Physics 14: Heat

1. Physics 14: Heat Christopher Chui Physics 14: Heat - Christopher Chui

2. 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

3. 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

4. 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

5. 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

6. 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