Introduction

Heat and temperature are part of everyday experience: every living thing depends on staying within a narrow temperature range to function at all, and life on Earth as a whole is only possible because the Sun keeps radiating energy at a steady, predictable rate. Because the two words get used almost interchangeably in ordinary speech, it is easy to blur what 'hot,' 'cold,' 'heat' and 'temperature' actually mean in a precise, physical sense. Thermodynamics is the branch of physics that puts this on a rigorous footing -- it studies exactly how heat and temperature behave and interact -- and this chapter's opening goal is simply to pin these terms down carefully: to understand what 'hot' and 'cold' really mean, and to see clearly why heat and temperature, though closely related, are two genuinely different quantities. Getting that distinction right at the very start is what the rest of the chapter, and the rest of thermodynamics, is built on.