_____ differences drive flow.

_____ differences drive flow.

• A. Pressure
• B. Temperature
• C. Viscosity
• D. Density

Answer: (A)

Flow is driven by pressure differences. A higher pressure somewhere pushes fluid toward regions of lower pressure, creating motion along a gradient in pressure. This pressure-driven push is the direct force underlying most fluid flows, such as water moving through a pipe where a pump raises pressure upstream so the fluid moves toward lower pressure downstream. Temperature differences can set up flow through buoyancy or convection, but they don’t act as the primary driving force in the same universal way as a pressure gradient. Viscosity describes how resistant the fluid is to deformation and flow—it modulates how much pressure drop is needed to sustain a given flow, rather than initiating the flow itself. Density differences can also cause motion via buoyancy in a gravitational field, but again that’s a secondary mechanism compared to the immediate push from pressure differences.

Flow is driven by pressure differences. A higher pressure somewhere pushes fluid toward regions of lower pressure, creating motion along a gradient in pressure. This pressure-driven push is the direct force underlying most fluid flows, such as water moving through a pipe where a pump raises pressure upstream so the fluid moves toward lower pressure downstream. Temperature differences can set up flow through buoyancy or convection, but they don’t act as the primary driving force in the same universal way as a pressure gradient. Viscosity describes how resistant the fluid is to deformation and flow—it modulates how much pressure drop is needed to sustain a given flow, rather than initiating the flow itself. Density differences can also cause motion via buoyancy in a gravitational field, but again that’s a secondary mechanism compared to the immediate push from pressure differences.