work
Work, in thermodynamics, is energy that crosses a boundary by pushing or pulling — by a force acting through a distance — rather than by a temperature difference. Picture a bicycle pump: you press the handle down, the air inside is squeezed, and your muscle's energy is delivered into the gas. That organized, directed transfer of energy, where something visibly moves against a force, is work.
The most common kind in chemistry is pressure–volume work: a gas expanding pushes its surroundings back, or the surroundings press in and compress the gas. In equations work is written w, counted as positive when energy is delivered into the system (the gas is compressed) and negative when the system gives energy out (the gas expands and pushes outward). Like heat, work is a path function — the amount depends on how the change is carried out.
Work and heat are the two doorways through which energy enters or leaves a system, and together they set the change in internal energy. The difference between them is in their orderliness: work is energy transferred through coordinated, large-scale motion, while heat is energy transferred through the random jostling of molecules. The same amount of energy can arrive by either road.
When gasoline burns in an engine cylinder, the hot gases expand and shove the piston down. That outward push on the piston is work — the engine turning chemical energy into motion.
Expanding gas pushing a piston is the classic example of pressure–volume work.
Sign conventions for work differ between textbooks. The modern physics convention (used here) counts work done on the system as positive, giving ΔU = q + w. Some older chemistry texts count work done by the system as positive and write ΔU = q − w. The physics is the same; only the bookkeeping sign flips.