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How does entropy change when heat enters a reservoir at constant temperature?

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Engraved portrait of Rudolf Julius Emanuel Clausius, a bearded man in formal attire with sideburns, facing forward. — thermodynamics Rudolf Clausius (1822 - 1888) is considered one of the central founders of the science of thermodynamics.

When an amount of heat $Q$ flows into a heat reservoir at a constant temperature $T$ , the entropy $S$ of that reservoir increases by the following equation:

$Δ S = \frac{Q}{T}$

This relationship, introduced by Rudolf Clausius, serves as a precise mathematical method for testing the second law of thermodynamics. In this context, entropy acts as a measure of a system's thermal energy per unit temperature that is unavailable for performing useful work.12