Temperature

Temperature is the hotness or coldness of a material. It is also the property of a matter that determines where the heat should flow. Heat flows from a point of higher pressure to a point with a lower pressure.

Temperature Scales There are three commonly used temperature scales. They are the Fahrenheit, Celsius, and Kelvin scales.

The Fahrenheit Scale Daniel Fahrenheit was the first to create a scale among the three. His basis was the freezing and boiling point of water. However, at the time he did his experiments, he did not realize the importance of using pure water. He had some impurities like salt in the water he used to define freezing at 0˚F and boiling at 200˚F. Today, this scale is relegated to American weather reports, home thermostats, and fever thermometers.

The Celsius Scale Unlike Fahrenheit, Anders Celsius was able to use pure water in his experiments. He assigned 0˚C to the temperature at which pure water freezes, and 100˚C to the temperature at which pure water boils. This is the most commonly used scale today.

The Kelvin Scale Developed by William Thomson, Lord Kelvin. This scale was introduced with the concept of absolute zero, the lowest possible temperature which is also the zero of his scale.

Absolute zero is the point where the substance has absolutely no energy to give up. Once he has set the baseline, his zero, every change in his scale is the same as the scale used by Celsius. So one number change in Celsius is the same as a number change in Kelvin. They just used a different zero.

Other Temperature Scales There are, in fact, more temperature scales than the other three commonly used ones. These are the Rankine (˚Ra), Rømer (˚Rø), Newton (˚N), Delisle (˚D), and Réaumur (˚Ré) scales. Their uses have died down within the past two centuries.

Temperature Conversions Celsius to Fahrenheit Fahrenheit to Celsius Celsius to KelvinK = ˚C

Thermal Expansion in Solids and Liquids Temperature affects some properties of matter. A change in temperature causes most solids to change in length by an amount proportional to their original lengths as well as the change in its temperature.

For example, if the original length of a rod is Lo, its change in length ΔL after its temperature is affected by ΔT is: Change in length = α (original length)(temperature change) ΔL = α Lo ΔT The quantity α is called the coefficient of linear expansion, a constant whose value depends on the nature of the rod.

Thermal Expansion in Gases Just like in solids and liquids, some properties of gases are affected by temperature.

Charles’ Law Kinetic Theory states that the speed of motion of the molecules in a substance decreases as the substance’s temperature decreases. Also, the speed of motion of the molecules in a substance increases as the substance’s temperature increases. If the pressure on a gas is kept constant, adding heat to the gas will cause it to expand or increase its volume. This leads to Charles’ Law for gases.

Boyle’s Law This law states that if temperature is held constant, the volume of the gas will vary inversely with its pressure. Mathematically:P 1 V 1 = P 2 V 2

General Gas Law

Effects of Thermal Expansion

Thermometers Thermometers depend upon the expansion (volume increase) of liquids as they are heated and the contraction (volume decrease) as they are cooled. Most materials expand as heat is added and temperature increases. As the material’s temperature increases, the molecules or atoms in the material jiggle faster and tend to move farther apart, on average. Nowadays, these kinds of thermometers are illegal as they use mercury, which is poisonous.

Construction As with the earlier example, strips of concrete and asphalt expand when heated. This is why when roads are constructed, the concrete must be poured in sections with small gaps in between, so that as things heat up there is room for the concrete to expand without cracking or breaking.