Temperature and Heat Study Guide
In this lesson, we will shift our attention from forces to energy, and we will learn about heat as a form of energy, characterized by temperature We will also discuss the various types of heat transfer between objects, and tha effect of heat transfer on solids, that is, thermal expansion.
Temperature and Temperature Scales
Compared to simpler quantities such as distance and mass, temperature is more difficult to explain because it is more abstract. It's not as visible or tangible a measurement as is mass. All things constant, the larger the object, the more the mass, or volume; that is, the larger the size of the cube, the greater the volume. Temperature, on the other hand, characterizes a state of the matter, and a thermometer placed in contact with an object at different times can show different values although the object itself remains the same. Temperature measures the motion of the particles in the matter, and we will learn about this dependence later in the book. An increase in the speed of motion produces an increase in the temperature.
Historically, only a few temperature scales have been defined and adopted, the most common ones being the Celsius, or centigrade scale, and the Fahrenheit scale. Both of these scales depend on some reference points. The Celsius scale defines 0° C as the temperature where water freezes at normal atmospheric pressure, and defines 100°C as the boiling point of water. One one-hundredth of this interval is called a Celsius degree. As you can see, the scale was defined taking into consideration a specific substance—water and its properties at two standard points (0 and 100).
The Fahrenheit scale considers the same specific substance, but the standard points were assigned at the values of 32 (freezing) and 212 (boiling). So, not only does the Fahrenheit scale have an offset with respect to Celsius, it's not a centigrade (or l00-grade) scale (212 – 32 = 180). Therefore, the conversion between the two is not through a conversion factor but is a linear dependence.
If we call the Celsius temperature t(°C) and the Fahrenheit temperature t(°F), then, as we said, there is an offset between the two and a difference in the degrees. The Celsius degree is larger than the Fahrenheit degree, for example, 100 degrees Celsius equals 180 degrees Fahrenheit.
In order to eliminate the dependence on the specific substance and standard points, a more universal scale is needed. The result of that search is the absolute Kelvin scale, and its definition is based on empirical observation of the temperature dependence of the pressure of a gas at a constant volume. By lowering the temperature of a gas, the pressure is shown to decrease linearly. The pressure cannot be measured for very low temperatures where the gas liquefies, but if we extrapolate the data, we can obtain the point where the pressure becomes zero.
This point is called the zero absolute temperature, and the scale defined is Kelvin temperature scale. We can think about the Kelvin scale as an offset of the Celsius scale by –273.15 degrees; hence, the conversion between the two scales is stated simply as:
T(K) = t(°C) + 273.15
When we talk about the state of an object, we say the temperature is so many degrees Celsius (°C) or Fahrenheit (°F). When we talk about a process, a process whereby temperature increases or decreases by a value, then one says the temperature changed by so many Celsius or Fahrenheit degrees.
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