Fig. 1
From: Quantum engine efficiency bound beyond the second law of thermodynamics
Visualisation of the concept of passive energy and ergotropy. The different kinds of energy contained in a quantum state visualised by means of a battery at a certain temperature. The battery charge (yellow bars) represents ergotropy \({\cal W}\) (extractable as work, here illustrated by a lighted bulb) and its temperature (colour of the battery: red—hot, blue—cold) represents passive (here: thermal) energy Epas—the higher the temperature the larger the passive energy. a The battery is partly charged and hot: this represents a non-passive state that allows for work extraction. As the battery is not completely charged, the light bulb appears dim. b The battery is discharged, but its temperature is the same as in a. This state is the passive state of a and, consequently, the light bulb does not shine. c The battery is in a non-passive state whose ergotropy is higher than in a (the battery is fully charged) but the passive energy is lower (the battery is colder). Although the total energy in a, c may be the same, more work can be extracted from the state c, causing the light bulb to shine brighter than in a
