Any change of a condition of gas is considered to be thermodynamic process. At the same time the simplest processes proceeding in ideal gas are called isoprocesses. At isoprocess the mass of gas and one more its parameter (pressure, temperature or volume) remain constants, the others change.
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1. Isoprocess in which pressure remains to constants is called isobaric. The existing dependence between the volume of gas and its temperature with a constant pressure of this gas was established by practical consideration to the French scientists L. Gey Lyussakom in 1808. It showed that the volume of ideal gas with a constant pressure increases with increase of temperature. In other words, the volume of gas is directly proportional to its temperature on condition of constant pressure.
2. The dependence described above was expressed in a formula: Vt = V0(1 + αt) where V0 is gas volume at a temperature of zero degrees, Vt is gas volume at a temperature of t which is taken on Celsius scale, α – the size of thermal coefficient of volume expansion. Absolutely for all gases α = (1/273 °C-1). So, Vt = V0(1 + (1/273) t). From here, t = (Vt - V0) / ((1/273)/V0).
3. Substitute basic data in this formula and count value of temperature with a constant pressure for ideal gas.
4. Pay attention that the received result is fair only for ideal gas. Real gases are subordinated to this dependence only in rather rarefied state, that is when indicators of pressure of air and its temperature have no critical value at which gas liquefaction process begins. Pressure of the majority of gases at the room temperature changes from 10 to 102 atmospheres.
5. Graphically represent dependence of temperature, pressure and volume of air. So, the schedule of dependence of volume and temperature will look in the form of a straight line which leaves T=0 point. This straight line is called an isobar.