[[Energy balances - variables and forms]] It is a function of (T,P) (see steam tables) ΔU = [[Heat (Q) is energy transferred between systems at different temperatures|Q]] - [[Work (W) is energy transfer|W]] * sometimes written as q + w, depending on how the sign of w is defined. * ΔU is the sum of energy transferred **from surroundings to system** U is also defined as the sum of kinetic and potential energy.