Temperature and Heat

1. Temperature and Heat • Learning Goals • Define temperature and convert from one scale to the other (Celsius, Kelvin, Fahrenheit) • Define heat and understand its connection to energy • Distinguish between specific heat and latent heat • Explain how specific heat relates heat and temperature • Describe and give examples of three methods of heat transfer • Identify the phases of matter and the molecular arrangements of each • Describe how temperature and molecular speed of gas are related • Solve exercises involving pV=nRT

2. Temperature • Measure of average kinetic energy of molecules • Measured with thermometer • Relies on physical property of a substance and how it changes with temperature • Dilation/contraction mainly • Three scales • Fahrenheit: 32 (freeze) – 212 (boil) • Celsius: 0 (freeze) – 100 (boil) • Kelvin: Celsius + 273

4. Conversion Between Units • TK = TC+273 • TF = (9/5)TC + 32 • TC = (5/9)(TF - 32) A typical cold day in Quebec is -30ºC. What is the equivalent temperature on the Fahrenheit scale? In the Kelvin scale?

5. Heat • Energy transferred from hotter to colder substance because of temperature difference • Transfer of kinetic energy between molecules of two substances • Always goes from the molecules of the substance having MORE kinetic energy to the ones having LESSHENCEfrom HOTTER to COLDER!

6. Calories = Heat! • Heat is measured in JOULES (it’s energy!) but can be measured in calories • 1 calorie = amount of heat to raise the temperature of 1kg of water by 1ºCelsius. • Food Calorie = 1 kilocalorie • Amount of energy generated when you burn food

7. Effect of Heat Transfer on Matter • Change in temperature • Change in length or volume (thermal expansion) • Bridges • Thermostats • Etc. • Change in phase • Solid, liquid gas

9. Phases • Solid • Definite shape and volume • Liquid • Definite volume • Gas • No definite shape or volume

10. Change in Temperature: Specific Heat Capacity • The same amount of heat transferred to different substances doesn’t mean the same increase in temperature • SPECIFIC HEAT (c): amount of heat (H) necessary to raise the temperature (DT) of one kg (m) of the substance by one degree Celsius.

12. Sand (700 J/kg Co) & Water (4186 J/kg Co)

13. Water + balloon + fire = Pop?

14. Example • How much energy does it take to heat 0.50kg of water from 20.ºC to 30.ºC?

15. Change in Phase: Latent Heat • Heating can change the PHASE of a substance • Energy is used to break bonds between molecule instead of raising temperature

16. Latent Heat • Amount of energy necessary to change 1kg of substance from one phase to another • Solid to liquid: latent heat of fusion, at the melting (or freezing) point • Liquid to gas: latent heat of vaporization, at vaporization (or condensation) point • Solid to gas: latent heat of sublimation, at sublimation (or deposition) point • While changing phase, the TEMPERATURE STAYS CONSTANT!

17. DISORGANIZATION

18. Latent VS Specific Heat • Each phase transition has a different latent heat 2100J/kg.Co 40.65 kJ/kg 4186 J/kg.Co 333.55 kJ/kg 2100 J/kg.Co

19. Examples of latent heat use • A dog opens its mouth to cool down • Hurricanes • Sweating • Condensation

20. Heat Transfer • Heat: transfer of energy through • Conduction: through molecular collisions • Convection: movement of a substance from one position to another • Radiation: by means of electromagnetic radiation (light!)

21. Conduction

22. Convection

23. Radiation

24. Kinetic Theory of Gases • Closed receptacle contain a finite number of gas molecules • Gases take the WHOLE volume of the enclosure they’re in • The faster the molecules move, the higher the temperature of the gas • Molecules hitting with the wall create pressure • Force per unit area, measured in Pascal, Pa • 1 Pa = 1N/m2 • High heels: higher pressure • Flat shoe: lower pressure

25. pV = NRT • If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules • If volume and number of gas molecules are kept constant, pressure p is proportional to temperature T • If the number of molecules and the temperature are kept constant, pressure p is inversely proportional to volume V

26. If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Pressure More Molecules

27. If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Temperature Higher Pressure

28. If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Pressure Smaller Volume

29. Numerical Examples • if you don’t add any more gas in your experiment, then • p1V1/T1 = p2V2/T2 • To do calculations • Pressure: in Pascal • Volume: in m3 • N: in moles • R = 8.314 472 m3·Pa/(K·mol) • T : in KELVIN!

30. Numerical Examples • A cylinder of gas is at room temperature (20°C). The air conditioner breaks down, and the temperature rises to 35°C. What is the new pressure of the gas relative to its initial pressure? • What are the constant quantities? • What has changed? • -V, the volume, is not changing • -p is changing, so T will be changing • -T increased from 273+20 =293K to 273+35 = 308K, that’s an increase of a factor 308/293=1.05 • -The pressure will hence increase by a factor 1.05.

31. Example • A constant volume and mass of helium gas at 27°C is heated so that the pressure of the gas doubles. What is the new temperature of the gas in degrees Celsius? pV = NRT • In that problem, V and N stay constant • p changed as T was changed! • If p is doubled, T had to be doubled too!

32. T was 27°C • In Kelvin: 273+27°C = 300K • Double that • Tx2 = 600K • Transform back in Celsius • 600K – 273 = 327°C

33. If the temperature of a quantity of ideal gas increases, then the • the number of molecules must increase. • pressure must increase. • volume must increase. • the product of the pressure and volume must increase.

34. Homework Chapter 5 • Short-answer questions • 5, 7, 11, 19, 23 • Exercises • 2, 20, 22 • Additional Homework Assignment: NOT GRADED • Multiple-choice questions: 2, 4, 5, 6, 9, • Fill in the blanks: 1, 2, 8, 9, 10 • Visual connection • Exercises: 1, 3, 19, 21