5. TEMPERATURE AND HEAT, IDEAL GASES DEC 1013 ENGINEERING SCIENCEs 5. TEMPERATURE AND HEAT, IDEAL GASES NAZARIN B. NORDIN nazarin@icam.edu.my

What you will learn: Heat and temperature Temperature scales Conversion between scales Transfer of heat: conduction, convection, radiation Heat capacity: specific heat capacity and latent heat (latent heat of fusion ; latent heat of vaporization) Phases of matter (solid, liquid, gas)

Temperature The energy transferred from a high-temperature object to a lower-temperature object is called heat. Celsius, Fahrenheit, Kelvin. ((C*9)/5)+32=f ((F-32)*5)/9=c C+273.15=k The specific heat capacity of a solid or liquid is defined as the heat required to raise unit mass of substance by one degree of temperature.

Temperature scale the SI unit of temperature, the kelvin, is defined as 1/273.16 of the temperature of the triple point of water. the triple point of water, which is the single temperature and pressure at which water, water vapor, and ice can coexist in equilibrium. it occurs at a temperature of 0.01C and a pressure of 4.58 mm of mercury.

Conversion between scales

Scale conversion

Conduction Thermal Conduction The energy transfer process most closely associated with a temperature difference is called thermal conduction or simply conduction. In this process, the transfer can be viewed on an atomic scale as an exchange of kinetic energy between microscopic particles—molecules, atoms, and electrons—with less energetic particles gaining energy as they collide with more energetic particles.

Convection The transfer of energy by the movement of a substance is called convection.

Radiation

Heat Early in the development of thermodynamics, before scientists realized the connection between thermodynamics and mechanics, heat was defined in terms of the temperature changes it produced in an object, and a separate unit of energy, the calorie, was used for heat. The calorie (cal) is defined as the energy necessary to raise the temperature of 1 g of water from 14.5° to 15.5°C. Likewise, the unit of heat in the U.S. customary system, the British thermal unit (Btu), was defined as the energy required to raise the temperature of 1 lb of water from 63°F to 64°F.

Sensible heat The specific heat capacity of a solid or liquid is defined as the heat required to raise unit mass of substance by one degree of temperature.

Remarks Notice the use of 2.01 m in the denominator of the last calculation, rather than 2.00 m. This is because, in effect, the strut was compressed back to the original length from the length to which it would have expanded. (The difference is negligible, however.) The answer exceeds the ultimate compressive strength of steel and underscores the importance of allowing for thermal expansion. Of course, it’s likely the strut would bend, relieving some of the stress (creating some shear stress in the process). Finally, if the strut is attached at both ends by bolts, thermal expansion and contraction would exert sheer stresses on the bolts, possibly weakening or loosening them over time.

Latent heat The unit of latent heat is the joule per kilogram ( J/kg). The word latent means “lying hidden within a person or thing.” The positive sign in Equation 11.6 is chosen when energy is absorbed by a substance, as when ice is melting. The negative sign is chosen when energy is removed from a substance, as when steam condenses to water. The latent heat of fusion Lf is used when a phase change occurs during melting or freezing, while the latent heat of vaporization Lv is used when a phase change occurs during boiling or condensing. For example, at atmospheric pressure the latent heat of fusion for water is 3.33 105 J/kg, and the latent heat of vaporization for water is 2.26 106 J/kg. The latent heats of different substances vary considerably, as can be seen in Table 11.2. Another process, sublimation, is the passage from the solid to the gaseous phase without going through a liquid phase. The fuming of dry ice (frozen carbon dioxide) illustrates this process, which has its own latent heat associated with it—the heat of sublimation.

To better understand the physics of phase changes, consider the addition of energy to a 1.00-g cube of ice at 30.0°C in a container held at constant pressure. Suppose this input of energy turns the ice to steam (water vapor) at 120.0°C. Figure 11.3 is a plot of the experimental measurement of temperature as energy is added to the system. We examine each portion of the curve separately.

Phase changes can be described in terms of rearrangements of molecules when energy is added to or removed from a substance. Consider first the liquid-to-gas phase change. The molecules in a liquid are close together, and the forces between them are stronger than the forces between the more widely separated molecules of a gas. Work must therefore be done on the liquid against these attractive molecular forces in order to separate the molecules. The latent heat of vaporization is the amount of energy that must be added to the one kilogram of liquid to accomplish this separation. Similarly, at the melting point of a solid, the amplitude of vibration of the atoms about their equilibrium positions becomes great enough to allow the atoms to pass the barriers of adjacent atoms and move to their new positions. On average, these new positions are less symmetrical than the old ones and therefore have higher energy. The latent heat of fusion is equal to the work required at the molecular level to transform the mass from the ordered solid phase to the disordered liquid phase. The average distance between atoms is much greater in the gas phase than in either the liquid or the solid phase. Each atom or molecule is removed from its neighbors, overcoming the attractive forces of nearby neighbors. Therefore, more work is required at the molecular level to vaporize a given mass of a substance than to melt it, so in general the latent heat of vaporization is much greater than the latent heat of fusion (Table 11.2).

Basic structure of solids, liquids and gases The hotter they are, the faster they move. This can explain freezing/melting, boiling/condensing and lots more. It is called the Kinetic Theory of Matter. Kinetic is something to do with movement. Solid Liquid Gases Heater

Basic structure of solids, liquids and gases

Basic structure of solids, liquids and gases

Basic structure of solids, liquids and gases

Basic structure of solids, liquids and gases

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