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Physics 14: Heat

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Physics 14: Heat

Christopher Chui

Physics 14: Heat - Christopher Chui

- 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

- 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

- 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

- 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 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