The statement that ‘heat cannot flow by itself from a body at a lower temperature to a body at a higher temperature’, is known as

The statement that heat cannot flow by itself from a body at a lower temperature to a body at a higher temperature is a fundamental expression of the Second Law of Thermodynamics [2]. This specific formulation is known as the Clausius statement [1]. While the First Law of Thermodynamics deals with the conservation of energy [2], it does not prohibit heat from flowing from cold to hot; it is the Second Law that imposes this directional restriction based on the principle of entropy. In natural processes, heat spontaneously flows from a hotter object to a colder one to increase the system's total entropy [1]. To reverse this flow and move heat from a lower to a higher temperature, external work must be performed on the system, as seen in refrigeration cycles.

Sources

1. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > 1.14 Environment and Ecology > p. 14
2. [1] https://satheejee.iitk.ac.in/article/physics/physics-clausius-statement/