27-01-2016, 10:44 AM
A heat engine – any device that is capable of converting thermal energy (heating) into mechanical energy (work). We will consider an important class of such devices whose operation is cyclic.
Heating – the transfer of energy to a system by thermal contact with a reservoir.
Work – the transfer of energy to a system by a change in the external parameters (V, el.-mag. and grav. fields, etc.).
The main question we want to address: what are the limitations imposed by thermodynamic on the performance of heat engines?
Perpetual Motion Machines of the first type – these designs seek to create the energy required for their operation out of nothing.
Perpetual Motion Machines of the second type - these designs extract the energy required for their operation in a manner that decreases the entropy of an isolated system.
Word of caution: for non-cyclic processes, 100% of heat can be transformed into work without violating the Second Law.
Example: an ideal gas expands isothermally being in thermal contact with a hot reservoir. Since U = const at T = const, all heat has been transformed into work.
Fundamental Difference between Heating and Work
- is the difference in the entropy transfer!
Transferring purely mechanical energy to or from a system does not (necessarily) change its entropy: S = 0 for reversible processes. For this reason, all forms of work are thermodynamically equivalent to each other - they are freely convertible into each other and, in particular, into mechanical work.
Work can be completely converted into heat, but the inverse is not true. The transfer of energy by heating is accompanied with the entropy transfer
Thus, entropy enters the system with heating, but does not leave the system with the work. On the other hand, for a continuous operation of a heat engine, the net entropy change during a cycle must be zero!
How is it possible?
An engine can get rid of all the entropy received from the hot reservoir by transferring only part of the input thermal energy to the cold reservoir.