# 能源和环境物理学|PHYS1211 Energy and Environmental Physics代写

A rambler is walking up a steep hillside in January. He is wearing clothing $1 \mathrm{~cm}$ thick, his skin temperature is $34^{0} \mathrm{C}$ and the exterior surface is close to freezing at $0^{\circ} \mathrm{C}$. Determine the rate of flow of energy outwards from his body, through thermal conduction, when:

• it is fine dry. Assume that the thermal conductivity for clothing, under dry conditions, is $0.042 \mathrm{Wm}^{-1} \mathrm{~K}^{-1}$.
• It has been raining heavily and the rambler is soaked. The thermal conductivity is now $0.64 \mathrm{Wm}^{-1} \mathrm{~K}^{-1}$. Assume that the walker has surface area of $1.84 \mathrm{~m}^{2}$.

Solution:

• Apply the Fourier’s law of thermal conduction.
$\mathrm{d} Q / \mathrm{d} t=-k A \cdot \mathrm{d} T / L=-0.042 \cdot 1.84 \cdot 34 / 0.01 \mathrm{~W}=-263 \mathrm{~W}$.
• Applying the Fourier’s law again, $\mathrm{d} Q / \mathrm{d} t=-4004 \mathrm{~W}$.
When clothing becomes wet, it becomes a better conductor for the outward dissipation of energy because water has a higher thermal conductivity than dry clothing. This is why jeans are inappropriate trouser-ware for strenuous outdoor pursuits in wet weather.

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