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