Temperature

From Thermal-FluidsPedia

The word “temperature” seems so familiar to most of us that we often take it for granted. While our built-in senses do provide us with a qualitative characterization of temperature, our senses can often be unreliable and misleading. On a cold winter day, for example, an iron railing seems much colder to the touch than a wooden fence post, yet both are the same temperature. This “deception” by our senses arises because iron conducts heat away from our fingers much more readily than wood does. Similarly, if you place one hand in a bowl of warm water and one hand in a bowl of ice water, then remove both hands from the bowls, the hand from the hot water feels cold, while the hand from the cold water feels warm, though both hands are really feeling the same air temperature.

Until Scottish scientist Joseph Black (1728-1799), a founder of modern chemistry and a pioneer in the study of heat, no one established a distinction between heat and temperature. Black distinguished between the quantity (caloric) and the intensity (temperature) of heat.

1 Temperature Scales
2 Thermal Properties
3 References
4 Further Reading
5 External Links

Temperature Scales

Thermometers are commonly used to measure temperatures. To be useful, thermometers have to be calibrated—which may be done by measuring the rise of a column of fluid (such as water, alcohol, or mercury) as it expands when exposed to an environment. Referring to the fluid analogy, temperature as a measure of coldness or hotness of an object could easily be interpreted as the “height” of the caloric fluid within the thermometer.

Using two reference points makes it possible to divide the distance between them into a number of equal intervals. The first thermometers were divided into 360 parts, like degrees of a circle (thus the term “degree”). In 1708, Gabriel Fahrenheit used spirits as the fluid and a mixture of ice, water, and salt, the lowest attainable temperature in a laboratory setting at the time as the zero mark. The second reference point he used was the temperature of the human body, which was assigned the value of 96°. The choice of 96° is believed to be due to the fact that it is divisible by 2, 3, 4, 8, 12, 16, and 32. Today, the Fahrenheit scale uses the freezing and boiling points of water as 32° and 212°. According to this scale, body temperature is 98.6°F.

Swedish astronomer Anders Celsius, who assigned the numbers 0 and 100 to the freezing and boiling points of water, used what is known as the centigrade scale. In 1948, the International Committee on Weights and Measures adopted this scale as the temperature standard, calling it Celsius in honor of its inventor. As we will see when we introduce the laws of thermodynamics, there is a lower limit to the temperature that can be reached. This temperature, called the Zero Absolute Temperature (-273.15° Celsius or zero kelvin), is understood to be the temperature of a perfect crystalline lattice. There is no upper boundary to temperature, although the highest predicted temperature is 10³⁸ K which was the suspected temperature at the very beginning of the creation of the universe, the Big Bang. Sustained fusion reactions require a temperature of 10⁸ K, and thermal plasmas occur at temperatures of 10⁴ to 10⁶ K. Table 1 gives formulas for converting temperatures between Celsius, Fahrenheit, and Kelvin scales.

Table 1: Temperature Conversion Forumlas

Thermal Properties

Different substances have different abilities to retain or transport heat. For example, some materials, like Styrofoam and cork, are better insulators than aluminum, and some materials, like sand and brass, warm more quickly than grass. Thermal properties of materials are best understood by three quantities: heat capacity, specific heat, and heat conductivity.

The heat capacity of a substance measures its ability to retain heat. It is defined as the quantity of heat required to raise a substance by one degree in temperature. Water has a higher heat capacity than sand because it takes more heat to warm up by the same amount than sand does. In other words, for the same amount of heat, sand temperature raises more than water temperature. Closely related to heat capacity is specific heat, which is the heat capacity per gram of substance. Thus, a bigger object will have a greater heat capacity, but the same specific heat. The more loosely the components of a solid are held together, the higher the substance’s specific heat. Graphite has a higher heat capacity than diamond because diamond’s lattice structure is more tightly bound than the graphitic structure of carbon. In SI units, specific heat is expressed in kJ/kg.K.

Question: Which do you think has the higher specific heat, lead or water?

Answer: Lead atoms are over three times heavier than water molecules. Thus, a given quantity (by mass) of water has more molecules than the same quantity (by mass) of lead. Consequently, a given quantity of heat is distributed among fewer lead atoms than water molecules. This means that lead will experience a greater temperature increase and thus has a lower specific heat.

Question: Licking a silver spoon that’s been sitting in a very hot cup of coffee probably won’t burn your tongue, but a spoonful of the same hot coffee dropped on your tongue could leave a blister. Why?

Answer: The heat capacities of water and silver are 1 and 0.06 cal/g.°C, respectively. That is, silver contains much less heat energy than water at a given temperature.

Thermal conductivity is a measure of a body’s ability to conduct heat. Table 2 gives values of thermal conductivity for several materials in order of their conductivity. As can be seen from this table, silver is a very good conductor of heat, with copper not too far behind. Air appears towards the end of the list, meaning it is a very poor conductor, but an effective insulator.