Thermometers
Physics 313Professor Lee
CarknerLecture 3
Exercise #2 Bridges Cable is hypotenuse of triangle:
L2 = 1302 + 8002, L = 810.5 feet New cable length:
L = LT = (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
130
L
800
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 0.006 atm
a = 273.16/XTP
T (X) = 273.16 (X/XTP)
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 = 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
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
maxT = 2.9 X 107
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 (5.6703 X 10-8 W/m2 K4) 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 (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
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