Write a differential equation stating the Fourier's law of conduction and label its variables. Using Fourier's law and FIGURE Q3 in APPENDIX Q3, derive the expression for the heat flow through a flat composite wall given by the EQUATION Q3. State any assumptions. Q=A \frac{T_{1}-T_{2}}{\frac{x}{k}} A window 1.0 m wide by 2.0 m high originally contained a single pane of glass 5 mm thick. It is proposed to install double glazing consisting of two sheets of glass separated by a stagnant air gap 10 mm thick at a cost of £450. If heating costs 4.0p per kWh, using a proper form EQUATION Q3, estimate the time taken to recover the cost of double glazing when the temperature difference between inside and outside surfaces of the glass window is 15°C.

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