18.0 - Introduction
Thermodynamics is the study of heat (“thermo”) and
the movement of that heat (“dynamics”) between
objects. A kitchen provides an informal laboratory for
the study of thermodynamics. Manufacturers offer
numerous kitchen devices designed to facilitate the
flow of heat: stovetops and ovens, convection ovens,
toasters, refrigerators, and more. Heat flow changes
the temperature of what is being cooked or cooled,
and that can be monitored with thermometers.
This chapter starts with a few basic thermodynamics
concepts, namely how temperature is measured,
what temperature scales are, and what is meant by
heat. It then begins the discussion of the relationship
between heat and temperature.
Kitchen appliances are engineered using the principles of thermodynamics.18.1 - Temperature and thermometers
Although temperature is an everyday word, like energy it is surprisingly hard to define.
For now, we ask that you continue to think of temperature simply as something
measured by a thermometer. Warmer objects have higher temperatures than cooler
objects.
Traditional thermometers rely on the important principle that any two objects placed in
contact with each other will reach a common temperature. For instance, when a
traditional fever thermometer is placed under your tongue, after a few minutes the flow
of heat causes it to reach the same temperature as your body.
While there are many different types of thermometers available, they all rely on some
physical property of materials in order to measure temperature. In the “old” days, body
temperature was measured with a glass thermometer filled with mercury, a material that
expands significantly with temperature and whose expansion is proportional to the
change in temperature.
Today, a wide variety of physical properties are used to determine temperature. Some
medical clinics use thermometers that measure temperature with plastic sheets
containing a chemical that changes color with temperature. Battery-powered digital thermometers rely on the fact that a resistor’s resistance
changes with temperature. Ear thermometers use thermopiles that are sensitive to subtle changes in the infrared radiation emitted by your
body; this radiation changes with your temperature.
Thermometers
Measure temperature based on physical
properties18.2 - Temperature scales
In the United States, the Fahrenheit system is the most common measurement system
for temperature. The units in this system are called degrees. In most of the rest of the
world, however, temperatures are measured in degrees Celsius. Physicists use the
Celsius scale or, quite often, another scale called the Kelvin scale. All three scales are
shown on the right.
There are two things required to construct a temperature scale. One is a reference
point, such as the temperature at which water freezes at standard atmospheric
pressure. As shown to the right, the three scales have different values at this reference
point. Water freezes at 273.15 kelvins (273.15 K), 0° Celsius (0°C) and 32°
Fahrenheit (32°F). Notice that the standard terminology for the Kelvin scale avoids the
use of “degrees.” Water freezes at 273.15 kelvins, not 273.15 degrees Kelvin.
The other requirement is to pick another reference point, such as the temperature at
which water boils at standard atmospheric pressure, and establish the number of
degrees between these two points. This determines the magnitude of the units of the
scale. The Celsius and Kelvin scales both have 100 units between the freezing and
boiling points of water. This means that their units are equal: a change of 1 C° equals a
change of 1 K. (Changes in Celsius temperatures are indicated with C° instead of °C.)
In contrast, there are 180 degrees between these temperatures in the Fahrenheit
system.
Temperature scales
Kelvin, Celsius, and Fahrenheit
Water freezes at 0°C
Absolute zero is 0 K
Unit of Celsius = unit of Kelvin