Introduction

Until this chapter, describing the equilibrium of a mechanical system or the motion of a body only ever needed three fundamental quantities: length, mass and time. Every other mechanical quantity -- velocity, force, energy, and so on -- can be built out of these three. This chapter studies a different family of phenomena, ones related to heat, and these genuinely need a fourth fundamental quantity: temperature.

We all have an everyday, qualitative sense of "hot" and "cold" -- a mother checks her child's forehead for fever by touch, a cook flicks water onto a hot pan to test it. But touch only lets us COMPARE two objects; it cannot give a precise number. To go from a vague sensation to a reproducible, numerical measurement, we need a physical quantity -- temperature -- that captures the "degree of hotness" of a body, and a device -- the thermometer -- to measure it on a fixed, agreed scale.

Phenomena and properties tied to temperature change and heat exchange are called thermal phenomena or thermal properties, and this chapter works through the main ones: how temperature is measured and put on scales, how the volume/pressure of an ideal gas relates to absolute temperature, how bodies expand when heated, how much heat different substances need to warm up by a given amount, how heat is exchanged and measured experimentally (calorimetry), how substances change state (melting, boiling, sublimation) and the latent heat involved, the three mechanisms by which heat moves from place to place (conduction, convection, radiation), and finally how a hot body cools according to Newton's law of cooling. Along the way we will be able to explain things like why sea breezes reverse direction between day and night, why a tight metal lid on a glass jar comes off more easily after running hot water over it, and why two metal vessels stuck together can be separated by heating only the outer one.