There are multiple valid ways to state the second law of thermodynamics. Some ways of expressing the law do so in terms of macroscopic notions such as heat and temperature.
Other descriptions employ the concept of entropy, which is based upon a statistical approach to thermodynamics. Some alternative macroscopic statements include:
- There can be no process with the sole result of absorbing heat and completely converting it into work.
- It is impossible to convert heat completely into work in a cyclic process.
- It is impossible to carry out a cyclic process using an engine connected to two heat reservoirs that will have as its only effect the transfer of a quantity of heat from the low-temperature reservoir to the high-temperature reservoir.