TFS · Practice Exam 1 · Problem 42PDFSolution in PDF ↓
TFS · Practice Exam 1 · Problem 42
Problem & Solution
Video Synthesis
- Problem: An exterior wall is made up of 4-inch brick cladding with a known thermal conductivity, and 3.5 inch mineral fiber bat insulation with a known conductance, and 5-8 inch gypsum board with a known condu...
- Approach: Let's start by drawing a picture.
- Key values: 3.5 in, 8 in, 4 in, 12 in
- ✅ Answer: A
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Student questions asked in live office hours about this problem
OH 75
Q: I used the reference handbook formula on page 287 to find the temperature at interface x in a composite wall problem, got the same R_total as the solution, but calculated T_x as 66°F instead of the correct answer—am I applying the equation correctly?
A: You're not using it correctly, and more importantly, you need to check your intuition: with 70°F inside and 0°F outside, 66°F at an interface beyond the insulation layer doesn't make physical sense—there should be a huge temperature drop across the insulation, so that point should be much closer to 0°F. The equation uses a proportion based on resistance ratios, so you can't just plug in numbers blindly; you likely made an error defining what R_x represents in your setup.