Temperature

Summary

Energy
- The energy due to the coordinated motion (kinetic energy) and average position (potential energy) of a large collection of particles is usually known as its mechanical energy, but is sometimes called its external energy.
- The sum of the energies due to the random motion (kinetic energy) and local position (potential energy) of a large collection of particles is known as its internal energy.
  - Two regions that can exchange internal energy are said to be in thermal contact.
  - The net transfer of internal energy between two regions in thermal equilibrium is zero.
  - Heat is the net transfer of internal energy from one region to another.
Temperature
- Temperature can be defined informally as the measure of a region's "hotness".
  - A region which is "hot" has a higher temperature than one that is "cold".
- Two regions have the same temperature when there is no net exchange of internal energy between them.
  - Heat flows from one region to another due to a difference in temperature. (Heat flows from "hot" to "cold".)
  - No heat flows between two regions with the same temperature.
- The symbol for temperature is T.
A device that can be used to measure temperature is called a thermometer.
- All thermometers measure the value of some thermometric variable that responds to changes in temperature.
- Thermometers can be classified according to the thermometric variable measured.
A temperature scale is built from …
- at least two fixed points (an upper fixed point and a lower fixed point) corresponding to the temperatures of a pair of reproduceable experiments and …
- a fundamental interval or span of numbers between the two fixed points.
The SI unit of temperature is the kelvin [K].
- Symbology
  - In current usage, the kelvin is always written in lowercase letters without a degree symbol [K].
  - In some early Twentieth Century sources it was common to see degree Kelvin [°K], but this is no longer considered acceptable.
- The kelvin is a fundamental unit; that is, it cannot be reduced to any simpler units.
- By definition, the kelvin is 1/273.16 of the thermodynamic temperature of the triple point of water; therefore …
  - the triple point of water is the upper fixed point,
  - absolute zero is the lower fixed point, and
  - 273.16 is the fundamental interval of the kelvin temperature scale.

The degree Celsius [℃] is an acceptable non SI unit.

Symbology
- Use degrees Celsius [℃] for temperatures (T).
- Use Celsius degrees [C°] for temperature intervals (ΔT).
The current definition of the degree Celsius is 1/273.16 of the thermodynamic temperature of the triple point of water like the kelvin, but …
- the triple point of water is assigned the value 0.1℃ and
- absolute zero is assigned the value −273.15 ℃.
The original definition of the degree Celsius is still approximately valid with …
- the normal boiling point of water as the upper fixed point,
- the normal freezing point of water as the lower fixed point, and
- 100 ℃ as the fundamental interval

The degree Celsius and kelvin have the same size, but assign zero to different values.

ΔT	[K]	=	ΔT	[℃]
T	[K]	=	T	[℃]	+ 273.15
T	[℃]	=	T	[K]	− 273.15

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No condition is permanent.