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Thermometers. Physics 313 Professor Lee Carkner Lecture 3. Exercise #2 Bridges. 130. L. Cable is hypotenuse of triangle: L 2 = 130 2 + 800 2 , L = 810.5 feet New cable length: D L = L aD T = (810.5)(6.5X10 -6 )(50) = 0.26 ft

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thermometers
Thermometers

Physics 313

Professor Lee Carkner

Lecture 3

exercise 2 bridges
Exercise #2 Bridges

130

L

  • 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
    • h2+8002 = L’2, h = 128.4 feet
  • Shrinking of towers
    • This is an insignificant change in tower height

800

800

L’

h’

thermometers1
Thermometers
  • A thermometer measures some property (pressure, volume, resistance … )
  • If you hold Y constant, X defines an isotherm
types of thermometers
Types of Thermometers
  • What is X?
    • Mercury:
    • Gas:
    • Resistance:
    • Blackbody radiation:
  • Different thermometers are better at different temperature ranges
thermometer calibration
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)

problems with thermometers
Problems With Thermometers
  • Non-constant Y
  • Most thermometers are only accurate for a restricted range of T
gas bulb thermometer
Gas Bulb Thermometer
  • Bulb connected to tube of mercury by capillary
  • Bulb gas volume must be kept constant
improving the gas bulb thermometer
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
ideal gas
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
blackbody radiation
Blackbody Radiation
  • Any thermally emitting object obeys Planck’s 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
alberio
Alberio
  • Double star
  • Which star is hotter?
  • Which is larger?
resistance thermometry
Resistance Thermometry
  • Resistance thermometers are practical
  • Harder to model sources of error
standard temperature scales
Standard Temperature Scales
  • A gas thermometer defines fixed points
  • Very close approximation to Kelvin scale
standard thermometers
Standard Thermometers
  • Low Temp (<10 K)
  • Medium Temp (10-1200 K)
  • High Temp (>1200 K)
four temperature scales
Fahrenheit

Rankine

absolute scale

Celsius

ice point = 0, steam point = 100

Kelvin

absolute scale

T (K) = T (C) + 273.15

Four Temperature Scales