#### Question

The heat current is written as `(ΔQ)/(Δt)`. Why don't we write `(dQ)/dt?`

#### Solution

The amount of heat crossing through any cross-section of a slab in time `Δt` is called heat current.

It is written as `(ΔQ)/(Δt)` and not `(ΔQ)/(Δt)` as complete derivative . This is because the amount of heat crossing through any cross section is a function of many variables like temperature difference, area of cross-section, etc. So, we cannot write it as a complete derivative with respect to time.

Is there an error in this question or solution?

Solution The Heat Current is Written as ( δ Q δ T ) Concept: Qualitative Ideas of Blackbody Radiation.