## What is the meaning of fluid?

The term fluid represents a type of substance that can easily change its shape under any external pressure/force and can flow. Fluids can acquire any shape in which it is stored and can resist normal stresses but cannot resist shear stress in rest condition. Whenever external shear stress is applied to fluids, it starts to flow. The fluids may be liquid or gas. Example- water, air, etc.

## What is the meaning of property?

The term property of a substance represents the characteristics, state, condition, and quality of the substance. Every substance has different characteristics and qualities. The identification/selection of material for any purpose would be done with the help of the material's properties. All the material in any state is defined with the help of its properties. Generally, the properties of the fluid can be classified into two types, intensive and extensive properties.

### Intensive properties

The properties of fluids (liquid/gas), which do not vary with respect to mass refer to the intensive properties of fluids. It means that the properties are independent of the mass of fluids. In other words, if we take a specific quantity of fluid and compare its properties with different quantity of the same fluid, then the properties should be the same (no changes). Example- temperature, pressure, density, etc.

### Extensive properties

The properties of fluids (liquid/gas), which vary with respect to mass, are dependent on the mass of fluids and are referred to as the extensive properties of fluids. In other words, if we take a specific quantity of fluid and compare its properties with another quantity of the same fluid, then the properties would not have the same value. Example- volume, energy, enthalpy, entropy, etc.

## Types of properties of fluids

The properties of fluids can be broadly classified into physical, kinematic, and thermodynamic properties. The basic details of these types of properties are given below.

### Physical properties

The term physical properties of fluids help to understand the physical state of the material of the fluid. The properties that can be physically seen, smell, taste, and feel, are physical properties. Example- color, odor, etc.

### Kinematic properties

The term kinematic properties of fluids help to understand the motion of the fluids' material. These properties can help to categorize into different categories like high velocity, medium velocity, and low velocity fluids. Example- velocity, acceleration, etc.

### Thermodynamic properties

The term thermodynamic properties of fluids help to understand the thermodynamic state of the fluids material. The thermodynamic state can be defined with the help of thermodynamic properties of fluids like pressure, temperature, specific energy, enthalpy, etc.

### Mass density

It is an intensive property of fluids that can be defined by its mass and volume ratio. The value of gas density depends on the value of pressure and temperature, whereas the liquid's density remains constant. The mathematical expression of density can be represented as,

$\rho =\frac{m}{V}$

Here, $\rho $ represents the mass density, $m$ represents the mass and $V$ represents the volume of fluid.

### Specific weight

It is also an intensive property of fluids that can be defined by its weight and volume ratio. The value of the specific weight of a fluid depends on the value of gravitational acceleration. The mathematical expression of specific weight can represent as,

$w=\frac{W}{V}$

Here, $w$ represents the specific weight, $W$ represents the weight of the fluid, and $V$ represents the fluid's volume.

### Specific gravity

The term specific gravity is a fluid property that relates the density of fluid with respect to the density of the standard fluid. For liquids, water is considered as a standard fluid, whereas, for gas, air is considered as a standard fluid. The mathematical expression of specific gravity of fluid can be represented as,

$S=\frac{{\rho}_{f}}{{\rho}_{s}}$

Here, $S$ represents the specific gravity of fluids, ${\rho}_{f}$ represents the density of a fluid, and ${\rho}_{s}$ represents the density of the standard fluid.

### Viscosity

The term viscosity represents a fluid's property due to the intermolecular force/molecular momentum transfer. Due to inter-molecular cohesive forces, there would be internal resistance to motion between layers of fluids. The internal resistance refers to viscosity. Inviscid fluids have zero resistance, and if the value of viscosity is low, then the flow of fluids would be easy. Viscosity is of two types, dynamic and kinematic viscosity.

The mathematical expression for Newtonian fluids can be represented as,

$\mu =\frac{\tau}{{\displaystyle \frac{du}{dy}}}$

Here, $\mu $ represents the dynamic viscosity, $\tau $ represents the shear stress, and $\frac{du}{dy}$ represents the rate of shear deformation.

### Temperature

The term temperature is a thermodynamic property of fluids that indicates the fluid material's hotness and coldness. It is generally measured in degrees Celsius, Kelvin, and Fahrenheit scale. The effects of temperature on fluids represent in the below figure.

### Specific volume

The term specific volume of fluid can be defined as the reciprocal of density of the fluid. The mathematical expression of the specific volume of the fluid can be represented as,

$v=\frac{V}{m}$

Here, $v$ represents the specific volume, $V$ represents the volume of fluid and $m$ represents the mass of fluid.

### Fluids pressure

The term fluid pressure is a fluid property that can be defined as the ratio of total force applied by a liquid fluid on a specific surface area. The mathematical expression of pressure can be represented as,

$P=\frac{F}{A}$

Here, $P$ represents the pressure, $F$ represents the total force on the surface and $A$ represents the area of surface.

Whenever a specific liquid is contained in a container up to a specific height, then the mathematical expression of liquid pressure on the bottom surface of the container is given by,

$P=\rho gh$

Here, $P$ represents the liquid pressure, $\rho $ represents the density of liquid, $g$ represents the gravitational acceleration and $h$ represents the height of fluid in container.

### Vapor pressure

The value of pressure applied by the vapor phase of a liquid on its free surface refers to vapor pressure. The value of vapor pressure depends on the temperature. Whenever the value of liquid pressure of a liquid drops below its vapor pressure, evaporation of liquid starts.

