Introduction

10.1 Introduction

We all carry an intuitive sense of what "hot" and "cold" mean. A kettle of boiling water feels hot; a block of ice feels cold. Temperature is the physical quantity that measures this "hotness" — it tells us how hot or cold a body is relative to some standard. But common sense alone is not enough for physics. We need precise definitions of heat, temperature, and the processes by which thermal energy moves.

This chapter builds those definitions from the ground up. You will learn what heat actually is (it is not the same as temperature), how it is measured, and the three fundamental ways heat flows from one body to another: conduction, convection, and radiation. Along the way, everyday phenomena will find their explanations — why a blacksmith heats an iron ring before fitting it onto a wooden wheel rim, and why the wind at a beach reverses direction after sunset.

The Central Puzzle: Phase Changes

One of the most striking observations about heat is what happens when water boils or freezes. During boiling, water stays at $100^\circ\text{C}$ even though a great deal of heat is flowing into it. During freezing, water stays at $0^\circ\text{C}$ even though heat is flowing out of it. The temperature does not change during these phase transitions, despite the continuous heat transfer. This puzzle — where does the heat go? — is a key question the chapter will answer.

What This Chapter Covers

The chapter proceeds systematically:

1. Temperature and its measurement — how we define temperature scales (Celsius, Fahrenheit, Kelvin) and the instruments (thermometers) that measure it.
2. Ideal-gas equation and absolute temperature — the link between temperature and the behaviour of gases, leading to the absolute (Kelvin) scale.
3. Thermal expansion — how solids, liquids, and gases change size with temperature, with formulas for linear, area, and volume expansion.
4. Specific heat capacity — the amount of heat required to raise the temperature of a unit mass of a substance by one degree.
5. Calorimetry — the measurement of heat transfer, using the principle of conservation of energy.
6. Change of state — the physics of melting, boiling, and the latent heat involved.
7. Heat transfer — the three modes: conduction (with thermal conductivity), convection, and radiation (including Stefan-Boltzmann law and Newton's law of cooling).

Why These Ideas Matter

The examples are not arbitrary. The blacksmith heats the iron ring so it expands; the hot ring slips easily over the wooden rim. As it cools, it contracts and grips the rim tightly — a perfect mechanical fit achieved by thermal expansion. The beach wind reverses because land and sea have different specific heat capacities. Land heats up and cools down faster than water. During the day, warm air rises over the land, drawing cooler air from the sea (sea breeze). At night, the land cools faster, so the air over the sea is warmer and rises, drawing air from the land (land breeze).

These are not just curiosities. They are direct consequences of the thermal properties we will now study in detail.