Temperature

• We've heard the TV meteorologist give the daily value of the temperature of theatmosphere (15 degrees Celsius, for example). 
• We know that a hot object has a high temperature, and a cold object has a low temperature. And we know that the temperature of an object changes when we heat the object or cool it.
• Scientists, however, must be more precise than simply describing an object as "hot" or "cold." An entire branch of physics, called thermodynamics, is devoted to studying the temperature of objects and the transfer of heat between objects of different temperatures. 
• let you study how temperature varies with height through the atmosphere.

• Turning to the large scale, the temperature of a gas is something that we can determine qualitatively with our senses. We can sense that one gas is hotter than another gas and therefore has a higher temperature. But to determine the temperature quantitatively, to assign a number, we must use some principles from thermodynamics:

1. The first principle is the observation that the temperature of an object can affect some physical property of the object, such as the length of a solid, or the gas pressure in a closed vessel, or the electrical resistance of a wire. You can explore the effects of temperature on the pressure of a gas at the animated gas lab.
2. The second principle is the definition of thermodynamic equilibrium between two objects. Two objects are in thermodynamic equilibrium when they have the same temperature.
3. And the final principle is the observation that if two objects of different temperatures are brought into contact with one another, they will eventually establish a thermodynamic equilibrium. The word "eventually" is important. Insulating materials reach equilibrium after a very long time, while conducting materials reach equilibrium very quickly.

• With these three thermodynamic principles, we can construct a device for measuring temperature, a thermometer, which assigns a number to the temperature of an object. When the thermometer is brought into contact with another object, it quickly establishes a thermodynamic equilibrium. By measuring the thermodynamic effect on some physical property of the thermometer at some fixed conditions, like the boiling point and freezing point of water, we can establish a scale for assigning temperature values.

• The Celsius scale, designated with a C, uses the freezing point of pure water as the zero point and the boiling point as 100 degrees with a linear scale in between these extremes. 
• The Fahrenheit scale, designated with an F, is a lot more confusing. It originally used the freezing point of sea water as the zero point and the freezing point of pure water as 30 degrees, which made the temperature of a healthy person equal to 96 degrees. 
• On this scale, the boiling point of pure water was 212 degrees. So Fahrenheit adjusted the scale to make the boiling point of pure water 212 and the freezing point of pure water 32, which gave 180 degrees between the two reference points. 
• 180 degrees was chosen because it is evenly divisible by 2, 3, 4, 5, and 6. On the new temperature scale, the temperature of a healthy person is 98.6 degrees F. Because there are 100 degrees C and 180 degrees F between the same reference condition:

1 degree C = 1 degree F * 100 / 180 

           = 1 degree F * 5 / 9

• Since the scales start at different zero points, we can convert from the temperature on the Fahrenheit scale (TF) to the temperature on the Celsius scale (TC) by using this equation:

TF = 32 + (9 / 5) * TC

• Of course, you can have temperatures below the freezing point of water and these are assigned negative numbers. When scientists began to study the coldest possible temperature, they determined an absolute zero at which molecular kinetic energy is a minimum (but not strictly zero!). 
• They found this value to be at -273.16 degrees C. Using this point as the new zero point we can define another temperature scale called the absolute temperature. If we keep the size of a single degree to be the same as the Celsius scale, we get a temperature scale which has been named after Lord Kelvin and designated with a K. Then:

K = C + 273.16

• There is a similar absolute temperature corresponding to the Fahrenheit degree. It is named after the scientist Rankine and designated with an R.

R = F + 459.69

• Absolute temperatures are used in the equation of state, the derivation of the state variables enthalpy, and entropy, and determining the speed of sound.