What must be supplied to transfer heat from a colder substance to a hotter substance according to the second law of thermodynamics?

To transfer heat from a colder substance to a hotter substance, external work must be supplied, in accordance with the second law of thermodynamics. This law establishes that heat naturally flows from areas of higher temperature to areas of lower temperature. In order to reverse this natural flow and move heat from a colder region to a hotter one, energy input is necessary, typically in the form of work.

In practical applications, such as in refrigeration systems, this is achieved by using a compressor to do work on a refrigerant, enabling the heat to be lifted from the cold reservoir and transferred to a hot reservoir, in opposition to the spontaneous direction of heat flow. Thus, the external work is essential for this process and directly aligns with the principles outlined in the second law of thermodynamics.