Everything about Adiabatic Process totally explained
» This article covers adiabatic processes in thermodynamics. For adiabatic processes in quantum mechanics, see adiabatic process (quantum mechanics). For atmospheric adiabatic processes, see lapse rate.
In
thermodynamics, an
adiabatic process or an
isocaloric process is a
thermodynamic process in which no
heat is transferred to or from the working
fluid. The term "adiabatic" literally means impassable (from Greek ἀ-διὰ-βαῖνειν, not-through-to pass), corresponding here to an absence of
heat transfer. For example, an
adiabatic boundary is a boundary that's impermeable to heat transfer and the system is said to be adiabatically (or thermally) insulated; an insulated wall approximates an adiabatic boundary. 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 that's
reversible is also called an
isentropic process. Additionally, an adiabatic process that's irreversible and extracts no work is in an
isenthalpic process, such as viscous drag, progressing towards a nonnegative change in entropy.
One opposite extreme—allowing heat transfer with the surroundings, causing the temperature to remain constant—is known as an
isothermal process. Since temperature is thermodynamically
conjugate to
entropy, the isothermal process is conjugate to the adiabatic process for reversible transformations.
A transformation of a thermodynamic system can be considered adiabatic when it's quick enough that no significant heat is transferred between the system and the outside. At the opposite extreme, a transformation of a thermodynamic system can be considered isothermal if it's slow enough so that the system's temperature remains constant by heat exchange with the outside.
Adiabatic heating and cooling
Adiabatic heating and cooling are processes that commonly occur from a change in the
pressure of a
gas.
Adiabatic heating occurs when the pressure of a gas is increased.
Diesel engines rely on adiabatic heating during their compression stroke to elevate the temperature sufficiently to ignite the fuel. Similarly
jet engines rely upon adiabatic heating to create the correct compression of the air to enable fuel to be injected and ignition to then occur.
Adiabatic heating also occurs in the
Earth's atmosphere when an
air mass descends, for example, in a
katabatic wind or
Foehn wind flowing downhill.
Adiabatic cooling occurs when the pressure of a substance is decreased as it does work on its surroundings. Adiabatic cooling doesn't 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. Adiabatic cooling also occurs in the
Earth's atmosphere with
orographic lifting and
lee waves, and this can form
pileus or
lenticular clouds if the air is cooled below the
dew point.
Rising magma also undergoes adiabatic cooling before eruption.
Such temperature changes can be quantified using the
ideal gas law, or the
hydrostatic equation for atmospheric processes.
It should be noted that no process is truly adiabatic. Many processes are close to adiabatic and can be easily approximated by using an adiabatic assumption, but there's always some heat loss.
There is no such thing as a perfect insulator.
Ideal gas (reversible case only)
The mathematical equation for an
ideal fluid undergoing a reversible (for example, no entropy generation) adiabatic process is
»
Graphing adiabats
An adiabat is a curve of constant entropy on the P-V diagram. Properties of adiabats on a P-V diagram are:
- Every adiabat asymptotically approaches both the V axis and the P axis (just like isotherms).
- Each adiabat intersects each isotherm exactly once.
- 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).
- If isotherms are concave towards the "north-east" direction (45 °), then adiabats are concave towards the "east north-east" (31 °).
- 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 towards the axes (towards the south-west), it sees the density of isotherms stay constant, but it sees the density of adiabats grow. 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 horizontal axis and pressure is the vertical axis.
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