Mercury rather than water is used in the manometer for arterial blood pressure. Mercury is 13.6 times denser than water, so what is the height of its column compared with a water column at the same pressure?
AIt becomes 13.6 times taller
BIt stays exactly the same height
CIt becomes 136 times shorter
DIt becomes 13.6 times shorter
Answer & Solution
Correct answer: D. It becomes 13.6 times shorter
1. The pressure supported by a liquid column is h rho g, so for a fixed pressure the height varies inversely with the density.
2. Mercury has 13.6 times the density of water.
3. Its column is therefore 1 divided by 13.6 as tall, that is 13.6 times shorter.
4. A taller mercury column would need mercury to be the lighter liquid, which reverses the physics.
5. A short column is exactly why mercury is chosen for pressures as large as arterial blood pressure.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement_
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