Thermal Properties of Materials 재료의 전자기적 성질 Part IV. Thermal Properties of Materials

Fundamentals of Thermal Properties 15 In the mid-1800s, Mayer, Helmholtz, and Joule discovered independently that heat is simply a form of energy. 1853: Wiedemann and Franz law (good electrical conductors are generally good thermal conductors) The ratio between heat conductivity and electrical conductivity (divided by the temperature) is essentially constant for all metals The thermal conductivity only varies within four orders of magnitude while the electrical property varies about 25 orders of magnitude. (Fig. 18.1)

Heat in physics is defined as energy transferred by thermal interactions. Heat flows spontaneously from systems of higher temperature to systems of lower temperature. When two systems come into thermal contact, they exchange energy through the microscopic interactions of their particles. When the systems are at different temperatures, this entails spontaneous net flow of energy from the hotter to the cooler, so that the hotter decreases in temperature and the cooler increases in temperature. This will continue until their temperatures are equal. Then the net flow of energy has settled to zero, and the systems are said to be in a relation of thermal equilibrium. Spontaneous heat transfer is an irreversible process. The first law of thermodynamics states that the internal energy of an isolated system is conserved. To change the internal energy of a system, energy must be transferred to or from the system. For a closed system, heat and work are the mechanisms by which energy can be transferred. For an open system, internal energy can be changed also by transfer of matter.[6] Work performed by a body is, by definition, an energy transfer from the body that is due to a change to external or mechanical parameters of the body, such as the volume, magnetization, and location of center of mass in a gravitational field.[2][7][8][9][10][11] When a body is heated, its internal energy increases. This additional energy is stored as kinetic and potential energy of the atoms and molecules in the body. [12] Heat itself is not stored within a body. Like work, it exists only as energy in transit from one body to another or between a body and its surroundings. (from wikipedia)

15.1. Heat, Work, And Energy First law of thermodynamics In this chapter, We limit our consideration to processes for which W can be considered to be Zero. Energy, work, and heat have same unit. 1 cal = 4.184 J 1 J = 0.239 cal

15.2. Heat Capacity c' Heat capacity : the amount of heat which needs to be transferred to a substance in order to raise its temperature by a certain temperature interval. Generally, it is interested in two kinds : at constant volume & at constant pressure Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure. at constant V at constant P H=E+PV These relationship is….

Proving and

15.3. Specific Heat Capacity, c Specific heat capacity is the heat capacity per unit mass 15.4. Molar Heat Capacity, cv Molar heat capacity is the heat capacity per mole

At room temperature, molar heat capacity at constant volume is approximately 25 J/mol·K for most solids. (discovered by Dulong and Petit) 298K Exception : carbon only reach 25J/mol·K at high temperature

All heat capacities are zero at T = 0K All heat capacities are zero at T = 0K. Near T= 0K, heat capacities are climb in proportion to T3 Debye Temperature θD: a temperature at which heat capacities reach 96% of their final value.

15.5. Thermal Conductivity, K Heat flux is proportional to the temperature gradient. The proportionality constant is called thermal conductivity . Negative sign indicates that the heat flows from the hot to the cold end.

15.6. The Ideal Gas Equation In ideal gas, This equation is a combination of two experimentally obtained thermodynamics. PV = constant ( discovered by Boyle and Mariotte ) V T, at constant P ( discovered by Gay-Lussac )

15.7. Kinetic Energy of Gases When a body is heated, its internal energy increases. This additional energy is stored as kinetic and potential energy of the atoms and molecules in the body. [12] Heat itself is not stored within a body. Like work, it exists only as energy in transit from one body to another or between a body and its surroundings. In Figure 19.2 A The number of particles reaching the end face The number of particles per unit time & unit area that hit the end face

The momentum change per unit time & unit area This yields, for the pressure, Inserting , (kinetic energy of a particle)