thermodynamics? ↓
The key equations for Thermodynamics are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Thermodynamics? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Thermodynamics for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.