Thermometers Physics 313 Professor Lee Carkner Lecture 3

Exercise #2 Bridges  Cable is hypotenuse of triangle:  L 2 = , L = feet  New cable length:   L = L  T = (810.5)(6.5X10 -6 )(50) = 0.26 ft   Shorter length L’ forms a new triangle with a different height, h  h = L’ 2, h = feet   Shrinking of towers   This is an insignificant change in tower height L h’L’

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

Types of Thermometers  What is X?  Mercury:  Gas:  Resistance:  Blackbody radiation:  Different thermometers are better at different temperature ranges

Thermometer Calibration  What is “a”?   Problem: hard to reproduce  Use triple point of water   at a pressure of atm  a = /X TP T (X) = (X/X TP )

Problems With Thermometers  Non-constant Y   Most thermometers are only accurate for a restricted range of T 

Gas Bulb Thermometer   Bulb connected to tube of mercury by capillary   Bulb gas volume must be kept constant

Improving the Gas Bulb Thermometer   The relationship between pressure and temperature is: T = (P/P TP )   P TP is the pressure measured for the triple point of water   All readings approach a common value as P goes to zero

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  Any thermally emitting object obeys Planck’s Law and will have a spectrum that depends on the temperature  max T = 2.9 X 10 7   The temperature of a thermal radiator also affects the total amount of power radiated, via the Stefan-Boltzmann law:  where:    is the Stefan-Boltzmann constant ( X W/m 2 K 4 )    T is the temperature

Alberio  Double star    Which star is hotter?  Which is larger?

Resistance Thermometry    Resistance thermometers are practical   Harder to model sources of error

Standard Temperature Scales   A gas thermometer defines fixed points   Very close approximation to Kelvin scale

Standard Thermometers  Low Temp (<10 K)   Medium Temp ( 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) 