Question

What is reversible heat flow, and how is it related to entropy? What is the difference between reversible and irreversible heat flow?

Answer 1

For further reading, see additional explanation here.

Heat flow can either be reversible or irreversible; all that means is that it is either perfectly conservative or we missed a spot and the heat is lost somewhere.

Reversible heat flow is the process of transferring heat infinitesimally slowly in such a way that no heat is lost, i.e. it is the MAXIMUM heat flow that can occur. That is the kind of heat flow described here:

`DeltaS = (q_"rev")/T`

Irreversible heat flow is basically inefficient heat flow plus the heat that was inadvertently missed or lost:

`q_"irr" + q_"lost" = q_"rev"`

That is, `q_"irr" < q_"rev"`. Let's then divide by `T`:

`q_"irr"/T + q_"lost"/T = q_"rev"/T`

The righthand side is equal to `DeltaS`:

`DeltaS = (q_"irr")/T + (q_"lost")/T`

If we write

`DeltaS >= q/T`,

we would then have that the equals sign represents `q_"rev"`, and the greater-than sign represents `q_"irr"`. i.e. we have:

`color(blue)(DeltaS > q_"irr"/T)`

`color(blue)(DeltaS = q_"rev"/T)`