Introduction: Heat and Temperature

Heat and Temperature

WBCHSE's Unit 7 "Properties of Bulk Matter" closes its third and final sub-topic by turning from the

mechanical response of solids and fluids (how they deform under stress, and how fluids exert pressure

and flow) to their thermal response -- how a solid, a liquid, or a gas behaves when it is made hotter

or colder. Two words that are often used loosely, and interchangeably, in everyday speech -- "heat" and

"temperature" -- must be sharply distinguished before any of the ideas in this chapter can make sense.

Heat is a form of energy -- specifically, it is energy that is in the process of being transferred

from one body to another (or between a body and its surroundings) purely because of a difference in

temperature between them. Heat is not something a body "contains"; rather, heat is energy in transit.

Once that transfer stops (because the two bodies have reached the same temperature, a state called

thermal equilibrium), it no longer makes sense to speak of "heat" at all -- the energy has simply

become part of each body's internal energy. Being a form of energy, heat is measured, in SI units, in

joules ($\text{J}$); an older but still occasionally used unit is the calorie, where

$1\ \text{cal} = 4.186\ \text{J}$ (approximately).

Temperature, on the other hand, is not energy. It is the physical quantity that tells us how hot or

cold a body is, and -- crucially -- it is temperature, and temperature alone, that decides the direction

in which heat will spontaneously flow when two bodies are placed in contact: heat always flows from the

body at the higher temperature to the body at the lower temperature, never the other way round on its

own. Temperature is measured with a thermometer, on one of several temperature scales (Celsius,

Fahrenheit, or the absolute Kelvin scale -- taken up in the next section).

A simple way to keep the two ideas apart: two cups of water, one large and one small, both boiling at

$100\,^\circ\text{C}$, are at the same temperature but do not contain the same quantity of heat energy

-- the larger cup, having more mass, holds more thermal energy and would take longer to cool down to

room temperature, even though a thermometer dipped into either cup reads exactly the same value at the

moment they are compared.

This chapter builds on that one distinction throughout: thermal expansion (Sections 10.3-10.5)

studies how a body's dimensions change with temperature; specific and latent heat, and calorimetry

(Sections 10.6-10.8) study how much heat must be exchanged to produce a given temperature change or a

given change of state; and heat transfer (Sections 10.9-10.15) studies the three distinct mechanisms

-- conduction, convection, and radiation -- by which heat energy actually moves from a hotter place to a

colder one.