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Physics 114 – Lecture 41

Physics 114 – Lecture 41. §14.5 Latent Heat Experimental observations: heat must be added to melt ice at 0 0 C to become water at 0 0 C Likewise heat must be removed from water at 0 0 C to produce ice at 0 0 C

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Physics 114 – Lecture 41

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  1. Physics 114 – Lecture 41 • §14.5 Latent Heat • Experimental observations: heat must be added to melt ice at 0 0C to become water at 0 0C • Likewise heat must be removed from water at 0 0C to produce ice at 0 0C • Likewise in going from water at 100 0C to steam at 100 0C and vice versa, heat must be added and removed respectively L41-s1,8

  2. Physics 114 – Lecture 41 • Heat added steadily to ice initially at T = - 40 0C • Resulting temperature and state of the sample L41-s2,8

  3. Physics 114 – Lecture 41 • The heat input required to change 1.00 kg of a substance from the solid to the liquid state without a change in temperature is defined to be the latent heat of fusion of that substance, Lf− units J/K • When m kg of a material is changed from a solid to a liquid without a change in temperature the heat added is • Q = mLf • Likewise the heat removed when m kg is changed from a liquid to a solid is • Q = mLf L41-s3,8

  4. Physics 114 – Lecture 41 • Likewise for vaporization, when the substance changes from the liquid to the gaseous state, or vice versa, from thegaseous to the liquid state, an amount of heat, Q, must be added or removed, where • Q = mLv • and where Lv is defined to be the latent heat of vaporization • Values for Lf and Lv are given in Table 14.2 • The calorimetry equation, heat lost = heat gained must be modified accordingly whenever a change of state occurs L41-s4,8

  5. Physics 114 – Lecture 41 • Study examples 14.7, 14.8 and 14.9 • Evaporation • Kinetic Theory of Latent Heat • §14.6 Heat Transfer: Conduction • Rate of heat flow is • where k is the thermal conductivity Study example 14.10 L41-s5,8

  6. Physics 114 – Lecture 41 • §14.6 Heat Transfer: Convection • No general quantitative expression can be given for this phenomenon • Examples of convection L41-s6,8

  7. Physics 114 – Lecture 41 • §14.6 Heat Transfer: Radiation • Example: the radiation from the sun • The rate of emission of radiation energy from a body of temperature, T, expressed in kelvins (K), is • This relation is known as the Stefan-Boltzmann equation, where e is the emissivity, T is expressed in K and σ is the Stefan-Boltzmann constant with a value σ = 5.67 X 10-8 W/m2. T4 L41-s7,8

  8. Physics 114 – Lecture 41 • If body 1 at temperature, T1, is surrounded by body 2 at temperature, T2, then both bodies emit radiation and the net radiation from body 1 becomes L41-s8,8

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