Trykforskellen

The term trykforskellen (literally "pressure difference" or "pressure differential") refers to the difference between two pressure values in a system. It determines the driving force for fluid motion and is a central concept in physics, engineering, and physiology.

Symbolically, ΔP = P1 − P2. The magnitude is measured in pascals (Pa) in the SI system, with common subunits such as kilopascals (kPa). In medical contexts, pressure differences are often expressed in millimeters of mercury (mmHg). Pressure readings can be absolute, gauge, or differential; the pressure difference itself is the driving force for flow regardless of the reference.

In fluids, ΔP drives flow. For viscous laminar flow through a cylindrical pipe, Hagen-Poiseuille's law states that Q = (π ΔP r^4) / (8 μ L). In general, flow rate is roughly proportional to ΔP and inversely related to resistance; this is analogous to Ohm's law in electrical circuits. Bernoulli's principle also relates ΔP to changes in fluid velocity and elevation, linking pressure differences to kinetic and potential energy in the fluid.

Applications span engineering and physiology. In engineering, ΔP moves liquids and gases through pipelines, HVAC systems, irrigation networks, and hydraulic machines. In biology and medicine, the heart maintains a pressure difference to drive blood through the circulatory system, and pressure gradients govern filtration and exchange across membranes. Measurement typically uses manometers or pressure transducers to determine ΔP between two points in a system.

Overall, the concept of trykforskellen is widely used wherever fluids or gases flow, because a difference in pressure is the fundamental driver of motion.