Two water columns of the same height but different volumes have ___.

• A. Different head pressures
• B. The head pressure is greater
• C. The same head pressure
• D. The head pressure is lower

Answer: (C)

Pressure in a static liquid comes from the weight of the liquid above a point, so it depends on how high the column is, not how much liquid there is. The hydrostatic pressure at depth h is P = ρgh (plus atmospheric pressure at the surface). If both water columns have the same height, same density, and the top is open to atmosphere, the pressure at the bottom must be the same. So the head pressure is identical. Changing the volume changes total water, not the pressure at a given depth. If the height were different, the head pressure would be different accordingly.