Which term refers to the normal force per unit area at a small hole in a duct wall?

The correct answer is static pressure. Static pressure is a fundamental concept in fluid dynamics that describes the pressure exerted by a fluid at rest relative to a surface. At a small hole in a duct wall, static pressure measures the normal force applied by the fluid per unit area at that specific point. This pressure is independent of the fluid's velocity and represents the potential energy of the fluid.

In air conditioning and refrigeration systems, understanding static pressure is crucial because it impacts airflow, system efficiency, and overall performance. Engineers and technicians analyze static pressure to ensure systems operate within designed parameters, ensuring adequate airflow and comfort.

Dynamic pressure, while related to fluid flow and velocity, does not apply here, as it reflects the kinetic energy of moving fluid rather than the force exerted by static fluid at a given point. Differential pressure involves the difference between static pressures at two points, rather than the pressure at a single point. A static tap refers to the physical connection point used to measure static pressure, but does not define the pressure itself at the hole in the duct wall. Hence, static pressure accurately represents the concept in the context of the question.