Conduction through a plane wall: How does the total heat flow rate depend on area, temperature difference, and thickness, according to elementary steady 1-D conduction relations?
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ADirectly proportional to the surface area of the body
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BDirectly proportional to the temperature difference across the body
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CInversely proportional to the thickness of the body
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DAll of the above
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EIndependent of material properties
Answer
Correct Answer: All of the above
Explanation
Introduction / Context:Fourier’s law for steady, one-dimensional conduction through a homogeneous plane wall leads to a simple proportionality that is widely used for first-cut thermal sizing of walls, insulations, and heat exchanger plates.
Given Data / Assumptions:
- Plane wall of thickness L and area A.
- Constant thermal conductivity k.
- Steady state with boundary temperatures T_hot and T_cold.
Concept / Approach:For 1-D conduction, q = -k * A * dT/dx. With linear temperature distribution and constant k, integration yields Q = k * A * (T_hot - T_cold) / L. This shows direct proportionality to area and temperature difference, and inverse proportionality to thickness; it also highlights dependence on material via k.
Step-by-Step Solution:
Start with q" = -k * dT/dx.Integrate across thickness L with ΔT = T_hot - T_cold.Obtain Q = k * A * ΔT / L.Interpret proportionalities directly from the expression.Verification / Alternative check:Compare two walls of the same material: doubling area doubles Q; doubling thickness halves Q; doubling ΔT doubles Q. Experimental results match these trends in the linear regime.
Why Other Options Are Wrong:
- (a), (b), and (c) each state a true but partial relationship; the complete statement is “All of the above”.
- (e) is false—material property k is central to conduction.
Common Pitfalls:Applying this 1-D formula to multilayer walls without using the appropriate series thermal resistance model; neglecting contact resistances and temperature-dependent k.
Final Answer:All of the above