Final answer:
The correct answer to the question about air entering a convergent-divergent nozzle is that the pressure decreases as it enters the nozzle due to the Bernoulli effect. This effect also explains how water can emerge from a nozzle against atmospheric pressure thanks to its kinetic energy overcoming the pressure difference.
Step-by-step explanation:
Regarding the student's question about air entering a convergent-divergent nozzle, the correct answer is a) The pressure decreases as it enters the nozzle. This is due to the Bernoulli effect, which in fluid dynamics states that as the velocity of a fluid increases, the pressure exerted by that fluid decreases, and this is seen as air moves through a nozzle. If air enters a nozzle at a pressure of 18×10⁵ Pa, the pressure will decrease as the fluid accelerates in the converging part of the nozzle.
The Bernoulli effect can also be applied to water flowing through a hose and nozzle. For example, when water with a flow rate of 40.0 L/s goes from a 9.00-cm-diameter fire hose into a 3.00-cm-diameter nozzle, the cross-sectional area of the flow decreases, which results in an increase in velocity and a corresponding decrease in pressure according to Bernoulli's principle and the continuity equation.
With regards to the student's potential confusion on pressures in the nozzle as related to the Bernoulli effect and atmospheric conditions, it is important to clarify that the pressure inside the nozzle can be less than atmospheric; however, owing to the kinetic energy of the water, it can still emerge against atmospheric pressure. This is because the kinetic energy of the water resulting from its increased velocity is used to overcome the atmospheric pressure opposing the water flow out of the nozzle.