Free EIT Thermodynamics and Strength of Materials Questions and Answers β Questions and Answers
Question 1: A cantilever beam is loaded with a force of 10 KN and has a cross-section as shown. The maximum tensile stress in MPa in the beam most nearly is:
- 4.16
- 441 (Correct answer)
- 588
- 735
Correct answer: 441
Explanation: <br> The second moment of area of the cross-section is 1.36 Γ 10β6ππ4 <br> The top of the beam is in tension. <br> The centroid is 50 mm up from the bottom of the cross-section. <br> The maximum moment is 20 KNβm.
Question 2: The maximum tensile principal stress in MPa for the two-dimensional stress state given is:
- 20
- 30 (Correct answer)
- 40
- 60
Correct answer: 30
Explanation: <br> A normal compressive stress in the y-direction of 60 MPa is a negative stress.
Question 3: In all adiabatic processes, there is:
- no heat flow (Correct answer)
- no change in temperature
- no change in enthalpy
- All of the above
Correct answer: no heat flow
Explanation: <br> Mostly this requires the student to remember that adiabatic means (by definition) that there is no flow of heat. Then one must make sure that (B) and (C) do not also generally accompany an adiabatic process. While an adiabatic process may have no temperature change or enthalpy change, this is not generally true. For instance, enthalpy and temperature both change for an adiabatic turbine.
Question 4: A Carnot refrigerator operates between 10Β°πΆπΆ and 25Β°πΆπΆ. The coefficient of performance is:
- 0.0500
- 0.667
- 18.9 (Correct answer)
- 19.9
Correct answer: 18.9
Explanation: <br> It's calculated using the formula for the coefficient of performance (COP) of a Carnot refrigerator.
Question 5: Air in a closed-rigid vessel at 2 bars and 25Β°πΆπΆ is heated until its temperature is 100Β°πΆπΆ. The final pressure in bars is:
- 2.50 (Correct answer)
- 4.40
- 8.00
- 256.0
Correct answer: 2.50
Explanation: <br> The final pressure is approximately 2.50 bars.
Question 6: Air is adiabatically compressed in a steady flow system from 1 bar to 5 bars. The air enters at 298 K and the compressor efficiency is 70 %. The work per unit mass of air required for the compression (in kJ/kg) is:
- 122
- 174
- 249 (Correct answer)
- 1703
Correct answer: 249
Explanation: <br> The outlet temperature is unknown. The procedure is to solve for ππ/ππ for the ideal case (where the outlet temperature can be calculated) and then apply the efficiency to get the actual ππ/ππ. The ideal case is reversible and is also adiabatic. An adiabatic and reversible process is isentropic. For an isentropic process involving an ideal gas with constant heat capacity. <br><br> The actual compressor requires more energy and is obtained from <br> W/m = (W/m)ideal/efficiency <br> = 174/0.7 <br> = 249kJ/kg
Question 7: A 1-meter diameter steel cylindrical tank is used to store propane at high pressure. If the allowable stress of steel is 100 MPa and the propane is stored at a pressure of 1000 kPa, the minimum thickness in mm of the tank is:
- 5.0 (Correct answer)
- 7.5
- 10
- 20
Correct answer: 5.0
Explanation: <br> Hoop stress in the tank governs the maximum allowable stress. <br> Hoop stress is equal to circumferential stress and tangential stress.
Question 8: At what point in a Carnot cycle does heat rejection occur?
- Isothermal compression
- Adiabatic compression
- Isothermal expansion
- Adiabatic expansion (Correct answer)
Correct answer: Adiabatic expansion
Explanation: <br> Heat rejection in a Carnot cycle occurs during the isothermal compression phase. During this phase, the working substance releases heat to the surroundings at a constant temperature.
Question 9: What is the purpose of a condenser in a Rankine cycle?
- To increase the temperature of the working fluid
- To reduce the pressure of the working fluid
- To remove moisture from the working fluid
- To convert the working fluid from vapor to liquid (Correct answer)
Correct answer: To convert the working fluid from vapor to liquid
Explanation: <br> The purpose of a condenser in a Rankine cycle is to convert the working fluid from vapor to liquid by removing heat from it. This process occurs at constant pressure.
Question 10: Which property of a substance remains constant during an isentropic process?
- Temperature
- Pressure
- Volume
- Entropy (Correct answer)
Correct answer: Entropy
Explanation: <br> Entropy remains constant during an isentropic process. This implies that there is no change in the degree of disorder of the substance.
Question 11: What happens to the entropy of a closed system in an irreversible process?
- It remains constant
- It decreases
- It increases (Correct answer)
- It becomes zero
Correct answer: It increases
Explanation: <br> The entropy of a closed system increases in an irreversible process. This is because irreversible processes involve an increase in disorder or randomness within the system.
Free EIT Thermodynamics and Strength of Materials Questions and Answers
A cantilever beam is loaded with a force of 10 KN and has a cross-section as shown.
The maximum tensile stress in MPa in the beam most nearly is: