Physics · Thermodynamics · NEET
Both are correct, but they use different sign meanings for W. The NCERT/NEET standard form is Q = dU + W, where W is work done BY the gas (on the surroundings). Rearranged, dU = Q - W. Some books write Q = dU - W when W means work done ON the gas. Stick to the NEET convention: Q = heat added to system, W = work done by system, so Q = dU + W. Memorize the words, not just letters: heat in equals energy stored plus work done out.
Q (heat) and W (work) are energy in transit (path functions) - they only exist during a process and depend on the path taken. dU (change in internal energy) is a state function - it depends only on the starting and ending temperature, not the path. So two different paths between the same two states can have different Q and different W, but always the same dU.
No. The First Law only says energy is conserved. It does not forbid heat flowing from cold to hot, or a gas cooling itself to do work. It is silent about direction and feasibility. That job belongs to the Second Law of Thermodynamics. The First Law is a bookkeeping rule: energy in must equal energy out plus energy stored.
Internal energy U is a state function. In a cyclic process the gas returns to its exact starting state (same P, V, T), so its internal energy is unchanged: dU = 0. Then the First Law gives Q = W. All the net heat absorbed comes out as net work. This is the working idea behind heat engines.
For an ideal gas, internal energy depends ONLY on temperature. So dU = n Cv dT for ANY process (not just constant volume). If temperature does not change (isothermal process), dU = 0 for an ideal gas, no matter how the volume or pressure changes. This is why in an isothermal process Q = W.
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:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Heat supplied to a system equals the increase in its internal energy plus the work done by the system: Q = dU + W. It is the law of conservation of energy applied to heat.
Q = dU + W. Q is heat added to the system (joules), dU is the change in internal energy, and W is the work done by the system. Rearranged: dU = Q - W.
Q is positive when heat is added to the system, negative when heat leaves. W is positive when the gas expands (does work on surroundings), negative when it is compressed. dU is positive when temperature (internal energy) rises.
Yes. Q = dU + W holds for every thermodynamic process - isothermal, adiabatic, isobaric, isochoric and cyclic - and for any substance, because it is just energy conservation.
At constant volume no work is done (W = 0), so Q = dU. All the heat added goes into raising internal energy and temperature.
It tells us energy is conserved but not the direction a process takes or whether it is feasible. It does not explain why heat flows from hot to cold. The Second Law fills this gap.
