1. Section 3.5: Temperature

2. Temperature • TemperatureProperty of an object that determines the direction of heat energy (Q) transferto or from other objects. Temperature Scales • Three Common Scales are used to measure temperature: Fahrenheit Scale (°F) Celsius (Centigrade) Scale(°C) Kelvin Scale (K)

3. Temperature Scales • TemperatureProperty of an object that determines the direction of heatenergy (Q) transferto or from other objects. • Three Common Scales are used to measure temperature: Fahrenheit Scale (°F) Used widely in the U.S. Divides the difference between freezing & boiling point of water at sea level into 180 steps. Celsius (Centigrade) Scale(°C) Used almost everywhere else in the world. Divides the freezing to boiling continuum into 100 equal steps. Kelvin Scale (K) Used by scientists. Created by Lord Kelvin. Starts with T = 0 K “Absolute Zero”.

4. 3 Common Scales are used to measure temperature. • However there have also been many other temperature scalesused in the past! Among these are: TheRankine Scale (°Ra).The Réaumur Scale(°Ré). The Newton Scale (°N). The Delisle Scale (°D). The Rømer Scale. (°Rø). Some Conversions:

5. Temperature Scale Comparisons Boiling Point of Water 212°F = 100°C = 373.15 K Melting Point of Ice 32°F = 0°C = 273.15 K “Absolute Zero” -459.67°F = -273.15°C = 0K Average Human Body Temperature: 98.6°F = 37°C = 310.16 K Average Room Temperature: 68°F = 20°C = 293.16 K

6. Common Conversions Celsius to Fahrenheit: F° = (9/5)C° + 32° Fahrenheit to Celsius: C° = (5/9)(F° - 32°)

7. The Kelvin ScaleSometimes Called the Thermodynamic Scale • The Kelvin Scale was created by Lord Kelvin to eliminate the need for negative numbers in temperature calculations. The Kelvin Scale isdefinedas follows: 1.The degree size isidenticalto that on the Celsius scale. 2.The temperature in Kelvin degrees at the triple point of water is DEFINED to be Exactly 273.16 K

8. How is Temperature Measured? • Of course,temperature is measured using a • Thermometer. • Thermometer Any object that has a property characterized by a • Thermometric Parameter • Thermometric ParameterAny parameter X, that varies in a known (calibrated!) way with temperature. Measure the value of X at TWOfixed points of temperature & interpolate & extrapolate as needed.

9. X • X2 • X1 Xm FP1 FP2 T Error! • Thermometric Parameter Any parameter X, that varies in a known (calibrated!) way with temperature. Measure the value of X at TWO fixed points of temperature & interpolate & extrapolate as needed. Two (or more) reference points can result inerrorswhen extrapolating outside of their range!!

10. Ranges of Various Types of Thermometers V P or V n.b.p.  normal boiling point

11. Reference Points for Temperature Scales & Some Brief History. • Daniel Fahrenheit (1724) • Ice, water & ammonium chloride mixture = 0 °F • Human body = 96 °F (now taken as 98.6 °F) • Anders Celsius (1742) • Originally: • Boiling point of water = 0 ºC! • Melting point of ice = 100 ºC! • Scale was later reversed. • Scale was originally called “centigrade”

12. Pt Resistance Thermometer For 0 ºC < t < 850 ºC Blundell and Blundell, Concepts in Thermal Physics (2006)

13. - + Unknown Temperature(hot junction) V Thermoelectric or Seebeck Effect Principle of Thermocouples Fixed Temperature(cold junction)

14. Metals used for Thermocouples Each type requires calibration against fixed points. Reports on Progress in Physics, vol. 68 (2005) pp. 1043–1094

15. Spectral Distribution of Thermal Radiation Planck Distribution Law Radiation Energy Density Infrared UV-Visible

16. Fixed Temperature Reference Points Melting points of metals and alloys Reports on Progress in Physics, vol. 68 (2005) pp. 1043–1094

17. Temperature Scale with a Single Fixed Point • Defining a temperature scale with a single fixed point requires a linear (monotonic) relationship between a • Thermometric Parameter X • & the • Temperature Tx: • c is a constant • By international agreement* in 1954, • The Kelvin or Thermodynamic • Temperature Scale • uses the triple point (TP) of water as the fixed point. There • The temperature is defined(notmeasured!) to be • Exactly 273.16 K.

18. • 0.006 The Triple Point of Water At the triple point of water: gas, solid & liquid all co-exist at a pressure of 0.006 atm.

19. Temperature Scale with • a Single Fixed Point • For Thermometric Parameter X at any • temperature Tx: So, • What thermodynamic variable should be measured to • use the thermodynamic temperature scale?

20. Gas P, V Unknown T TP = 273.16K The Ideal Gas Temperature Scale The Ideal Gas Law: Hold V & n constant!

21. A Constant-Volume Gas Thermometer

22. Defining the Kelvin and Celsius Scales • “One Kelvin degree is 1/273.16 of the • temperature of the triple point of water.” • Named after William Thompson (Lord Kelvin). • Relationship between °C and K: • °C = K - 273.15 • Note that careful measurements find that at 1 • atm. water boils at 99.97 K above the melting • point of ice (i.e. at 373.12 K). • 1 K is not exactly equal to 1° centigrade.

24. Comparison of temperature scales