Consider this figure:

From there we get:

dS = d Q_R / T

d Q_R = T dS

Q_R = ²ʃ₁ T Ds

= area under the curve 1 – 2

= Heat transfer in the process.

• In x-axis we take entropy (s) and in y-axis we take Kelvin temperature (T).
• This is basically known as called T – 5 diagram.
• T 1 – 2 is a reversible process.