What term is defined as a measure of the average kinetic energy of the particles of a substance

The expression your wrote down for the energy is the expression for the ensemble average kinetic energy of a monatomic ideal gas. Therefore, we see that for this system, the average energy of the system is simply proportional to the temperature.

For a general statistical mechanical system, however, it might not even make sense to talk about the "average kinetic energy" of the system. For example, take a quantum system consisting of a single spin $1/2$ particle interacting with a magnetic field. It is possible to define a temperature for this system when it is in contact with a heat bath even if the particle is not moving around. In such cases, one appeals to more general definitions of temperature such as that to which John Rennie refers in his answer.

The measure of average kinetic energy of molecules is called .​

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So, instead of a full explanation of entropy, I will just give some interesting aspects of it. Thermal equilibrium is not a purely energy phenomena. Energy is conserved when two objects reach thermal equilibrium, but it would also be satisfied if one object got hot and the other one became cold. Thermal equilibrium is a statistical process. It just so happens that the most probable outcome for two objects in contact is that they reach the same temperature. The other weird cases (one getting hot and one getting cold) can also technically happen, but their chances are way less than you winning the lottery (and your chance of winning the lottery is essentially zero).

Since temperature is really a statistical quantity, you can't have a temperature of a single particle. So, the next time someone talks about the temperature of a single electron—or worse, the temperature of a photon—maybe you should just walk away.

Which Temperature Scale Is The Best?

There are quite a few temperature scales, but these are three most common: Celsius, Fahrenheit (which I can never spell correctly), and Kelvin. I know that most of the civilized world uses Celsius, but I just have trouble training my brain to think of temperature in this scale. I'm probably too old to change. Also, I always think of this graphic display of the temperature scales which says that 0 degrees Celsius is cold, but at a temperature of 100 degrees Celsius you would be dead (the temperature of boiling water).

How do you calibrate a temperature scale? The Celsius scale is easy. The zero value is at the freezing point of water and the 100 value is at the boiling point. That's fairly easy to reproduce but these values do depend on atmospheric conditions, so it's not a perfect method to calibrate a thermometer. The Kelvin scale is just like the Celsius scale, but it is shifted by 273.15 such that 0 Kelvin (there are no degrees on the Kelvin scale) is equal to 273.15 degrees Celsius. With the Kelvin scale, you don't get negative temperatures—so that's useful in lots of calculations.

But what about the Fahrenheit scale? I think that everyone will agree that it is based on two measurements: the temperature of a human body (around 98 degrees Fahrenheit) and the temperature of salt and ice (0 °F). Actually, this is something that's interesting. If you mix ice and salt (and a little water), the coldest you can get the mixture is zero. That is surprisingly cold and why you use salt-ice mixture to make homemade ice cream.

Still, there does not seem to be complete agreement as to why the human body temperature measures at 98 °F instead of 100 °F. One idea is that the scale is broken into three parts, each of 32 °s, since 32 is the temperature of freezing water. This wouldn't quite work fitting in the human body temperature at 100 °F, but it would be close. Oh well, I guess we won't know until someone invents a time machine.

What Is So Special About -40°?

If you convert -40°F to Celsius, you get -40°C. But the correct answer to the significance of -40° is that it is the temperature on Hoth. OK, if you look at Wookiepedia (the Star Wars Wikia) it says that Hoth gets down to -60°C at night. So, I'm going to guess that maybe during the day it is -40°C (or °F). Anyway, when the MythBusters tested the thermal properties of a tauntaun they used a temperature of -40—so there.

Now for some math. How do you convert from °F to °C? Since both of these are linear temperature scales, I can find a function for the Celsius temperature as a function of Fahrenheit temperature. To do this, I need two data points to make a line. Good thing I already have them—they are the boiling and melting point of water. This gives two x-y points (except x is Fahrenheit temperature and y is the Celsius temperature) that are (32,0) and (212,100). Now I can use these points to find the slope of the line and the point-slope formula to find the equation of the line. I will skip the details (you can do it at home for fun), but I get the following equation.