Thermometry

1. Thermometry Textbook Chp8, Pg 143-152

2. Overview of Thermal Physics • Temperature vs Heat • Thermometric Properties • Thermocouple Topics

3. 2 categories – Microscopic (small scale) and Macroscope (large scale) • Pressure, Volume and Temperature are macroscopic phenomenon. These are observable in real life. • Collisions, vibrations and movement of molecules are microscopic phenomenon. These are theoretical (not observable in real life) • In Kinetic Theory, we used microscopic theories to explain macroscopic phenomenon Overview of Thermal Physics

4. Temperature is a macroscopic quantity (observable) • It is a measure of how hot and how cold an object is • Microscopically, the temperature is determined by the average kinetic energy of the molecules • Hotter the object, faster the molecules What is Temperature?

5. Heat is a very specific term in physics • Heat refers to the amount of thermal energy when thermal energy is being transferred from a hotter to colder region • The term “Thermal Energy” is used when there is no transferring. • http://www.youtube.com/watch?v=yxBTEMnrZZk • We will study heat more in the next chapter Temperature vs Heat

6. There is a need to measure temperature accurately, and the human body cannot do that. • Hence the need for thermometers, instruments designed to measure temperature. • In your syllabus, you are required to be familiar with 3 kinds thermometers and how they work. • Each kind of thermometer vary by their thermometric property (what changes when temperature changes) Thermometric Properties

7. You should already be very familiar with the liquid in glass thermometer (either the alcohol or the mercury versions) • The thermometric property is the volume of a fixed mass of liquid Liquid in Glass Thermometer Alcohol-in-glass Thermometer Mercury-in-glass Thermometer

8. To make a thermometer, you don’t just need some alcohol or mercury, you need to read it against a scale • To construct a scale, you need two fixed points of temperature • Usually the two points are the • Ice Point (melting point of pure water) • Steam Point (boiling point of pure water at 1 atm) Calibrating a Thermometer

9. There is a complex looking formula in your textbook. Recommend NOT to memorize this formula. • Instead follow this thinking process: • What is the total length from 0°C to 100°C? • What is length for each°C? • Hence, solve the unknown length/temperature. Calibrating a Thermometer

10. The length of mercury is 1.0 cm at ice point and 11.0 cm at steam point. What is the length of mercury at 70°C? • What is the length from 0°C to 100°C? Ans: 10.0 cm. • What is the length for each °C? Ans: 0.1 cm • 1°C → 0.1 cm • 70°C → 7 cm • So is the answer 7 cm? • No! Because at 0°C, the length of mercury is not 0 cm, but 1 cm. The answer is 8 cm. Example 1

11. The length of an alcohol thermometer is 5 cm in ice point and 85 cm in steam point. What temperature is it when the length is 65.4 cm? Example 2

12. The thermometric property of the resistance thermometer is electrical resistance • When temperature increases, electrical resistance (in Ohms Ω) increases Resistance Thermometer

13. The resistance of a resistance thermometer is 50.0 Ω at ice point. If the resistance is 51.5 when the temperature is 50°C, what is the resistance at steam point? Example 3

14. The thermometric property for a thermocouple is voltage (or e.m.f.) in Volts. • E.m.f. is proportional to the difference in temperature between the two junctions V Thermocouple Steam Point Ice Point

15. When the cold junction is in melting ice and when the hot junction is in boiling water, the e.m.f produced is 2.00 V. If the hot junction is placed in water of 50 C, what is the e.m.f.? Example 4

16. Overview of Thermal Physics • Temperature vs Heat • Thermometric Properties • Thermocouple Summary

17. 10 min Quiz!