Thermometers

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Thermometers

• Physics 313
• Professor Lee Carkner
• Lecture 3

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Exercise 2 Bridges

• Cable is hypotenuse of triangle
• L2 1302 8002, L 810.5 feet
• New cable length
• DL LaDT (810.5)(6.5X10-6)(50) 0.26 ft
• Shorter length L forms a new triangle with a
  different height, h
• h28002 L2, h 128.4 feet
• Shrinking of towers
• This is an insignificant change in tower height

130
L
800
800
h
L
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Thermometers

• A thermometer measures some property (pressure,
  volume, resistance )
• If you hold Y constant, X defines an isotherm

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Types of Thermometers

• What is X?
• Mercury
• Gas
• Resistance
• Blackbody radiation
• Different thermometers are better at different
  temperature ranges

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Thermometer Calibration

• What is a?
• Problem hard to reproduce
• Use triple point of water
• at a pressure of 0.006 atm
• a 273.16/XTP
• T (X) 273.16 (X/XTP)

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Problems With Thermometers

• Non-constant Y
• Most thermometers are only accurate for a
  restricted range of T

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Gas Bulb Thermometer

• Bulb connected to tube of mercury by capillary
• Bulb gas volume must be kept constant

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Improving the Gas Bulb Thermometer

• The relationship between pressure and temperature
  is
• T 273.16 (P/PTP)
• PTP is the pressure measured for the triple point
  of water
• All readings approach a common value as P goes to
  zero

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Ideal Gas

• This situation is called an ideal gas
• PV nRT
• The ideal gas law is an equation of state
• Other equations of state can be used if greater
  accuracy is needed

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Blackbody Radiation

• Any thermally emitting object obeys Plancks Law
  and will have a spectrum that depends on the
  temperature
• lmaxT 2.9 X 107
• The temperature of a thermal radiator also
  affects the total amount of power radiated, via
  the Stefan-Boltzmann law
• where
• s is the Stefan-Boltzmann constant (5.6703 X 10-8
  W/m2 K4)
• T is the temperature

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Alberio

• Double star
• Which star is hotter?
• Which is larger?

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Resistance Thermometry

• Resistance thermometers are practical
• Harder to model sources of error

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Standard Temperature Scales

• A gas thermometer defines fixed points
• Very close approximation to Kelvin scale

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Standard Thermometers

• Low Temp (lt10 K)
• Medium Temp (10-1200 K)
• High Temp (gt1200 K)

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Four Temperature Scales

• Fahrenheit
• Rankine
• absolute scale

• Celsius
• ice point 0, steam point 100
• Kelvin
• absolute scale
• T (K) T (C) 273.15