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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

Christopher Chui

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 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

• 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

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

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

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