Foundation of refrigeration cycles: The fundamental principle that makes refrigeration (heat pumping from cold to hot) possible is based on which law of thermodynamics?

Introduction:
Refrigeration and heat pump systems move heat from a lower-temperature reservoir to a higher-temperature reservoir by doing external work. Understanding which thermodynamic law underpins this possibility is crucial for analyzing vapor-compression, absorption, and gas refrigeration cycles used in HVAC and process industries.

Given Data / Assumptions:

• Idealized cyclic operation of a refrigerator/heat pump.
• Work input is available from an external source (compressor, pump, etc.).
• No violation of thermodynamic laws.

Concept / Approach:
The second law of thermodynamics states that heat does not spontaneously flow from cold to hot. However, it can be forced to do so when external work is supplied. Refrigeration cycles exploit this by using a working fluid that absorbs heat at low temperature (evaporator) and rejects it at higher temperature (condenser), with a compressor providing the necessary work to overcome the natural direction of heat flow.

Step-by-Step Solution:
Identify the task: transfer heat from cold to hot reservoir.Invoke the second law: spontaneous transfer is impossible; work input is required.Conclude that refrigeration is governed by the second law, not merely energy conservation (first law).

Verification / Alternative check:
Coefficient of performance (COP) definitions and Carnot refrigerator analysis come directly from second-law statements and entropy balances.

Why Other Options Are Wrong:

• Zeroth law: Defines temperature and thermal equilibrium, enabling thermometry.
• First law: Energy conservation; necessary but not sufficient to enable cold-to-hot heat flow.
• Third law: Concerns entropy at absolute zero.
• Equipartition: A statistical concept, not the governing thermodynamic law here.

Common Pitfalls:
Confusing “possible with work input” (second law compliant) with “forbidden without work input.”

Final Answer:
second