11.5.5 Adiabatic

Adiabatic literally means zero heat transfer, i.e. $\displaystyle Q=0$ .

If we’re talking about an adiabatic compression, then $\displaystyle {{W}_{{ON}}}>0$ since the volume decreases.

Applying the first law of thermodynamics,

$\displaystyle \displaystyle \overset{{}}{\mathop{{\Delta U}}}\,=\overset{0}{\mathop{Q}}\,+{{\overset{{\text{+ve}}}{\mathop{W}}\,}_{{ON}}}$

we can deduce that ΔU is positive. An adiabatic compression always results in an increase in temperature.

On the P-V diagram, an adiabatic compression is represented by a curve that climbs from a lower isotherm to a higher isotherm.

In theory, an adiabatic compression can be achieved by insulating a gas cylinder perfectly, and then compressing the gas. In practice, we can simply thrust the piston into the cylinder very quickly. For that short duration of time, the amount of heat transfer is negligible compared to the work done on the gas. So the process is practically adiabatic.

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Demonstration

Fire Syringe and Cloud in Bottle

Concept Test

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