--- title: "Definitions of basic fluid properties" subject: "Hydro Power" chapter: 1 key: "semester_6/hydro_power:fluid-property-definitions" source: https://www.bctengineer.com/q/semester_6/hydro_power/fluid-property-definitions site: BCT Engineering --- # Definitions of basic fluid properties Hydro Power · Chapter 1: Introduction to Basic Fluid Mechanics and Hydraulics - Asked in: 2081 Chaitra, 4 marks; 2078 Chaitra, 4 marks; 2074 Bhadra, 2 marks (asked 3 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/hydro_power/fluid-property-definitions - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-1 ## Question Define the terms specific weight, specific gravity, specific mass, specific volume, viscosity, surface tension, capillarity and compressibility. ## Short answer Specific weight is weight per unit volume (γ = ρg). Specific gravity is density ratio to water. Specific mass is mass per unit volume. Specific volume is volume per unit mass. Viscosity resists shear (τ = μ du/dy). Surface tension is force per length on a surface. Capillarity is liquid rise/fall in a tube (h = 4σ cosθ/(ρ g d)). Compressibility is fractional volume change per pressure change. ## Answer These are the basic physical properties used to describe a fluid. - **Specific weight (weight density), $w$ or $\gamma$:** weight per unit volume, $\gamma = \rho g$. Unit N/m³. Water: 9810 N/m³. - **Specific gravity, $S$:** ratio of the density (or specific weight) of a fluid to that of a standard fluid (water at 4 °C for liquids, air for gases). $S = \rho/\rho_w$; no unit. Mercury: 13.6. - **Specific mass (mass density), $\rho$:** mass per unit volume, $\rho = m/V$. Unit kg/m³. Water: 1000 kg/m³. - **Specific volume, $v$:** volume per unit mass, $v = 1/\rho$. Unit m³/kg. Mostly used for gases. - **Viscosity:** the property by which a fluid resists the relative motion (shear) between its layers. By Newton's law, $\tau = \mu \dfrac{du}{dy}$, where $\mu$ is the dynamic viscosity (N·s/m² or Pa·s). Kinematic viscosity $\nu = \mu/\rho$ (m²/s). - **Surface tension, $\sigma$:** the tensile force per unit length acting on the free surface of a liquid, caused by cohesion between liquid molecules. Unit N/m. Water–air at 20 °C: about 0.073 N/m. - **Capillarity:** rise or fall of a liquid in a thin tube due to surface tension, adhesion and cohesion. Rise $h = \dfrac{4\sigma \cos\theta}{\rho g d}$. Water rises (adhesion > cohesion); mercury falls. - **Compressibility:** the reciprocal of bulk modulus; it measures the fractional change in volume per unit change of pressure, $\beta = \dfrac{1}{K} = -\dfrac{dV/V}{dp}$. Unit m²/N. Liquids have very low compressibility, so they are treated as incompressible. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the relation between specific weight (gamma) and specific mass (rho)? A. gamma = rho / g B. gamma = rho g, in N/m^3 C. gamma = 1/rho, in m^3/kg D. gamma = rho g, in kg/m^3 Answer: B. Specific weight is weight per unit volume, gamma = rho g, with unit N/m^3 (9810 N/m^3 for water). (Key point: Specific weight vs specific mass) 2. Specific volume of a fluid is defined as: A. Weight per unit volume B. Volume per unit mass, v = 1/rho C. Ratio of density to that of water D. Mass per unit volume Answer: B. Specific volume is the reciprocal of density, v = 1/rho, in m^3/kg. (Key point: Specific volume) 3. In tau = mu du/dy, what does mu represent and what is the kinematic viscosity? A. mu is surface tension; nu = mu g B. mu is dynamic viscosity; nu = mu/rho C. mu is dynamic viscosity; nu = mu rho D. mu is bulk modulus; nu = 1/mu Answer: B. mu is dynamic viscosity (Pa.s) and kinematic viscosity is nu = mu/rho in m^2/s. (Key point: Newton's law of viscosity) 4. Why does mercury fall in a thin glass tube while water rises? A. Mercury is denser than water B. For mercury cohesion exceeds adhesion; for water adhesion exceeds cohesion C. Mercury has lower surface tension D. Water is compressible Answer: B. Water wets glass (adhesion > cohesion) and rises; mercury does not wet glass (cohesion > adhesion) and is depressed. 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