Adiabatic processes are processes in which no heat is gained or lost in the working fluid. The term "adiabatic" describes things that are impermeable to heat transfer; for example, an adiabatic boundary is a boundary that is impermeable to heat transfer. An insulated wall approximates an adiabatic boundary condition. Another example is the adiabatic flame temperature, which is the temperature that would be achieved by a flame in the absence of heat loss to the surroundings. An adiabatic process which is also reversible is called an isentropic process.
The opposite extreme, in which the maximum heat transfer with its surroundings occurs, causing the temperature to remain constant, is known as an isothermal process.
There are three rates of adiabatic cooling for air.
- The ambient atmosphere lapse rate, which is the rate that air cools as one goes up in altitude.
- The dry adiabatic lapse rate, -10°C per 1000m rise.
- The wet adiabatic lapse rate, about -6° per 1000m rise.
Adiabatic cooling does not have to involve a fluid. One technique used to reach very low temperatures (thousandths and even millionths of a degree above absolute zero) is adiabatic demagnetisation, where the change in magnetic field on a magnetic material is used to provide adiabatic cooling.
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2 Graphing Adiabats
3 Adiabatic (quantum mechanics)
The mathematical equation for an adiabatic process is
where P is pressure, V is volume, and
being the molar specific heat for constant pressure and
being the molar specific heat for constant volume.
For a monatomic ideal gas, .
Derivation of Formula
When heat transfer to the system is zero, .
Then, according to the first law of thermodynamics,
It is desired to know how the values of and
relate to each other as the adiabatic process proceeds.It will now be assumed that the system is a monatomic gas, so that
Given and then
Graphing AdiabatsProperties of adiabats on a P-V diagram are:
(1) every adiabat asymptotically approaches both the V axis and the P axis (just like isotherms).
(2) each adiabat intersects each isotherm exactly once.
(3) an adiabat looks similar to an isotherm, except that during an expansion, an adiabat loses more pressure than an isotherm, so it has a steeper inclination (more vertical).
(4) if isotherms are concave towards the "north-east" direction (45 °), then adiabats are concave towards the "east north-east" (31 °).
(5) If adiabats and isotherms are graphed severally at regular changes of entropy and temperature, respectively (like altitude on a contour map), then as the eye moves outwards away from the axes (towards the north-east), it sees the density of isotherms stay constant, but it sees the density of adiabats drop. The exception is very near absolute zero, where the density of adiabats drops sharply and they become rare (see Nernst's theorem).
The following diagram is a P-V diagram with a superposition of adiabats and isotherms:
The isotherms are the red curves and the adiabats are the black curves. The adiabats are isentropic. Volume is the abscissa and pressure is the ordinate.
Adiabatic (quantum mechanics)
In quantum mechanics, an adiabatic change is a sufficiently slow change in the Hamiltonian which would result only in a change of eigenvalues, not eigenstates. Hence, if a system starts in the ground state, it will remain in the ground state of the system during the change, despite the fact that the properties of the ground state may change. If, in such a process, there is a qualitative change in the properties of the ground state (for example the spin), the change is called a quantum phase transition.