9.1. INTRODUCTION

In this chapter, we shall study some common physical properties of liquids and gases. Liquids and gases can flow and are therefore called fluids. It is this property that distinguishes liquids and gases from solids in a basic way.

Fluids are defined as substances that can flow, which allows them to take the shape of their containers. This characteristic of flowing is what fundamentally sets fluids apart from solids, which retain a definite shape. Liquids, such as water, and gases, like air, can adapt to the shapes of the vessels they occupy due to their ability to move freely.

Think about how when you pour water into a glass, it conforms perfectly to the shape of the glass. In contrast, if you try to deform a solid object, like a spoon or a rock, you'll notice that it maintains its shape. This quality of liquids and gases is what we refer to when we say they are 'fluids'.

Fluids are everywhere around us. Earth has an envelope of air and two-thirds of its surface is covered with water. Water is not only necessary for our existence; every mammalian body constitutes mostly of water. All the processes occurring in living beings, including plants, are mediated by fluids. Thus understanding the behaviour and properties of fluids is important.

Fluids, encompassing both liquids and gases, play crucial roles in our environment and biological systems. For instance, the atmosphere (air) is a gas, which surrounds our planet and is essential for breathing. Similarly, water, a liquid, is vital not just for humans but for all life forms; it facilitates biological processes, nutrient transportation, and various chemical reactions in organisms.

Consider how plants absorb water through their roots. The water moves through the plant tissues (which are mostly liquid), transporting essential nutrients. Without this fluid movement, plants would not be able to grow and thrive, demonstrating the critical role fluids play in life.

How are fluids different from solids? What is common in liquids and gases? Unlike a solid, a fluid has no definite shape of its own. Solids and liquids have a fixed volume, whereas a gas fills the entire volume of its container. We have learnt in the previous chapter that the volume of solids can be changed by stress.

Fluids and solids differ primarily in their shape and volume characteristics. Solids have both a definite shape and a fixed volume, allowing them to resist changes when a stress is applied. In contrast, fluids do not maintain a fixed shape; they change shape based on the container they occupy. Gases are unique in that they expand to fill the entirety of their container, irrespective of the container's shape.

Imagine a balloon filled with air (gas). When you squeeze the balloon, the shape changes to accommodate your grip, unlike a rock, which remains rigid and doesn’t change shape no matter how much you press on it.

The volume of solid, liquid, or gas depends on the stress or pressure acting on it. When we talk about the fixed volume of solid or liquid, we mean its volume under atmospheric pressure. The difference between gases and solids or liquids is that for solids or liquids the change in volume due to change of external pressure is rather small.

The concept of compressibility refers to how much a substance can change its volume when subjected to pressure. Solids and liquids are generally incompressible, meaning their volumes do not change significantly with pressure changes. In contrast, gases can be compressed significantly, leading to much larger volume changes. This property is essential for various applications, including in engines and pneumatic systems.

Think about a sponge (gas) being pressed down into water. The sponge can be compressed and will take up less space. However, if you tried to compress a block of wood (solid), it would not change shape or size noticeably, showing that solids resist compressibility.

Shear stress can change the shape of a solid keeping its volume fixed. The key property of fluids is that they offer very little resistance to shear stress; their shape changes by application of very small shear stress.

The response of materials to shear stress is a fundamental difference between solids and fluids. A solid can change shape when subjected to shear stress without a change in volume. However, fluids experience significant shape changes with minimal applied shear stress, which is why they flow. This characteristic is essential when considering how fluids behave in motion and under different forces.

Imagine trying to push a box across the floor (solid); it requires significant force to change its position. However, if you pour syrup on a surface, it will flow and spread out almost effortlessly, quickly changing shape under just a light touch.

The shearing stress of fluids is about a million times smaller than that of solids.

This statement highlights how much more responsive fluids are to applied forces than solids. When you apply shear stress to a fluid, it can lead to immediate movement and changes in shape, far easier than with solids. This indicates a significant difference in how fluids and solids interact with forces.

Picture mixing paint with water; as you stir (apply shear), the two quickly combine and change form. In contrast, shifting a rock requires a lot of effort, emphasizing the contrast between solid and fluid response to stress.

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