thermometers
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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Thermometers
Physics 313Professor Lee
CarknerLecture 3
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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’
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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 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
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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 (<10 K)
Medium Temp (10-1200 K)
High Temp (>1200 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