What is Temperature? — Physics

Definition 15.2 Primary & Middle School Physics · Chapter 15 — Temperature and Thermometers

The temperature of a thing is the number that says exactly how hot or cold it is. Higher number, hotter thing; lower number, colder thing. Temperature is measured in degrees Celsius, written ${}^{\circ}\mathrm{C}$ , with a thermometer.

A thermometer: the red liquid climbs when warmed. Here it stands at the mark 25 — the temperature is 25\, C. — A thermometer: the red liquid climbs when warmed. Here it stands at the mark $25$ — the temperature is $25\,{}^{\circ}\mathrm{C}$ .

A real thermometer on its wall: the red column climbs the scale, and anyone who reads the mark gets the same number.

Examples

Example 15.4 (How the glass judge works)

Inside the thermometer’s thin tube sits a trapped colored liquid. Warmth makes that liquid swell just a little — and in a tube so thin, a little swelling pushes the column visibly up. Cold shrinks it back down. The liquid cannot be fooled like a hand: it swells the same for the same warmth, today, tomorrow, for everyone.

Example 15.6 (Temperatures to live by)

Some numbers worth knowing by heart: a pleasant room is about $20\,{}^{\circ}\mathrm{C}$ ; your body keeps itself at about $37\,{}^{\circ}\mathrm{C}$ , in summer and winter alike (much above that is a fever — the doctor’s thermometer knows); the refrigerator holds about $4\,{}^{\circ}\mathrm{C}$ ; a hot summer day reaches $30\,{}^{\circ}\mathrm{C}$ ; bath water is pleasant near $35\,{}^{\circ}\mathrm{C}$ .

Read in context →

Definition 20.4 University Physics — Year 1 · Chapter 20 — Kinetic Theory and the Perfect Gas

Two systems in contact through a wall that lets energy pass reach, in time, a common state: they are then at the same temperature, and two bodies each in equilibrium with a third are in equilibrium with each other (the zeroth law). The kelvin scale is fixed by $k_B = 1.380\,649 \times 10^{-23}\,\mathrm{J}/\mathrm{K}$ exactly; Celsius: $\theta = T - 273.15$ . The kinetic theory below gives $T$ its microscopic meaning.

Read in context →