# Thermodynamic cost of external control

## TL;DR

This paper derives a refined second law for small artificial molecular machines driven by external control, accounting for the thermodynamic cost of control, revealing unexpected regimes where control work can be less than extracted work.

## Contribution

It introduces a refined second law that includes the thermodynamic cost of external control in small machines, highlighting new regimes of work relationships.

## Key findings

- Control work can be smaller than the extracted work.
- Work to generate control can be less than the control work itself.
- New entropic regimes emerge beyond the irreversible control limit.

## Abstract

Artificial molecular machines are often driven by the periodic variation of an external parameter. This external control exerts work on the system of which a part can be extracted as output if the system runs against an applied load. Usually, the thermodynamic cost of the process that generates the external control is ignored. Here, we derive a refined second law for such small machines that include this cost, which is, for example, generated by free energy consumption of a chemical reaction that modifies the energy landscape for such a machine. In the limit of irreversible control, this refined second law becomes the standard one. Beyond this ideal limiting case, our analysis shows that due to a new entropic term unexpected regimes can occur: The control work can be smaller than the extracted work and the work required to generate the control can be smaller than this control work. Our general inequalities are illustrated by a paradigmatic three-state system.