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 is one of the seven base units of the International System of Units.
    • The kelvin is now defined by relating the defined value of Boltzmann's constant (k = 1.380649 × 10⁻²³ J/K) to the definitions of the meter, kilogram, and second.
  • The original definition of the kelvin temperature scale is still approximately true with…
    • the triple point of water as the upper fixed point
    • absolute zero as the lower fixed point
    • 273.16 K as the fundamental interval
• 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 degree Celsius and kelvin have the same size, but assign zero to different values.
    • Absolute zero is assigned the exact value −273.15 °C.
  • The original definition of the Celsius temperature scale is still approximately true with…
    • the normal boiling point of water as the upper fixed point
    • the normal freezing point of water as the lower fixed point
    • 100 °C as the fundamental interval