Chemistry · Thermodynamics · NEET
They are NOT the same. Both heat and work change the internal energy (U) of a system, but they are two different roads for energy to travel. Heat travels ONLY when there is a temperature difference. Work travels when a force acts through a distance (like a gas pushing a piston), with no temperature difference needed. Same destination (change in U), different path.
A state function depends only on the start and end state, not the route taken. Internal energy U is a state function. But q and w depend on HOW you go from start to end. The same change of state can be reached with lots of heat and little work, or little heat and lots of work. Since their values change with the path, q and w are called path functions, not state functions.
No. This is a very common mistake. A system does not contain heat and does not contain work. It only contains internal energy U. Heat and work are energy IN TRANSIT — they only exist while energy is crossing the boundary. Once energy is inside, it is just internal energy. So never say 'the gas has 100 J of heat'; say 'the gas gained 100 J as heat'.
Look at the cause. If energy crosses the boundary because one side is hotter than the other, it is heat. If energy crosses because of an organised force moving through a distance (a piston moving, a paddle stirring), it is work. Joule showed that stirring water with a paddle wheel raises its temperature — that is energy entering as work, even though no hotter object touched it.
Follow the IUPAC chemistry convention used in NCERT. Heat: q is POSITIVE when heat enters the system (surroundings to system), NEGATIVE when heat leaves. Work: w is POSITIVE when work is done ON the system (gas compressed), NEGATIVE when work is done BY the system (gas expands). Then the first law is written as ΔU = q + w.
Because sign shows direction. When a gas EXPANDS, it pushes the surroundings and gives energy away, so the system loses energy — that is why w is negative in the chemistry convention. When a gas is COMPRESSED, the surroundings push energy into it, so w is positive. The sign is not about heat vs work; it is about whether the system gained or lost energy.
At a certain temperature T (K), during a process, 500 J is absorbed by the system and work of 200 J is done by the system. Then the change in internal energy of the system is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Heat is energy transferred because of a temperature difference; work is energy transferred by any other means, such as a force moving through a distance.
Heat is a way of transferring energy, not a stored form. The stored form inside a system is internal energy (U). Heat only exists while energy is crossing the boundary.
Because most errors are sign errors, not formula errors. NEET sets traps where you must decide if heat enters or leaves and if work is done on or by the system before using ΔU = q + w.
Yes. Both can raise or lower internal energy U. That is exactly why the first law adds them together: ΔU = q + w. They are just two different channels for the same energy change.
It proves you can raise a system's temperature purely by doing work (stirring), with no hotter body involved. It also showed adiabatic work is path-independent, which lets us define internal energy as a state function.
