HVAC · Thermodynamics · Problem 10PDFSolution in PDF ↓
HVAC · Thermodynamics · Problem 10
Problem & Solution
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Problem: Thermoten, what quantity of heat is released per unit mass when copper is cooled from 250 degrees Fahrenheit to 75 degrees Fahrenheit?
Approach: And we know the delta t because we know the two temperatures.
Calc: So we're actually expecting the answer to be in B2's per pound.
Calc: Specific heat capacity B2 for pound degree F.
Result: Which is answer choice A.
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Student questions asked in live office hours about this problem
OH 43: HVAC: Thermo-10
Q: Where can I find the formula Q = mcp∆T in the reference handbook?
A: It's in section 9.1.7 on page 428 under the water heat transfer formula, though it applies to any substance with constant specific heat. You'll use it so frequently during the exam that you'll naturally memorize it anyway.
OH 69: HVAC: Thermo #10
Q: Why is my method using the volume ratio formula (T2/T1 = (V2/V1)^(K-1)) incorrect when the worked solution uses pressure ratio with compression ratio instead?
A: You need to distinguish between compression ratio (P2/P1, pressure-based) and volumetric compression ratio (V2/V1, volume-based)—they're only equivalent if temperature is constant, which we can't assume here. Since temperature is changing in this process, these are separate and distinct parameters, and you must identify which one the problem actually gives you.