Question

In the carburetor of an aircraft or automobile engine, air flows through a relatively small aperture and then expands. In cool, foggy weather, ice sometimes forms in this aperture even though the outside air temperature is above freezing. Why?

Short answer

Due to the temperature drop, sometimes ice may form in the air venting aperture even if the outside air temperature is above freezing point.

$W=-∆U$

Step-by-step solution

Adiabatic process

In an adiabatic process, there is no heat exchange

$∆\underset{~}{O}=0$

According to the first law of thermodynamics, we have

$∆\underset{~}{O}=∆U+W$

Therefore, we get

$\begin{array}{l}0=∆U+W\\ W=-∆U\end{array}$

In the carburetor of an aircraft or automobile engine, air flows through a relatively small aperture and then expands. In this process, the air is expanded hence the system is doing work. Since this process is adiabatic, so there is no heat exchange,

Therefore, the work done by the system will decrease the internal energy of the system.

Therefore, there is a decrease in the temperature of the air Due to this temperature drop, sometimes ice may form in the air venting aperture even if the outside air temperature is above freezing point.

$W=-∆U$

Conclusion

Hence, due to the temperature drop, sometimes ice may form in the air venting aperture even if the outside air temperature is above freezing point.

$W=-∆U$