# Heat transfer direction demonstration

I have here a demonstration of heat transfer direction from my course

Suppose that we have to body (H) and (C) in a isolated system:

$$\Delta U = Q_C + Q_H = 0 \Longleftrightarrow Q_C = -Q_H$$

$$\Delta S_{sys} = \Delta S_{C} + \Delta S_{H} = S_{creation}$$

$$\Delta S_{C} + \Delta S_{H} > 0$$

$$\frac{Q_H}{T_H} + \frac{Q_C}{T_C} = Q_H(\frac{1}{T_H}-\frac{1}{T_C}) > 0$$

First line is first principle, second line is second principle in an isolated system (no exchange term) but in my course it's written that we admit a globally irreversible process while we are ignoring the creation term from each system from equation 3 to 4

How the process can be globally irreversible if the two only process happening are reversible ?

• Your second through fourth equations only apply to an irreversible process. They equal zero for a reversible process. What is it you don’t understand? Feb 11, 2020 at 20:46