Saturday, December 28, 2019
Isochoric Process Definition and Use
An isochoric process is a thermodynamic process in which the volume remains constant. Since the volume is constant, the system does no work and W 0. (W is the abbreviation for work.) This is perhaps the easiest of the thermodynamic variables to control since it can be obtained by placing the system in a sealed container which neither expands nor contracts. First Law of Thermodynamics To understand the isochoric process, you need to understand the first law of thermodynamics, which states: The change in a systems internal energy is equal to the difference between heat added to the system from its surroundings and work done by the system on its surroundings. Applying the first law of thermodynamics to this situation, you find that: delta-Since delta-U is the change in internal energy and Q is the heat transfer into or out of the system, you see that all of the heat either comes from internal energy or goes into increasing the internal energy. Constant Volume It is possible to do work on a system without changing the volume, as in the case of stirring a liquid. Some sources use isochoric in these cases to mean zero-work regardless of whether there is a change in volume or not. In most straightforward applications, however, this nuance will not need to be consideredââ¬âif the volume remains constant throughout the process, it is an isochoric process. Example Calculation The websiteà Nuclear Power, a free, nonprofit online site built and maintained by engineers, gives an example of a calculation involving the isochoric process. Assume anà isochoric heat additionà in an ideal gas. In anà ideal gas, molecules have no volume and do not interact. According to theà ideal gas law,à pressureà varies linearly withà temperatureà and quantity, and inversely withà volume. The basic formula would be: pV nRT where: pà is the absolute pressure of the gasnà is the amount of substanceTà is the absolute temperatureVà is the volumeRà à is the ideal, or universal, gas constant equal to the product of the Boltzmann constantà and the Avogadro constantK is the scientific abbreviation forà Kelvin In this equation the symbol R is a constant called theà universalà gas constantà that has the same value for all gasesââ¬ânamely, R à 8.31à Joule/moleà K. The isochoric process can be expressed with the ideal gas law as: p/T constant Since the process isà isochoric,à dVà 0, theà pressure-volume work is equal to zero. According to theà ideal gas model, the internal energy can be calculated by: âËâ U m cvà âËâ T where the propertyà cvà (J/mole K)à is referred to asà specific heatà (orà heat capacity) at a constant volume because under certain special conditions (constant volume) it relates the temperature change of a system to the amount of energy added by heat transfer. Since there is no work done by or on the system, theà first law of thermodynamicsà dictatesà âËâ U âËâ Q.à Therefore: Q à m cvà âËâ T
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