### Surface tension

The term surface tension represents a tendency of a liquid molecule to contract its surface area to a minimum value. It is due to cohesive forces between molecules of the liquid at the interface of two fluids. The surface tension value can be obtained by taking the surface force ratio on a liquid molecule at the interface and the length of the surface.

### Capillarity

Whenever a glass tube of a smaller diameter is inserted in a specific liquid, there would be a rise or fall of liquid in the tube. This phenomenon is referred to as capillarity, and it varies with the value of surface tension, tube diameter. Due to the cohesive and adhesive force in a liquid, the phenomenon of capillarity exists.

### Compressibility

The term compressibility is a fluid property that represents the relative change in volume of a fluid with respect to the relative change in pressure of the fluid. In other words, it represents how much a liquid can be compressed by a specific pressure change. The expression of the coefficient of compressibility is given as,

$\beta =-\frac{1}{V}\frac{dV}{dP}$

Here, $\beta $ represents the coefficient of compressibility, $V$ represents the volume of fluid, $dV$ represents the volume of fluid change and $dP$ represents the change in pressure of fluid.

### Expansivity

The term expansivity is a fluid property that represents the relative change in volume of a fluid with respect to the change in temperature of the fluid. In other words, it describes how much a liquid expands by changing the fluid's temperature. The expression of the coefficient of expansivity is given as,

$\alpha =\frac{1}{V}\frac{dV}{dT}$

Here, $\alpha $ represents the coefficient of expansivity, $V$ represents the volume of fluid, $dV$ represents the change in volume of the fluid, and $dT$ represents the change in temperature of the fluid.

### Specific heat

The term specific heat of liquid represents the amount of required heat that can increase the temperature of the liquid by one-degree centigrade. It is of two types for gas fluids: specific heat at constant pressure and specific heat at constant volume. The value of specific heat helps to obtain the amount of heat required by a particular amount of fluid to change its temperature by a particular amount.

The specific heat for liquid is represented by the symbol $C$. Specific heat for gas at constant volume is ${C}_{v}$ and at constant pressure is ${C}_{p}$.

### Internal energy

The internal energy of fluid represents the amount of energy of fluid due to its molecular activity. It is a microscopic form of energy and represents the sum of kinetic energy and potential energy of fluid molecules. Whenever the fluid temperature starts increasing, the kinetic energy of each molecule of fluid starts increasing and results in an increase in internal energy.

## Common Mistakes

- Students sometimes get confused about whether gases come under the category of fluids or not. However, liquids and gases both come under the category of fluids.
- Sometimes, students also get confused about the difference between the adhesive and cohesive force. However, cohesive force represents a force that exists between the same fluid molecules whereas, adhesive force represents a force that exists between different types of fluid molecules.
- The student also gets confused about the difference between the liquid pressure of a static liquid and vapor pressure. However, liquid pressure represents the pressure exerted by the liquid on a surface whereas, the pressure exerted by the vapor phase of a specific liquid (above the free surface of the liquid) on the free surface of the liquid represents the vapor phase.

## Contexts and Applications

Properties of fluids are very significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

- Bachelor of Technology in Mechanical Engineering
- Bachelor of Technology in Civil Engineering
- Master of Technology in Mechanical and Civil Engineering
- Doctor of Philosophy in Mechanical Engineering

## Related Concepts

- Shear stress in fluids
- Thermal conductivity of fluids
- Modulus of elasticity of fluids
- Continuum
- Absolute pressure of fluids
- Normal pressure of static fluids
- Shear strain
- Cavitation
- Vacuum pressure

## Practice Problems

**Q1**. Which of the following option is correct regarding specific volume of fluids which is inverse of :

a. Mass density

b. Weight density

c. Volume

d. Mass

Correct option:** (a)**

**Explanation:** The relation between specific volume and mass density is a linear one. The mass density of the fluid can obtain by taking the ratio of the mass of fluid and its volume. So, the specific volume would be the ratio of fluid volume and its mass.

**Q2**. The specific gravity of fluids can be measured by :

a. Hygrometer

b. Hydrometer

c. Viscometer

d. None of these

Correct option:** (b)**

**Explanation:** The measurement of a fluid property as specific gravity can be done with the help of a hydrometer, whereas the viscometer helps measure the viscosity of the fluid.

**Q3**. Which of the following option is correct if cohesive force is higher than adhesive force?

a. Capillary rise

b. Capillary fall

c. No capillary action

d. None of these

Correct option:** (b)**

**Explanation:** The decision of capillary rise and fall decide by the relation between cohesive and adhesive force. Cohesive force means the attraction force between the same types of molecules.

**Q4**. Which of the following option is correct if adhesive force is higher than cohesive force?

a. Capillary rise

b. Capillary fall

c. No capillary action

d. None of these

Correct option:** (a)**

**Explanation:** Adhesive force means the force that acts between different types of molecules. If the adhesive force is more in comparison to cohesive force, then capillary rise takes place.

**Q5**. Which of the following option is correct regarding the concept of surface tension?

a. Due to density of fluid

b. Due to pressure

c. Due to adhesive force

d. Due to cohesive forces

Correct option:** (d)**

**Explanation:**The concept of surface tension of a specific fluid is associated with the cohesive force that works between the same fluid elements.

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