Mechanical Properties of Fluids

10.1 Introduction

The chapter begins by addressing our intuitive understanding of heat and temperature, which we experience in everyday life. For example, we know that a kettle with boiling water is hotter than a box containing ice. However, physics requires precise definitions of these concepts. Temperature is a measure of the 'hotness' of a body, but it is not an absolute quantity; it is a relative measure that helps us understand heat flow. Heat itself is a form of energy transfer that occurs due to temperature differences between bodies or systems. This chapter explores what heat is, how it is measured, and the processes by which heat flows from one body to another. It also explains practical phenomena such as why blacksmiths heat iron rings before fitting them onto wooden wheels and why winds at the beach change direction after sunset. Additionally, the chapter discusses phase changes like boiling and freezing, noting that temperature remains constant during these processes despite heat flow.

• Temperature is a measure of 'hotness' but is relative, not absolute.
• Heat is energy transferred between systems due to temperature difference.
• Physics requires precise definitions beyond everyday intuition.
• The chapter covers heat measurement and modes of heat transfer.
• Real-world examples illustrate thermal phenomena.
• Phase changes occur at constant temperature despite heat flow.
• 📌 Temperature: A relative measure of the hotness or coldness of a body.
• 📌 Heat: Energy transferred between systems due to temperature difference.

10.2 Temperature and Heat

Temperature is a relative measure indicating how hot or cold an object is. Terms like 'hot' and 'cold' are relative, similar to 'tall' and 'short'. We perceive temperature by touch, but this sense is unreliable and limited for scientific purposes. Heat transfer occurs when two bodies at different temperatures are in contact; heat flows from the hotter to the colder body until thermal equilibrium is reached. For example, ice-cold water left on a hot day warms up as heat flows from the environment to the water, while hot tea cools down as heat flows from the tea to the surroundings. Heat is thus defined as the form of energy transferred between systems or a system and its surroundings due to temperature difference. The SI unit of heat is joule (J), and temperature is measured in Kelvin (K), with degrees Celsius (°C) commonly used. Heating an object can cause temperature rise, expansion, or phase change, which are studied in later sections.

• Temperature indicates relative hotness or coldness.
• Heat flows from higher to lower temperature bodies.
• Heat transfer continues until thermal equilibrium is reached.
• Heat is energy transferred due to temperature difference.
• SI unit of heat is joule (J); temperature unit is Kelvin (K).
• Temperature changes can cause expansion or phase changes.
• 📌 Thermal equilibrium: State when two bodies have the same temperature and no heat flows between them.
• 📌 Heat: Energy transferred due to temperature difference.