First Law of Thermodynamics

First law of thermodynamics: ΔU = Q − W. Energy conservation applied to a thermodynamic system.

Q = heat ADDED to the system (positive when heat enters).

W = work done BY the system on surroundings (positive when gas expands).

ΔU = change in internal energy.

Alternative sign convention: ΔU = Q + W where W = work done ON the system. Use one convention consistently.

For an ideal gas: ΔU = nC_v·ΔT, regardless of process.

Different processes redistribute Q and W differently — but ΔU = nC_v·ΔT always (for ideal gas).

Cannot create or destroy energy — only transform between heat, work, and internal energy.