Thermodynamics: Temperature
May 31, 2013
THERMODYNAMICS - Temperature: Temperature (T) is a measure of the
average molecular kinetic energy of a substance.
This implies that Temperature is a measure of the molecular motion of a given
substance. For example...
• When the temperature of water is high, this implies that the individual
molecules of water, on average, have a lot of kinetic energy; meaning that the
molecules are moving with a high speed.
• When the temperature is low, this implies the molecules have, on average, a
small amount of kinetic energy; meaning that the molecules are moving with
a low speed.
Temperature Scales: Fahrenheit
• The official temp. scale in the US; currently defined by the boiling point of
water (212 °F) and the freezing point of water (32 °F) at STP; 180° of
separation between these temperatures indicates the size of 1 F degree is the
fraction (1/180) the temperature difference between freezing and boiling.
• Based on a temperature scale suggested by Daniel Fahrenheit
> Freezing point of "brine" (equal parts water, ice, salt) defined as 0 °F
> Temperature of water and ice in 1:1 ratio is 32 °F
> Human body temperature is 96 °F
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Thermodynamics: Temperature
May 31, 2013
Temperature Scales: Celsius (aka Centigrade)
• Most common temperature scale in the world; defined traditionally by the
boiling (0° C) and freezing (100° C) point of water.
• Now is better defined based on Absolute Zero (-273.15° C) and the Triple
Point of a specially prepared sample of water (0.01° C).
> Absolute Zero is the temperature at which all molecular motion stops.
Temperature Scales: Kelvin (aka Absolute)
• The SI unit of temperature
• The so-called Absolute temperature scale, defined by Absolute Zero (0 K) and
the Triple Point of water (273.16 K).
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Thermodynamics: Temperature
May 31, 2013
Temperature Conversion: To convert from one temp scale to another, we use
the following conversion factors:
Converting To...
Converting From...
Equations
Fahrenheit
Celsius
TF = 1.8TC + 32
Fahrenheit
Kelvin
TF = 1.8TK - 459.67
Celsius
Fahrenheit
TC = (TF - 32)/1.8
Celsius
Kelvin
TC = TK - 273.15
Kelvin
Celsius
TK = TC + 273.15
Kelvin
Fahrenheit
TK = (TF + 459.67)/1.8
A few "standard" temperatures:
• "Room" Temp = 68 °F = 20 °C = 293.15 K
• "Body" Temp = 98.6 °F = 37 °C = 310.15 K
• Intersection of F and C scales: -40 °F = -40 °C = 233.15 K
Examples:
1.
A swimming pool thermometer reads a temperature of 80 °F. What is the
temp in °C, K?
2.
A water sample in a lab is heated to a temperature of 65 °C. What is this
temp in K, °F?
3.
The average temperature of the Universe is 2.7 K. Express this temp in
°C, °F.
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