What describes the characteristics of a system at thermal equilibrium?

A system at thermal equilibrium is characterized by uniform temperatures throughout. This means that there are no temperature differences within the system, and as a result, there is no net heat flow between different parts of the system. When thermal equilibrium is reached, all parts of the system are at the same temperature, which indicates that energy in the form of heat is transferred evenly among the components.

In contrast, constant heat flow between objects would imply that there are temperature differences, which is not a characteristic of thermal equilibrium. Similarly, the presence of temperature gradients indicates that different areas within the system are at varying temperatures, suggesting that the system has not yet reached equilibrium. Lastly, the idea that heat is always lost to the surroundings contradicts the concept of thermal equilibrium, as heat transfer can occur only when there is a temperature difference, and at equilibrium, the system is effectively isolated in terms of net heat exchange. Therefore, the only option that accurately depicts a system at thermal equilibrium is the uniform temperature throughout.