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