2/2/15 Oregon State University PH 212, Class #13 1

Tools that Use Effects of Static Fluid Pressure

Archimedes’ Principle:

･ All flotation devices

Pressure at depth:

･ Pressure gauges

･ Drinking straws and barometers

Pascal’s Principle:

･ Hydraulics

2/2/15 Oregon State University PH 212, Class #13 2

Pressure Gauges

Static pressure varies directly with the depth in an incompressible fluid (no matter the container shape). This makes it very handy to use in a pressure gauge, which measures pressure differences between two points, as indicated by a column of fluid.

Most gauges measure pressure against the most convenient reference—atmospheric pressure:

Pabs. = Pgauge + Patm.

or: P – Pgauge = Patm.

(If you don’t see the word “gauge” used, you may assume the pressure indicated is absolute pressure.)

2/2/15 Oregon State University PH 212, Class #13 3

Drinking Straws and Barometers

There is no such thing as “suction.” You cannot effectively move much fluid by “pulling” on it—you can only push on it.

When you drink from a straw, you are creating a low-pressure chamber in the straw, so the surrounding atmosphere pushes the liquid up the column. Thus, you can’t drink out of a straw on the moon—or any place where there is no atmospheric pressure.

The ultimate example of a drinking straw is the barometer, which indicates atmospheric pressure via a completely evacuated chamber above the fluid column; the pressure in there is essentially zero.

2/2/15 Oregon State University PH 212, Class #13 4

A swimming pool is filled 10.0 m deep with pure water. If the atmospheric pressure above a swimming pool then decreases by 1000 Pa, by how much does the pressure decrease on the bottom of the pool?

1. 970 Pa

2. 1000 Pa

3. 1970 Pa

4. 2000 Pa

5. None of the above

2/2/15 Oregon State University PH 212, Class #13 5

Hydraulics

A very powerful tool involving static fluid pressure results from Pascal’s Principle:

“If you change the pressure at any point in a trappedcontainer of fluid, that pressure change will occurthroughout the entire container—on all of the fluidand the walls of the container.”

2/2/15 Oregon State University PH 212, Class #13 6

The power of Pascal’s Principle becomes apparent when you artificially “press on” a container of liquid. That added pressure instantly transmits to all points in the container. But since pressure is the ratio of force to area, the fluid can thus transmit an enormous output force from a small input force. It all depends on the area over which that force acts. This is how hydraulic systems operate.

The same pressure (same depth in a fluid) can allow a small piston pushed by a relatively small force to support a large platform bearing a large load: P1 = P2

F1/A1 = F2/A2

F2 = F1(A2/A1)

2/2/15 Oregon State University PH 212, Class #13 7

HW4 hints

2h:

5b:

6f: