IB Physics HL Practice Exam Prep - Practice Test & Study Guide

Thermal equilibrium signifies a condition where two systems in heat exchange reach the same temperature, resulting in no net heat transfer between them. This is encapsulated in the concept that once equilibrium is achieved, the systems are in balance, and any heat exchange that occurs is equal in both directions, effectively resulting in no overall change in heat energy.

In proposition C, although it hints at a scenario where systems are different initially, it actually implies that there is no heat exchange once the systems reach thermal equilibrium. However, this option only details the outcome without highlighting the crucial fact that thermal equilibrium fundamentally requires an equalization of temperatures between systems.

The correct implication is more closely aligned with the principle that at thermal equilibrium, the temperatures of both systems are equal, and consequently, any potential for heat flow ceases because there is no temperature gradient to drive the transfer. Therefore, thermal equilibrium is best defined by equal temperatures in two systems involved in heat exchange, making the condition of temperature equality central to understanding thermal equilibrium.