Physics · Thermodynamics · NEET
The system is the specific part of the universe you choose to study, for example the gas inside a cylinder. The surroundings are everything outside the system that can exchange energy or matter with it, like the air and the cylinder walls. Together the system plus surroundings make the whole universe for that problem. You draw an imaginary line (the boundary) to decide what is inside (system) and what is outside (surroundings).
The boundary is the real or imaginary surface that separates the system from the surroundings. It can be fixed, like the rigid walls of a sealed box, or movable, like a piston that slides up when gas expands. The boundary also decides what can cross it: heat, work, or matter. When a piston moves, the boundary moves, and the gas does work on the surroundings.
In a CLOSED system energy (heat and work) can cross the boundary but matter cannot, for example gas sealed in a metal cylinder that can be heated. In an ISOLATED system NOTHING crosses the boundary, neither energy nor matter, for example a perfect thermos flask (ideal). So a closed system still exchanges heat with surroundings, while an isolated system is completely cut off. This is the most common NEET confusion.
Open system: a cup of hot tea without a lid, since steam (matter) and heat both escape. Closed system: gas in a sealed piston cylinder, energy passes as heat and work but no gas leaves. Isolated system: an ideal thermos flask that lets neither heat nor matter through. Note that a truly perfect isolated system does not exist in reality; it is an ideal model.
Thermodynamics deals with BULK (macroscopic) properties like pressure, volume, temperature, and internal energy of a large collection of particles. It does not track individual atoms or their speeds. The study of individual particle motion is kinetic theory or statistical mechanics. So in thermodynamics you describe the state of a system using only a few measurable large-scale variables.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Thermodynamics is the study of heat, work, and energy transfer between a system and its surroundings, using bulk properties like pressure, volume, and temperature.
Open (exchanges both matter and energy), Closed (exchanges only energy), and Isolated (exchanges neither matter nor energy).
The human body is an OPEN system because it takes in food and air (matter) and releases heat and waste (matter and energy) to the surroundings.
No, a real thermos is only an approximation. A perfect isolated system exchanges nothing, but a real flask slowly loses some heat, so it is a near-isolated system, not a perfect one.
Because it describes the average behaviour of a huge number of particles using just a few measurable quantities like P, V, and T, instead of tracking every atom.
