9. First Law of Thermodynamics

First Law of Thermodynamics Heat Capacity Heat capacity (C) of a body is the amount of thermal energy (Q) required to raise the temperature of the whole body by one kelvin (or one degree celcius). T QC  Specific Heat Capacity Specific heat capacity (c) of a substance is the amount of thermal energy (Q) required to raise the temperature of unit mass of the substance by one kelvin (or one degree celcius). Tm Qc  Latent Heat Latent heat is the amount of heat absorbed or released by the whole body of the substance during a change in its physical phase that occurs at constant temperature. Specific Latent Heat Specific latent heat of a substance is the amount of heat absorbed or released by unit mass of the substance during a change in its physical phase that occurs at constant temperature. QL m  Vaporization: when it changes physical phase from liquid to vapour at constant temperature. Equivalently, it is the amount of heat released by unit mass of the substance when it changes physical phase from vapour to liquid at constant temperature.  Fusion: when it changes physical phase from solid to liquid at constant temperature . Equivalently, it is the amount of heat released by unit mass of the substance when it changes physical phase from liquid to solid at constant temperature.  Sublimation: when it changes physical phase from solid to vapour at constant temperature. Equivalently, it is the amount of heat released by unit mass of the substance when it changes physical phase from vapour to solid at constant temperature Kinetic Theory of Gas to Melting and Boiling  What does melt at constant temperature mean? o Mean kinetic energy o f molecules is proportional to temperature. Constant temperature mean no change in mean kinetic energy of molecules o Hence, the energy supplied during melting must be solely used to increase the potential energy due to intermolecular forces among molecules  Why, in general, specific latent heat of vaporization is larger than specific latent heat of fusion? o Increasing in potential energy among molecules would mean increasing in average separation distance among molecules, thus increasing in volume. o However, the increase in volume from liquid to solid is higher than that from solid to liquid. o Significant work against atmospheric pressure must be done to change from liquid to solid.

Temperature against Time Curves (An example) c Note:  Assume constant power o The longer the horizontal line, the more the energy input to the body  Can read off o Melting point o Boiling point Internal Energy The internal energy of a system is the sum of the random kinetic and potential energies of the individual atoms or molecules of the system. First Law of Thermodynamics It states that the increase in internal energy U of a closed system is equal to the sum of the heat supplied to the system Q and the external work done on the system W. U Q W  The sign conventions for the various quantities are: Positive Negative U Increase in Internal Energy Decrease in Internal Energy Q Heat absorbed by system Heat given out from system W Work done on system Compression Work done by system Expansion p-V Diagram (An Example) Notes  Direction of arrows indicate process directions, here ABCA  AB, isochoric/isovolumetric process; W = 0  CA, isobaric process; W = -pΔV, i.e. W = -p1(V1 – V2)  W is obtained from area under p-V diagram. o WCA represented by red area (positive, ‘cos compression) o WBC represented by green area (Negative)  ΔUABCA = 0. (Cyclic process) T t Horizontal lines means change of states at constant temperature Gradient is inversely proportional to c p V A B C V2 V1 p1 p2 No work done for AB, no matter how large p1 or p2 are BC is NOT isothermal, unless otherwise stated or it follows an isotherm