Transcript OLL#9: Temperature in Thermal Systems

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Thermal systems are designed to move heat energy
from warmer regions to cooler regions
Usually to help remove excessive heat (ex. radiator of
car)
Temperature difference (∆T) is the prime
mover/force-like quantity in thermal systems similar
to voltage in electrical systems and force in
mechanical systems.
Heat energy always flow from high to low heat
energy regions
i.e. system tries to reach equilibrium so that temp is
the same across the entire system
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Temperature depends on the presence or absence of heat
Measured either in Fahrenheit(F) or Celsius(C)
If measuring temperature than written as °F or °C
(degree symbol is written BEFORE symbol)
If measuring temperature difference(change in
temperature) then must write C ° or F ° (placement of °
symbol changes so it is written AFTER the symbol)
Temperature is a scalar quantity
It is a measure of the amount of average energy that
molecules in motion have
The more motion, the greater the energy, the higher the
temperature (cool to warm to hot)
1a. Thermometers
 Most common device is the liquid-in-glass
thermometer
 Glass bulb containing capillary tube filled with
liquid (most commonly alcohol…or mercury)
 As temp increases, fluid expands and rises up
capillary tube
 Height of fluid is measured using scale on outer
surface of glass – either Celsius(SI/metric unit) or
Fahrenheit(English unit)
2a. Thermocouples
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Thermocouple is another type of device used to measure
temperature (temperature difference)
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Depends on changes in electrical properties of certain metals (ex.
Iron, Constantan, Chromel)
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when 2 of these metals are twisted together (like a twist-tie) they
make a thermocouple junction
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A small voltage develops across the connecting joint when the
other ends of the wires are at different temperatures
Amount of voltage generated depends on the types of metal used
and the temperature difference being measured
2a. Thermocouples cont.
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2 thermocouple junctions connected together make a thermocouple
thermometer
WIRE
A
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Contains 2 junctions(TR and T) connected to a Voltmeter
One of the junctions is a reference junction (TR)
The other junction is the measurement junction (T)
When the junction of the two metals for TR and T is heated or cooled a
voltage is produced that can be correlated back to the actual
temperature.
If temp at T is greater than at TR ,a higher positive voltage will be read
by the voltmeter. A negative voltage will be read if temp at T< TR
1b. Thermometer Scales and Calibration
 To ensure proper measurements thermometers must first
be calibrated
 Done using 2 reference points – boiling and freezing
 In Celsius: Boiling pt water = 100°C(at sea level)
Freezing pt water = 0°C which means 100° Celsius
degree(°) points on scale between boiling and freezing
 In Fahrenheit: Boiling pt of water = 212F and Freezing pt
water = 32F which means 180 Fahrenheit degree points
between boiling and freezing
 Now complete this: Student Exercises Q1-8 on pp.97-98
in workbook. YOU MUST COPY EACH QUESTION
and draw the table for Q#2 and use GFS for Q#5.
1c. Converting Among Celsius and Fahrenheit
Scales (absolute values)
 To convert from Fahrenheit to Celsius:
T(°C) = 5/9[T(°F) – 32°]
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To convert from Celsius to Fahrenheit:
T(°F) = 9/5 T(°C) + 32°
PRACTICE: Now try Example “1a” on the last slide…
1d. Converting Among Celsius and Fahrenheit
Scales For Temperature Difference
 With temp.difference you must use relationship:
1C° = 1.8F°
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Set up correct proportion depending on given
PRACTICE: Now try Examples “1b & c” on the last slide...
1a) Outside temperature is 42°C. Use GFS to find
what is the temperature in °F?
b) The next day the temperature drops to 93.2 °F.
Use GFS to find the temperature
change/difference?
c) Convert your answer from “b” to C°
PRACTICE WORK: Complete Q1-6 on pg.101 in workbook using GFS. Read the
examples on pp.99-100 if you need help.
Note: This work must all be done by the end of class