In Chapter 3, heat and work were identified as two forms of energy that are of primary interest in the study of thermodynamics and it was shown that the first law of thermodynamics relates the heat and work interactions at the boundary of a system to the change of energy within the system. The first law views heat and work as quantitatively equivalent to each other but places no restrictions on the conversion of heat into work or work into heat. In other words, the first law places no constraints on the direction of conversion.