Physics · Thermodynamics · NEET
No. Internal energy (U) is energy already stored inside the gas: the kinetic energy of its moving molecules (and potential energy of interactions). Heat (Q) is energy that FLOWS in or out only because two bodies are at different temperatures. You cannot say a gas 'contains 50 J of heat'; you can only say 50 J of heat FLOWED into it. Once inside, that energy becomes part of the internal energy. So heat is a transfer process, internal energy is a stored amount.
Heat and work only exist while energy is crossing the boundary of the system. A gas sitting alone has an internal energy, but it has no heat and no work at that moment. Heat and work are like the act of transferring money; internal energy is the balance in the account. That is why we write a small change as delta U (a change in a stored quantity), but we just write Q and W (amounts transferred), not delta Q or delta W.
Internal energy (U) is a STATE function: it depends only on the current state (mainly temperature for an ideal gas), not on how the gas got there. Heat (Q) and work (W) are PATH functions: their values depend on the route taken on the P-V diagram. Two different paths between the same start and end points give the SAME delta U but usually DIFFERENT Q and W. This is exactly what NEET 2025 tested.
Not always. By the first law, delta U = Q - W. If the gas absorbs heat Q but also does work W by expanding, the internal energy rises only by the leftover (Q - W). In an isothermal expansion the gas absorbs heat but ALL of it becomes work, so delta U = 0 and temperature does not change. In an adiabatic expansion Q = 0, yet the gas still does work, so internal energy (and temperature) FALLS.
No. A gas never 'contains' work, just as it never 'contains' heat. Work is energy transferred when the gas volume changes against a pressure. Once the piston stops moving, there is no work anymore; the transferred energy has become part of the internal energy (or was supplied by it). Only internal energy is stored in the gas.
At a certain temperature, during a process, 500 J is absorbed by the system and 200 J of work is done by the system. The change in internal energy of the system is
An electric heater supplies heat to a system at a rate of 100 W. If the system performs work at a rate of 75 J/s, the rate at which internal energy increases is
Two gases A and B are filled at the same pressure in separate cylinders with movable pistons of radius r_A and r_B. On supplying an equal amount of heat to both gases reversibly at constant pressure, the pistons of A and B are displaced by 16 cm and 9 cm respectively. If the change in their internal energy is the same, then the ratio r_A / r_B is
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Heat and work are energy in TRANSIT across the boundary (heat due to temperature difference, work due to volume change), while internal energy is the energy STORED inside the system.
All three are forms of energy, so all are measured in joules (J). Heat is also sometimes given in calories, where 1 cal = 4.2 J.
Only internal energy is a state function; it depends solely on the state (mainly temperature for an ideal gas). Heat and work are path functions that depend on the process taken.
Yes. In an adiabatic process Q = 0, but if the gas is compressed, work is done ON it and its internal energy (and temperature) rises. So delta U = -W.
Because U is a stored state quantity, so a change in it makes sense (delta U). Heat and work are amounts transferred, not stored, so we write just Q and W, meaning 'the heat transferred' and 'the work done'.
