Forming three sides of a square, a steel frame ( K = 45 W m^ -1 ^ ^...

The rate of heat flow through a conducting rod is given by Fourier's law of heat conduction:

$H = \dfrac{KA \Delta T}{L}$

Given:
Thermal conductivity, K = 45 \text{ W m}^{-1} ^{}^{{}^{\circ}}\textC^{-1}
Cross-sectional area, $A = 0.20 \text{ cm}^2 = 0.20 \times 10^{-4} \text{ m}^2 = 2 \times 10^{-5} \text{ m}^2$
Total length of the frame, $L = 60 \text{ cm} = 0.6 \text{ m}$
Temperature difference between the ends, \Delta T = 40^{}^{{}^{\circ}}\text{C} - 20^{}^{{}^{\circ}}\text{C} = 20^{}^{{}^{\circ}}\text{C}

Substituting the values into the formula:

$H = \dfrac{45 \times 2 \times 10^{-5} \times 20}{0.6}$

$H = \dfrac{1800 \times 10^{-5}}{0.6}$

$H = 3000 \times 10^{-5} \text{ W}$

$H = 0.03 \text{ W}$