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51.3: Pressure

Last updated

Feb 3, 2024
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- 51.2: Floating Bodies
- 51.4: Change in Fluid Pressure with Depth

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Pressure $P$ is defined to be force divided by the area over which that force is applied:

$P=\frac{F}{A}$

For a fluid, imagine placing a small area $A$ inside the fluid. Then the pressure at the location of $A$ is the force due to the fluid on one side of $A$ on the fluid on the other side of $A$ , divided by the area $A$ .

Pressure in SI units is measured in Pascals ( $\mathrm{Pa}$ ), named for the French mathematician and physicist Blaise Pascal. One pascal is equal to $1 \mathrm{~N} / \mathrm{m}^{2}$ . Other common units are:

atmospheres $(1 \mathrm{~atm}=101,325 \mathrm{~Pa})$
torr ( 1 torr $=1 \mathrm{mmHg}=133.3223684210526315789 \mathrm{~Pa})$
$\operatorname{bar}(1 \mathrm{bar}=100,000 \mathrm{~Pa} ; 1$ millibar $=100 \mathrm{~Pa})$
pounds per square inch $(\mathrm{psi})(1 \mathrm{psi}=6894.757293168361336723 \mathrm{~Pa})$
inches of mercury $(1 \mathrm{inHg}=3386.388157894736842105 \mathrm{~Pa})$
dynes per square centimeter $\left(\right.$ dyne $\left./ \mathrm{cm}^{2}\right)\left(1\right.$ dyne $\left./ \mathrm{cm}^{2}=0.1 \mathrm{~Pa}\right)$

The pressure $P$ is sometimes called the absolute pressure; this is to distinguish it from the gauge pressure $P_{g}$ , which is the difference between absolute pressure and atmospheric pressure $P_{a}: P_{g}=P-P_{a}$ .

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