Pressure drop is in what proportion to tubing length?

• A. Direct
• B. Inverse
• C. Square
• D. Cubic

Answer: (C)

The main idea is that pressure drop through a tube increases in direct proportion to its length. When flow rate and tube radius stay the same, each additional unit of length adds the same amount of frictional resistance, so the total pressure you must provide goes up linearly with length. In laminar flow, this shows up in the formula ΔP = (8 μ L Q) / (π r^4): the pressure drop scales with L (length) directly. So doubling the tubing length will double the pressure drop, all else equal. The reason the other options don’t fit is that they imply nonlinear scaling with length, but resistances in series add up linearly for a uniform tube under steady flow.