Under which condition$($s$)$ does the equation below apply?

$\frac{1}{A}\frac{\text{d}\text{e}\text{l}}{\text{d}\text{e}\text{l}\text{x}}(kA\frac{\text{d}\text{e}\text{l}\text{T}}{\text{d}\text{e}\text{l}\text{x}})+e_{\text{g}\text{e}\text{n}}=c\frac{\text{d}\text{e}\text{l}\text{T}}{\text{d}\text{e}\text{l}\text{t}}$