According to the Second Law of Thermodynamics, what must be supplied to transfer heat from a colder substance to a hotter substance?

The Second Law of Thermodynamics states that heat cannot spontaneously flow from a colder area to a hotter area. To facilitate the transfer of heat against this natural flow, external work must be supplied. This process typically occurs in systems like refrigerators and heat pumps, where work is done on the refrigerant to absorb heat from a colder source and release it to a warmer sink.

Mechanical refrigeration systems illustrate this principle well. When work is inputted into the system, such as through the action of a compressor, it allows for the heat to be effectively moved from a lower temperature to a higher temperature environment. This is a fundamental aspect of how refrigeration and air conditioning systems operate, utilizing mechanical energy to transfer heat in a direction it would not naturally flow.

While options like internal energy, mechanical refrigeration, and physical labor could theoretically contribute to certain aspects of heat transfer or energy transfer, they do not specify the requirement for external work needed to move heat from a cold to a hot substance as clearly as the correct choice does. Thus, understanding that external work is essential to override the natural tendencies described by the Second Law is key to grasping the mechanics of thermodynamic systems.