Why this topic is important for ECET
Fluids are the lifeline of civil engineering – from water supply systems, dams, canals, and pipelines to irrigation and hydrology. ECET 2026 Civil frequently tests fundamentals like density, viscosity, pressure head, Pascal’s law, and manometers. These are scoring topics, because once formulas are clear, answers come quickly.
📘 Concept Notes
1. Properties of Fluids
- Density (ρ): Mass per unit volume.
Specific Weight (γ): Weight per unit volume.
Specific Gravity (SG): Ratio of density of fluid to density of water.
Viscosity (μ): Resistance to flow.
Shear stress in a Newtonian fluid:
Kinematic Viscosity (ν):
Compressibility (β): Reciprocal of bulk modulus.
2. Pressure in Fluids
- Pressure: Force per unit area.
Hydrostatic Pressure: At depth h in a liquid:
Pressure Head: Height of liquid column equivalent to given pressure.
3. Pascal’s Law
“Pressure applied at any point in a confined fluid is transmitted equally in all directions.”
This is the principle behind hydraulic jacks, lifts, and brakes.
4. Pressure Measurement Devices
- Piezometer: Measures pressure head directly in terms of liquid column rise.
Simple but limited to moderate pressures. - Manometer: Uses liquid columns to measure pressure differences.
- Simple U-tube manometer – compares pressure with atmosphere.
- Differential manometer – compares pressures at two points.
Formula:
Bourdon Gauge: Mechanical device, widely used in industry.
Example:
Water pressure at 5 m depth:
⚙️ Formulas
🔟 10 MCQs
Q1. The SI unit of viscosity is:
a) Ns/m²
b) kg/m²
c) N/m²
d) Pa/s
Q2. A liquid of SG = 1.2 has density = ? (water density = 1000 kg/m³)
a) 800 kg/m³
b) 1200 kg/m³
c) 1000 kg/m³
d) 1.2 kg/m³
Q3. Hydrostatic pressure at 10 m depth in water is:
a) 9.81 kPa
b) 49.05 kPa
c) 98.1 kPa
d) 100 kPa
Q4. Kinematic viscosity is:
a) μ × ρ
b) μ / ρ
c) ρ / μ
d) 1 / μ
Q5. Which law forms the basis of hydraulic machines?
a) Boyle’s law
b) Charles law
c) Pascal’s law
d) Bernoulli’s law
Q6. In a U-tube manometer containing mercury (ρ = 13,600 kg/m³) connected across water pipe, difference in mercury level = 0.2 m. Pressure difference = ?
a) 1.962 kPa
b) 19.62 kPa
c) 2.672 kPa
d) 26.72 kPa
Q7. Specific weight of water is approx:
a) 9.81 N/m³
b) 981 N/m³
c) 9810 N/m³
d) 1000 N/m³
Q8. A piezometer cannot measure:
a) Low pressures
b) High pressures
c) Negative pressures
d) Atmospheric pressures
Q9. Pressure head equivalent of 20 kPa in water is:
a) 2.04 m
b) 4.08 m
c) 20.4 m
d) 200 m
Q10. Differential manometer measures:
a) Absolute pressure
b) Gauge pressure
c) Difference of pressure between two points
d) Vacuum
✅ Answer Key
| Q | Answer |
|---|---|
| 1 | d |
| 2 | b |
| 3 | c |
| 4 | b |
| 5 | c |
| 6 | b |
| 7 | c |
| 8 | c |
| 9 | b |
| 10 | c |
🧠 Explanations
- Q1: SI unit of dynamic viscosity is Pa·s (same as Ns/m²) → (d).
- Q2: SG = 1.2 → ρ = 1.2 × 1000 = 1200 kg/m³ → (b).
- Q3: p = ρgh = 1000×9.81×10 = 98.1 kPa → (c).
- Q4: ν = μ/ρ → (b).
- Q5: Pascal’s law governs hydraulic lifts → (c).
- Q6: Δp = (ρm – ρw)gh = (13600-1000)×9.81×0.2 ≈ 19.62 kPa → (b).
- Q7: γ = 1000×9.81 = 9810 N/m³ → (c).
- Q8: Piezometer cannot measure negative pressures (vacuum) → (c).
- Q9: hp = p/γ = 20000/9810 ≈ 2.04 m → (b).
- Q10: Differential manometer → pressure difference → (c).
🎯 Motivation / Why Practice Matters
Hydraulics is core Civil Engineering – examiners love asking pressure measurement and fluid properties because they are direct formula-based.
👉 In ECET 2026, speed matters. Solving such MCQs quickly gives you confidence and time for tougher questions.
👉 Consistent practice builds formula recall and reduces silly mistakes in units (Pa, kPa, m head).
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