Transcript A Brief History of Planetary Science

Thermometers
Physics 313
Professor Lee Carkner
Lecture 3
Exercise #2 Bridges
Cable is hypotenuse of triangle:
L2 = 1302 + 8002, L = 810.5 feet
1
3
0
New cable length:
L
DL = LaDT = (810.5)(6.5X10-6)(50) = 0.26 ft

Shorter length L’ forms a new triangle
with a different height, h
800
800
L’
h’
h2+8002 = L’2, h = 128.4 feet

Shrinking of towers

This is an insignificant change in tower
height
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


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
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
