Work, heat and the first law

NCERT's sign convention: work that the surroundings do on the system counts as positive w, and work the system does on its surroundings counts as negative w.

Likewise q is positive when heat flows into the system and negative when heat leaves it.

First law: ΔU = q + w. Energy of an isolated system stays constant; energy can change form but is not created or destroyed.

If a gas absorbs 100 J of heat and does 300 J of work on the surroundings, ΔU = +100 + (−300) = −200 J.

For heat exchanged at constant volume and with no other work, w = 0 and ΔU = qᵥ.

Adiabatic work done on a system raises its internal energy by exactly that amount: ΔU = w_ad.

Heat and work are two different ways of transferring energy; neither is stored in the system.