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# Thermometers

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

## Thermometers

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### Presentation Transcript

1. Thermometers Physics 313 Professor Lee Carkner Lecture 3

2. Exercise #2 Bridges 130 L • Cable is hypotenuse of triangle: • L2 = 1302 + 8002, L = 810.5 feet • New cable length: • DL = LaDT = (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 800 800 L’ h’

3. Thermometers • A thermometer measures some property (pressure, volume, resistance … ) • If you hold Y constant, X defines an isotherm

4. Types of Thermometers • What is X? • Mercury: • Gas: • Resistance: • Blackbody radiation: • Different thermometers are better at different temperature ranges

5. 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)

6. Problems With Thermometers • Non-constant Y • Most thermometers are only accurate for a restricted range of T

7. Gas Bulb Thermometer • Bulb connected to tube of mercury by capillary • Bulb gas volume must be kept constant

8. 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

9. 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

10. 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

11. Alberio • Double star • Which star is hotter? • Which is larger?

12. Resistance Thermometry • Resistance thermometers are practical • Harder to model sources of error

13. Standard Temperature Scales • A gas thermometer defines fixed points • Very close approximation to Kelvin scale

14. Standard Thermometers • Low Temp (<10 K) • Medium Temp (10-1200 K) • High Temp (>1200 K)

15. Fahrenheit Rankine absolute scale Celsius ice point = 0, steam point = 100 Kelvin absolute scale T (K) = T (C) + 273.15 Four Temperature Scales

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