Air is contained in a vertical piston-cylinder assembly fitted with an electrica

Question

Air is contained in a vertical piston-cylinder assembly fitted with an electrical resistor. The atmosphere exerts a pressure of 14.5 psi on top of the piston which has a mass of 100 lbm and a face area of 1 ft2. Electric current passes through the resistor, and the volume of the air slowly increases by 1.6 ft3 while its pressure remains constant. The mass of the air is 0.6 lbm, and its specific internal energy increases by 8 Btu/lbm. The air and piston are at rest initially and finally. The piston-cylinder material is a ceramic composite and thus a good insulator. Friction between the piston and cylinder wall can be ignored, and the local acceleration of gravity is 32.0 ft/s2. Determine the heat transfer from the resistor to the air in Btu for a system consisting of the air alone. Hint: To determine the pressure of the system, use a force balance in which the upward force exerted by the air on the bottom of the piston equals the weight of the piston plus the downward force of the atmosphere acting on the top of the piston.

Explanation / Answer

Fair + Fpiston = P1A => Patm + mg/A = P1 =>14.5 + 22.2 =36.7 psi

Change in Internal Energy = Mass * Increase in Specific Internal Energy = 8* 0.6 = 4.8 Btu

Work Done = Pressure * Change in Volume = 36.7 * 1.6 * 123 = 101468.16/778 = 130.4 Btu

dQ = dU + dW => 130.4 + 4.8 = 135.2 Btu

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.