Heat and Thermodynamics

9.2 The Spontaneous Process

The conversion of mechanical work into heat is spontaneous. Thus when two blocks of wood are rubbed together, both of them become heated and this happens without our intervention thereby converting the mechanical work into heat. But these two blocks when placed together will not rub against each other, though each of them has a good stock of energy in the form of heat. So, conversion of heat into mechanical work is not a spontaneous process; otherwise, ocean-going vessels would no longer need to carry large quantities of fuel. The ocean itself has an immeasurable store of energy in the form of heat and this heat could be spontaneously converted into mechanical work of moving the vessel against the friction of water. ...