The Physics
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# Temperature

## Summary

• Internal 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 mechanical energy, but is sometimes called 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.
• The symbol for internal energy is U.
• The SI unit for internal energy is the joule [J].
• Heat
• 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.
• The symbol for heat is Q (probably from "quantity of heat").
• The SI unit for heat is the joule [J].
• 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 reproducible 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 20th 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 K is the fundamental interval of the kelvin temperature scale.
• The degree Celsius [°C] is an acceptable non SI unit for temperature.
• Symbology
• The reversed phrase Celsius degree [C°] is sometimes used for temperature intervals (ΔT).
• The original name for this unit was the degree centigrade [°C], but this is no longer considered acceptable.
• 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 0.01°C and
• absolute zero is assigned the value −273.15 °C.
• The original definition of the degree Celsius is still 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 °C as the fundamental interval.
• The degree Celsius and kelvin have the same size, but assign zero to different values.
 ΔT [K] = ΔT [°C] T [K] = T [°C] + 273.15 T [°C] = T [K] − 273.15