Entropy Change Calculator
Calculate entropy change ΔS = Q ÷ T for a reversible heat transfer at constant absolute temperature.
How it works
- 1Enter the heat transferred Q in joules — positive for heat added, negative for heat removed.
- 2Enter the absolute temperature T at which the transfer occurs, in °C or °F (converted to kelvin).
- 3The calculator applies ΔS = Q ÷ T and returns the entropy change in joules per kelvin (J/K).
Use cases
- Finding the entropy change when a boiler absorbs heat at a fixed temperature.
- Analysing the entropy expelled in a refrigeration cycle.
- Checking that a proposed process satisfies the second law via the sign of ΔS.
Frequently asked questions
What is the formula for entropy change?
For a reversible heat transfer at constant absolute temperature, ΔS = Q ÷ T, with Q in joules and T in kelvin. Adding 1000 J at 250 K gives ΔS = 1000 ÷ 250 = 4 J/K.
What does the second law say about entropy?
The total entropy of an isolated system never decreases. Real (irreversible) processes always produce positive entropy; only ideal reversible processes reach ΔS = 0 for the universe. A subsystem’s ΔS can be negative if offset elsewhere.
Why must the temperature be in kelvin?
ΔS = Q ÷ T is valid only for absolute temperature measured from absolute zero. Dividing by 0 °C (273.15 K) or a negative Celsius value would give a wrong answer or flip the sign. The tool converts °C/°F to kelvin automatically.
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