TFS · Thermodynamics · Problem 2PDFSolution in PDF ↓
TFS · Thermodynamics · Problem 2
Problem & Solution
Video Synthesis
- Problem: What is the ratio of specific heats for 120 degree air?
- Key step: What is the ratio of specific heats for 120 degree air?
- Watch out: So look up in the reference handbook specific gas constant and not only will you find the value which is that are equals 53.35 and that has units of a...
- Result: And right k equals Cp over Cv both of which are now known and have the same units.
- ✅ Answer: A
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Student questions asked in live office hours about this problem
OH 107
Q: You show Cp with units of BTU/(lbm·°R), but the reference handbook table 1.2.7 'Properties of Air at Low Pressure' lists Cp in BTU/(lbm·°F) — am I looking at the right table?
A: Yes, you're looking at the right table — and it's perfectly fine regardless of which temperature unit appears. The size of one Fahrenheit degree equals the size of one Rankine degree, so BTU/(lbm·°F) and BTU/(lbm·°R) are numerically identical. You only need to distinguish between Fahrenheit and Rankine when you're working with absolute temperature values in equations like the ideal gas law; for differences (like ΔT) and specific heat capacity, the units are interchangeable.