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Physics 14: Heat. Christopher Chui. Heat as Energy Transfer. 1 calorie is the amount of heat to raise the temperature of 1 gm of water by 1 degree C 1 kcal is the heat needed to raise 1 kg of water by 1 o C 1 kcal = 1 dietary Cal = 1 Cal = 4.186 x 10 3 J

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Physics 14: Heat

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Physics 14 heat l.jpg

Physics 14: Heat

Christopher Chui

Physics 14: Heat - Christopher Chui


Heat as energy transfer l.jpg

Heat as Energy Transfer

  • 1 calorie is the amount of heat to raise the temperature of 1 gm of water by 1 degree C

  • 1 kcal is the heat needed to raise 1 kg of water by 1oC

  • 1 kcal = 1 dietary Cal = 1 Cal = 4.186 x 103 J

  • 1 BTU is the heat needed to raise the temperature of 1 lb of water by 1oF

  • Heat is energy transferred from one body to another because of a difference in temperature

Physics 14: Heat - Christopher Chui


Temperature heat internal energy l.jpg

Temperature, Heat, & Internal Energy

  • The total sum of all energy of all molecules in an object is its thermal energy or internal energy

  • Temperature is a measure of the average kinetic energy of individual molecules

  • Heat refers to a transfer of energy from a hot object to a cold object

  • Internal energy of an ideal gas = 3/2 nRT where n is the number of moles, R is the gas constant, T: temp in K

  • Specific heat, c, is the heat needed to raise 1 kg of water by 1 oC

  • Equation of heat flow: Q = mc DT

Physics 14: Heat - Christopher Chui


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Calorimetry

  • Heat lost by sample = heat gained by water + cup

  • Heat of fusion, LF, is the heat needed to change 1 kg of a substance from the solid state to the liquid state

  • Heat of vaporization, Lv, is the heat needed to change 1 kg of a substance from the liquid state to the gas state

  • Both heat of fusion and heat of vaporization are latent heats. Equation: Q = mL

Physics 14: Heat - Christopher Chui


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Problem Solving Techniques

  • Understand the problem by reading it carefully twice

  • Apply conservation of energy: heat gained=heat lost

  • If no phase changes occur, Q(gained) = mc(Tf – Ti) and Q(lost) = mc (Ti – Tf)

  • If phase changes occur, add latent heat, Q = mL

  • When the system reaches equilibrium, there is only one final temperature, Tf

  • Solve the energy equation for the unknown

Physics 14: Heat - Christopher Chui


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Heat Transfer

  • Heat is transferred by 3 ways: conduction, convection, and radiation

  • DQ/Dt = kA( T1 – T2)/length, k: thermal conductivity

  • Wind can cause much greater heat loss

  • Convection is the heat transferred by mass movement of molecules from one place to another

  • Fans provide forced convection of air

  • Radiation is heat transferred by electromagnetic waves

  • Stefan-Boltzmann equation: DQ/Dt = esAT4 where e: emissivity, and s is Stefan-Boltzmann constant

  • Net flow of radiation energy: DQ/Dt = esA(T14 – T24)

Physics 14: Heat - Christopher Chui


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