Thermodynamic Systems, Equilibrium, and State Variables
A thermodynamic system is a specified quantity of matter or region separated by a boundary; at equilibrium its macroscopic state variables are time-independent, with intensive properties independent of size and extensive properties additive with system size.
Why this shows up in the exam
Classifying pressure, volume, temperature, mass, and internal energy · Applying the zeroth law and thermal-equilibrium reasoning · Counting independent state variables from an equation of state
Learn the idea
A thermodynamic state is fixed by macroscopic variables, while equilibrium makes those variables well defined. Describe the chosen matter and its boundary first; then decide which measured properties specify its state and which depend on sample size.
🧠 Memory hook: State belongs to the system; path belongs to the journey.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- PV = nRT — ideal-gas equation of state for equilibrium states; P is absolute pressure and T is kelvin temperature
- x_specific = X/m — an extensive property X divided by mass becomes an intensive specific property
How to approach it
- 1Draw the system boundary
- 2Classify each quantity as state/path and intensive/extensive
- 3Apply the equation of state only to equilibrium endpoints
Common slip-ups that cost marks
- •Calling heat or work a state variable
- •Treating temperature in degrees Celsius inside gas-law ratios
- •Assuming every extensive-to-extensive ratio is extensive
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A gas absorbs 500 J of heat and does 200 J of work. What is the change in its internal energy?
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Ideal Gas Law and Equation
Learn the ideal gas equation, its relation to physical quantities like pressure, volume, temperature, and density, and its use in describing the behavior of ideal gases.
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Entropy and Its Changes
Understand entropy as a measure of disorder, how it changes in physical and chemical processes, and its calculation in various scenarios including phase transitions and isothermal processes.