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Chapter 14--Heat

Chapter 14--Heat. Sections 1-9. 14.1 Heat as Energy Transfer. Two objects at different temperatures transfer heat; hot cold heat flow spontaneously from hot to cold. 14.1 Heat as Energy Transfer. calorie (c or cal)--amount of heat necessary to raise the temperature of 1-g of water 1 C

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Chapter 14--Heat

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  1. Chapter 14--Heat Sections 1-9

  2. 14.1 Heat as Energy Transfer • Two objects at different temperatures transfer heat; hotcold • heat flow spontaneously from hot to cold

  3. 14.1 Heat as Energy Transfer calorie(c or cal)--amount of heat necessary to raise the temperature of 1-g of water 1C Calorie (C) or kilocalorie (kcal)--1000 calories; dietary calorie BTU (British thermal unit)--heat needed to raise 1 lb. of water 1 F • 0.252 kcal = 1 BTU = 1055 J

  4. 14.1 Heat as Energy Transfer Mechanical equivalent of heat • James Joule conducted experiments to show that work is done by heat transfer (& vise versa) • 4.186 J = 1 cal • joule =SI energy unit

  5. 14.1 Heat as Energy Transfer Heat • energy that is transferred from one body to another due to a difference in temperature

  6. 14.2 Distinction Between Temperature, Heat and Internal Energy Thermal energy (internal energy) • sum of all the energy of all the molecules in an object • units = joules • total energy of all the molecules in an object

  7. 14.2 Distinction Between Temperature, Heat and Internal Energy Temperature • measure of the average kinetic energy of individual molecules • units = kelvin

  8. 14.2 Distinction Between Temperature, Heat and Internal Energy Heat • transfer of energy (such as thermal energy) from one object to another due to a temperature difference

  9. 14.2 Distinction Between Temperature, Heat and Internal Energy • If 50-g of water at 30C is mixed with 200-g of water at 25C heat will flow from water at 30C to water at 25C even though there is more internal energy in 200-g of water (25C ) than in 50-g water (30C)

  10. 14-3 Internal Energy of an Ideal Gas Internal energy of an ideal monatomic gas • U = N(1/2mv2) = 3/2NkT = 3/2 nRT • where: • U = internal energy (J) • N = # of atoms • m = mass of atom • v = average speed • n = number of moles • T = temperature (K) • k = 1.38 x10-23 J/K (Boltzmann constant) • R = 8.315 J/molK or 0.0821 L atm/mol K

  11. 14-3 Internal Energy of an Ideal Gas • Internal energy of an ideal gas  T and n • if the gas is polyatomic then rotational and vibrational energy (?) of the molecules must be taken into account (U will be greater than it is for monatomic gases but still  T)

  12. When Heat is added to a substance it either: • Increases its temperature • Energy goes into increasing Kinetic Energy—causes particles to move faster Or • Changes its phase • Energy goes into increasing Potential Energy—causes particles to become further apart (overcome intermolecular forces)

  13. 14.4 Specific Heat Q = mcT where: • Q = heat lost/gained • m = mass of substance (kg) • T = change in temperature (T2- T1) • c = specific heat • ***this heat added causes a change in temperature***

  14. 14.4 Specific Heat Specific Heat • characteristic of material (changes slightly with temperature) • water: 1kcal/kgoC or 4186 J/kg oC (20oC) • ice: 2100 J/kg oC (-5oC) • steam: 2010 J/kg oC (110oC)

  15. 14.4 Specific Heat When: • Q = -; T = -; heat is transferred out of substance • Q = +; T = +; heat is transferred into of substance

  16. 14.5 Calorimetry • In an isolated system energy is conserved • If heat is lost by one • component of the system it is gained by another component of the system. • -(Q+) = +(Q-) • maca(T2-T1) = - mbcb(T1-T2)

  17. 14.5 Calorimetry Calorimetry • Technique used to quantitatively measure heat exchange Calorimeter • Device used to quantitatively measure heat exchange within a system • Used to find specific heat of an unknown substance • Well insulated so no heat exchange with environment

  18. 14.5 Calorimetry If 200 mL of tea at 95oC is poured into a 150 g cup at 25oC what will be the final temperature of the tea? (cwater = 4186 J/kgoC cglass = 840 J/kgoC )

  19. 14.5 Calorimetry • Answer: 85.8oC

  20. 14.6 Latent Heat Latent Heat • Heat required to change the phase of a substance • solid  liquid, liquid  gas • Units: kJ/kg, J/kg, J/g, Kcal/kg

  21. 14.6 Latent Heat Latent Heat Two types: • Heat of fusion—heat required to melt 1kg of a substance • Lf H2O = 333 kJ/kg • Heat of vaporization—heat required to vaporize 1 kg of a substance • Lv H2O =2260 kJ/kg

  22. 14.6 Latent Heat Heating Curve • A graph of Temperature vs. Heat added • Shows what happens as heat is added to a sample of substance

  23. 14.6 Latent Heat Heating Curve: Water • What happens in each part? • What equation do we use to find heat? T Q

  24. 14.6 Latent Heat How much heat is needed to change the temperature of 1,350 mL of water from –17°C to 145°C?

  25. 0 14.6 Latent Heat Answer: 4.24 x 106 J

  26. 0 There are three ways heat can move from one place to another: • Conduction • Convection • Radiation

  27. 0 14.7 Conduction Conduction • Heat moves in an object without the net movement of its particles. • Results of molecular collisions • Hot molecules collide with cold molecules and ???? • Only occurs when there is a temperature difference • Rate of flow  temperature difference, area in contact , 1/length(of motion)

  28. 0 14.7 Conduction Conductors • Substances where heat moves quickly Insulators • Substance when heat moves slowly

  29. 0 14.7 Conduction At room temperature why does a carpet feel warmer than a tile floor? Why are storm doors so good at insulating if glass is a good conductor of heat? Why do you layer clothes to keep warm? Building materials: R-Value—rates how resistant a material is to the flow of heat. • R = l/k

  30. 0 14.8 Convection Convection • Heat is transferred by the mass movement of molecules from one place to another • Forced vs Natural convection • Convection Currents

  31. 0 14.8 Convection Convection • In the human body 20% of food energy is used to do work, 80% goes into heat • If this heat was not dissipated body temperature would increase by 3°C per hour • Blood moves heat by convection to beneath skin and it is conducted to the surface

  32. 0 14.9 Radiation Radiation • Transfer of heat without a medium • Most heat from fire comes by way of radiation • Infrared Radiation form the Sun is responsible for heating the Earth

  33. 0 14.9 Radiation Stefan-Boltzmann equation • Shows the rate at which an object radiates energy • Q/t = eAT4 where: •  = Stefan-Boltzmann constant (5.67 x 10-8 W/m2•K4) • e = emissivity; number between 0 &1 that shows how well an object absorbs (emits) radiation (1 = pure black)

  34. 0 14.9 Radiation Stefan-Boltzmann equation • So rate of emission of radiation (Q/t) is proportional to Temperature4!

  35. 0 14.9 Radiation Radiation from the Sun strikes the Earth at 1350 J per second per square meter or 1350 W/m2 (solar constant) • During cloudy conditions about 70% reaches ground when clear 1000W/m2

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