Heat

1. Heat Unit 5- Lecture 3

2. Temperature • is a measure of the average kinetic energy of the particles in the object. • based on the motion of the particles in a substance • fast motion = high temperature • slower motion = lower temperature

3. Temperature • relatively describes how the particles collide with the surface of the thermometer • collisions transfer energy • measured in K (kelvin) in SI system • no degrees mark, same size divisions as Celcius

4. Temperature Scales • Fahrenheit • Common in the US • Freezing point of water is 32ºF • Boiling point of water is 212ºF • Celsius • Common in the rest of the world (SI) • Freezing point of water is 0ºC • Boiling point of water is 100ºC

5. Temperature Scales • Kelvin • used for science, as there are NO negative values • 0K is set at Absolute Zero, the temperature at which all particle motion stops • Freezing point of water is 273K • Boiling point of water is 373K • NOTE – 100 K between freezing and boiling, so Kelvin uses the same degrees as Celsius

7. Converting Temperatures • Fahrenheit to Celsius Cº = (5/9) * (Fº - 32º) • Celsius to Fahrenheit Fº = (9/5) * ºC + 32 • Celsius to Kelvin K = ºC + 273 Adjusts for differences in Zero temperature

8. Heat • is a flow of energy from objects of higher thermal energy to objects of lower thermal energy • heat is measured in Joules (J) because it is a form of energy • described as a flow from hot to cold • no such thing as “cold”

9. Specific Heat • amount of energy needed to raise 1 kg of something by 1 ºC • measured in: • joules per kilogram degree Celcius • J / (kg * ºC) • is measured using a calorimeter • represented by the variable “Cp”

10. Specific Heat • note: water has an incredibly high specific heat value due to the bonds between water molecules • this is why water is used as a coolant – because it will absorb a lot more energy before its temperature increases

11. Thermal Energy • sum of the potential and kinetic energy of the object’s particles • more particles  more mass  more thermal energy • relationship: inverse / direct? • faster moving  greater temperature more kinetic energy  more thermal energy • relationship: inverse / direct?

12. Thermal Energy • changes in thermal energy : • mass * change in temperature * specific heat • kg * ºC * (J / [kg * ºC]) • Q = m * C * (Tf - Ti) • Q = m * C * ∆T

13. Because heat is a form of energy, heat is measured in … • Watts • Newtons • Joules • Mishbohah

14. Heat always flows… • From low E to high E • From high E to low E • Across layers of density • Downwards, like gravity

15. The average motion of particles in a substance defines that substance’s… • Heat • Energy • Potential Energy • Temperature

16. The temperature scale used in the USA is • Fahrenheit • Celsius • Kelvin • Thermocline

17. The temperature scale used in SI is • Fahrenheit • Celsius • Kelvin • Thermocline

18. The temperature scale used in science is • Fahrenheit • Celsius • Kelvin • Thermocline

19. Thermal Expansion • as substances increase in temperature, the particles move apart • substances expand and become less dense when heated • substances shrink and become more dense when cooled • explains expansion joints inbridges and concrete sidewalks • water is one very important exception

20. Examples of Thermal Expansion • Notice the cracks that have formed in the concrete after some time of thermal expansion

21. Transferring Thermal Energy • conduction – transfer by DIRECT particle contact • heat [and electricity] moves faster in solids [to heat or cool] • little to no resistance • low specific heat • best are metals because of “sea of electrons” that move between elements

22. Transferring Thermal Energy • insulators • resist the flow of heat [& electricity] • high specific heat • resisted heat builds up and can cause substance to burn • typically non-metals • large molecules, no free electrons

23. Transferring Thermal Energy • convection – transfer by mass movement of particles • transfer by fluid movement between warmer and cooler locations • currents: • warm expands [higher – less dense], • cold contracts [lower – more dense]

24. Transferring Thermal Energy • Radiation – transfer by invisible radiation • electromagnetic waves • frequently called radiant energy

25. More on Radiation • Emitters are substances that give off radiation • stars, campfires, space heaters, etc. • Absorbers capture radiation • black shirts, asphalt, water • Reflectors return most of the radiation that is emitted towards them • mirrors, aluminum foil, ice & snow

26. Layers and Heat Transfer • Layers trap air in a “dead air space” • More layers mean it’s harder to change temperatures between layers • Ex. double-paned windows, dressing in layers, layers of construction on a house

27. What type of heat transfer describes cooking on an electric range? • Conduction • Convection • Radiation

28. What type of heat transfer describes why a dark shirt feels warmer on sunny days? • Conduction • Convection • Radiation

29. What type of heat transfer describes how a A/C system changes room temperature? • Conduction • Convection • Radiation

30. What type of heat transfer describes how lake water is hot on top and cold on bottom? • Conduction • Convection • Radiation

31. What type of heat transfer describes cooking in an electric oven? • Conduction • Convection • Radiation

32. Homework If you’re not working well…you [individuals] will have an additional assignment • If assigned and completed correctly, no penalty, no benefit • If assigned and not completed correctly, will be – 15pts to your current accuracy score. • Pg. 163 – Questions # 1- 4 • Pg. 170 – Questions # 1 – 4

33. Homework If you’re working well….complete only • Heat Calculations w/s • Temperature conversions w/s