Thermodynamics Temperature and Heat

Thermodynamics Introduction: Thermodynamics Thermodynamics is the branch of physics that studies the exchange of energy between the system and the environment with the aim of identifying the conditions in which the system is in equilibrium or transforms itself. Temperature and Heat Heat and temperature are well-defined properties and in close relation with the structure of matter. While in solids the constituent particles can oscillate around fixed positions, in liquids and gases the particles are animated by a chaotic motion called thermal agitation. The energy transfer from a warmer to a colder object is called heat or thermal energy. Heat is an energy in transit that passes from one object to another. Heat transmission occurs spontaneously only from a higher temperature object to a lower temperature object. Temperature is measured by means of instruments called thermometers. The thermometer commonly used is the clinical one, formed by glass and mercury. The thermometer is a level transducer, it provides an indication of the temperature of mercury by measuring the level of a liquid. The most commonly used thermometric scales are the Celsius scale and the Kelvin scale. In the International System of Units, Kelvin scale is used. Heat, being a form of energy, is measured in joules. When heat is supplied to an objects, the resultant temperature increase depends on the mass and material of which it is made. The specific heat of a substance is defined as the amount of heat to be supplied to the unit of mass of the substance to raise its temperature by 10°. Thermal Expansion Thermal expansion: as the temperature increases, the volume of the objects increases. We can distinguish between linear expansion, area expansion and volume expansion. The effect of thermal expansion is a change in object volume that increases with increasing temperature. Linear thermal expansion: We speak of linear thermal expansion when the object will mainly lengthen when the temperature rises. Example: a copper wire because its length is not negligible. Alpha (a) is the coefficient of linear thermal expansion. LO is the length of the object before it expands. AT is the temperature change. Area thermal expansion: We speak of area thermal expansion when the object will mainly extend when the temperature rises. Example of an objects: a metal axis because its surface is not negligible. The coefficient of area expansion is equal to twice the coefficient of thermal expansion. Sigma (σ) is the coefficient of area expansion. SO is the area of the object before it expands. AT is the temperature variation. Volume expansion: We speak of volume expansion when an objects expands in all directions when the temperature rises. The coefficient of volumic dilatation is equal to three times the coefficient of thermal dilatation. Beta (β) is the coefficient of volumetric expansion. VO is the volume of the object before it dilates. AT is the temperature variation Changes in the State of Matter The three main states of the matter are: Solid State, Liquid State and Gas State. The three states are distinguished by the interaction between the molecules that constitute a substance. If the interaction is strong the substance will tend to the solid, if weak a liquid, if the molecules are far enough away from each other, the matter will be gaseous. N.B.: solids have a higher density of liquids and gases. The solids and liquids maintain their volume and are incompressible at pressures that are not very high. Gases, on the other hand, can be compressed or expanded by varying the pressure Here are the status steps: Fusion: solid → liquid Solidification: liquid → solid Evaporation: liquid → gas Condensation or liquefaction: gas liquid Sublimation: solid → gas Deposition: gas → solid Stirling cycle The Stirling cycle is a thermodynamic cycle that describes the operation of a class of generating or operating machines. The Stirling engine is also called an external combustion engine. The cycle is reversible, meaning what can be achieved by generators, where the application of heat and a cold source can be obtained mechanical energy, or applying vice versa mechanical energy can be obtained hot or cold in separate locations. The Stirling Cycle is a closed cycle, i.e. the fluid that carries out the cycle is permanently contained in the apparatus that carries out the cycle, which is therefore not exchanged with the outside. The cycle is divided into four phases: 1 - Isothermal expansion. The expansion chamber is heated from the outside and the gas contained in it has an isothermal expansion. 2 - Transfer of hot gas at a constant volume, the gas passes through the regenerator, releasing part of the heat to the regenerator. 3 - Isothermal compression, the fluid in the compression space is cooled. 4 - Heat transfer at a constant volume; the fluid flows back through the regenerator, recovering the heat from the regenerator itself. There is less difficulty in practicing the reverse cycle, i.e. producing a pulsation of the confined fluid through the administration of mechanical energy, thus obtaining an expansion site (therefore a cooling) and a compression site (therefore a heating). This is done in the Stirling refrigeration machine. The Perfect Gas The perfect gas is a rather rarefied gas, whose temperature is far above the liquefaction temperature. Perfect Gas obeys 3 Laws: Boyle's Law Boyle's Law: it analyses transformations in which no temperature variation occurs. It says that the product between pressure and volume remains constant - Gay-Lussac's first law Gay-Lussac's first law: in a transformation in which the pressure remains constant (isobar), the volume of a gas increases (or decreases) as its temperature increases (or decreases). VO is the initial volume, a is the coefficient of volumetric expansion, and t is the temperature variation. - Gay-Lussac's second law Gay-Lussac's second law: in a transformation in which the volume remains constant (isochoras), the pressure of a gas increases (or decreases) as its temperature increases (or decreases). p0 is the initial pressure, a is the coefficient of volumetric expansion, and t is the temperature variation.