The concept of temperature is rooted in qualitative ideas based on our sense of touch. A body that feels “hot” usually has a higher temperature than a similar body that feels “cold.”
Many properties of matter that we can measure i.e. :—
- the length of a metal rod
- steam pressure in a boiler
- the ability of a wire to conduct an electric current
- the color of a very hot glowing object
depend on temperature.
Temperature and kinetic energy
Temperature is also related to the kinetic energies of the molecules of a material. We will discuss the relation between kinetic energy of the molecules of a material and its temperature in Kinetic Theory of Gases.
In this article we’ll develop a macroscopic definition of temperature :-
“It is the quantity by the aid of which the hotness or coldness of a body can be determined.”
To use temperature as a measure of hotness or coldness, we need to construct
a temperature scale. To do this, we can use any measurable property of a system that varies linearly over a wide range with its “hotness” or “coldness.”
For example, the length of mercury column filled in the thermometer, changes linearly with temperature.
When the system becomes hotter, the liquid (usually mercury or ethanol) expands and rises in the tube, and the value of L increases.
Another simple system to measure temperature, is a quantity of gas in a constant volume container. The pressure p, measured by the gauge, increases or decreases as the gas becomes hotter or colder.
A third example is the electrical resistance R of a conductor wire, which changes when the wire is hot or cold (the changes must be linear).
Each of these properties gives us a number (L, p, or R) that changes linearly with warmth and coolness, so each property can be used to make a thermometer.