Temperature Part 2

1
Temperature -- Part 2

• Physics 313
• Professor Lee Carkner
• Lecture 3

2
Thermometers

• A thermometer measures some property (pressure,
  volume, resistance )
• how is this related to temperature?
• Consider a plot of X versus Y
• If you hold Y constant, X defines an isotherm
• T (X) aX

3
Types of Thermometers

• What is X?
• Mercury volume
• Gas pressure
• Resistance electrical resistance
• Blackbody radiation radiative flux
• Different thermometers are better at different
  temperature ranges

4
Thermometer Calibration

• What is a?
• Can determine a from ice or steam point of
  water
• Problem hard to reproduce
• Use triple point of water
• ice, water and steam co-exist
• set equal to 273.16 K
• a 273.16/XTP
• T (X) 273.16 (X/XTP)

5
Problems With Thermometers

• Non-constant Y
• Non-uniform X-Y relation
• Most thermometers are only accurate for a
  restricted range of T
• A gas bulb thermometer is generally the most
  accurate

6
Gas Bulb Thermometer

• Bulb full of gas placed in substance to be
  measured
• Bulb connected to tube of mercury by capillary
• With temperature change, the pressure changes,
  raises or lowers Hg
• Bulb gas volume must be kept constant

7
Improving the Gas Bulb Thermometer

• The relationship between pressure and temperature
  is
• T 273.16 (P/PTP)
• if V is constant
• PTP is the pressure measured for the triple point
  of water
• If PTP is changed or the type of gas is changed T
  changes but
• All readings approach a common value as P goes to
  zero

8
Ideal Gas

• All gasses have different properties, but in the
  limit of zero pressure they converge
• This state is called an ideal gas
• PV nRT
• n is number of moles
• R is gas constant

9
Blackbody Radiation

• Plancks Law
• If you can measure the spectrum, you can find the
  temperature

10
Alberio

• Double star
• Both at same distance
• Size proportional to flux
• Which star is hotter?
• Which is larger?

11
Resistance Thermometry

• The resistance of a wire varies with temperature
• Resistance thermometers are practical
• Useful over a wide range
• Harder to model sources of error

12
Standard Temperature Scales

• In order to calibrate lab instruments standard
  thermometers are used
• A gas thermometer defines fixed points
• More practical secondary thermometers find
  temperatures in between
• Very close approximation to Kelvin scale

13
Standard Thermometers

• Low Temp (lt10 K)
• He Vapor
• Medium Temp (10-1200 K)
• Platinum Resistance
• High Temp (gt1200 K)
• Optical Pyrometer

14
Four Temperature Scales

• Fahrenheit
• ice point 32, steam point 212
• Rankine
• absolute scale
• T (R) T (F) 459.67

• Celsius
• ice point 0, steam point 100
• Kelvin
• absolute scale
• T (K) T (C) 273.15
• T (R) (9/5) T (K)
• T (F) (9/5) T (C) 32

15
Concepts

• Defining temperature with other properties
• Problems with thermometers
• Types of thermometers
• gas bulb
• radiation
• resistance
• Ideal gas approximation