EIT Electrical, Thermodynamics & Fluid Systems 2 — Questions and Answers
Question 1: A 3-phase balanced load draws 30 kW at 0.8 power factor lagging from a 480 V (line-to-line) source. What is the line current?
- 45.1 A (Correct answer)
- 36.1 A
- 52.0 A
- 28.9 A
Correct answer: 45.1 A
P = √3 × V_L × I_L × PF → I_L = 30000 / (√3 × 480 × 0.8) ≈ 45.1 A.
Question 2: Which thermodynamic cycle best approximates the operation of a gas turbine engine?
- Brayton cycle (Correct answer)
- Rankine cycle
- Otto cycle
- Diesel cycle
Correct answer: Brayton cycle
Gas turbines operate on the Brayton cycle, which consists of isentropic compression, constant-pressure heat addition, and isentropic expansion.
Question 3: For fully developed laminar flow in a circular pipe, the friction factor f is related to Reynolds number Re by:
- f = 64/Re (Correct answer)
- f = 0.316/Re^0.25
- f = 0.184/Re^0.2
- f = 32/Re
Correct answer: f = 64/Re
The Hagen-Poiseuille result gives the Darcy friction factor f = 64/Re for laminar pipe flow.
Question 4: A capacitor is charged to 100 V and then connected in series with a 10 kΩ resistor. After one time constant, the voltage across the capacitor is approximately:
- 36.8 V (Correct answer)
- 63.2 V
- 50.0 V
- 13.5 V
Correct answer: 36.8 V
For a discharging RC circuit, V(τ) = V₀ e^(−1) ≈ 0.368 × 100 V = 36.8 V.
Question 5: Steam enters an adiabatic turbine at 3 MPa and 350°C and exits at 100 kPa. If the isentropic efficiency is 85%, the actual work output compared to ideal is:
- 85% of the isentropic work (Correct answer)
- 100% of the isentropic work
- 115% of the isentropic work
- 70% of the isentropic work
Correct answer: 85% of the isentropic work
Isentropic (turbine) efficiency η_t = w_actual / w_isentropic, so actual work = 0.85 × ideal work.
Question 6: Bernoulli's equation is strictly valid for flow that is:
- Steady, incompressible, and inviscid along a streamline (Correct answer)
- Turbulent and compressible
- Viscous and unsteady
- Rotating and compressible
Correct answer: Steady, incompressible, and inviscid along a streamline
Bernoulli's equation assumes steady, inviscid, incompressible flow along a single streamline.
Question 7: In a DC circuit, two resistors R₁ = 4 Ω and R₂ = 12 Ω are connected in parallel across a 24 V source. What is the total current drawn from the source?
- 8 A (Correct answer)
- 2 A
- 6 A
- 4 A
Correct answer: 8 A
I₁ = 24/4 = 6 A, I₂ = 24/12 = 2 A, total I = 6 + 2 = 8 A.
EIT Electrical, Thermodynamics & Fluid Systems 2
A 3-phase balanced load draws 30 kW at 0.8 power factor lagging from a 480 V (line-to-line) source.
What is the line current?