Chapter-11 Continued…

11.4 Pressure Gauges

Pressure gauges are used to measure pressures.

Tire pressure

gauge

Atmospheric pressure is measured with a mercury barometer.

Blood pressure is measured with a sphygmomanometer.

Gauge Pressure

• Gauge pressure is the pressure measured by a pressure gauge.

• It is the difference between the absolute pressure and atmospheric pressure.

Gauge pressure = Absolute pressure - atmospheric

pressure

Atmospheric Pressure

Atmospheric pressure at sea level is 1.013 × 105 Pa, which is sufficient to crumple a can if the inside air is pumped out.

Mercury Barometer

Mercury Barometer

At sea level,

Height of mercury = h = 76 cm.

Atmospheric pressure = 76 cm of Hg.

(76 cm = 760 mm = 29.9 inch)

.10013.18.9600,1376.0 25

mNghPa

Blood pressure is measured with a sphygmomanometer.

For a young, healthy heart:

Systolic pressure = 120 mm of Hg

Diastolic pressure = 80 mm of Hg

Pressure at the peak of the beating cycle- Systolic.

Pressure at the low point of the beating cycle- Diastolic.

11.5 Pascal's Principle

Any change in the pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and the enclosing walls.

Car Brakes

Hydraulic Car Lift

.; 11

22

1

1

2

2 FAA

FAF

AF

Archimedes of Syracuse

(287BC-212BC)

Much of Archimedes fame comes from his relationship with Hiero, the king of Syracuse, and Gelon, Hiero's son.

At one time, the king ordered a gold crown and gave the goldsmith the exact amount of gold to make it.

When Hiero received it, the crown had the correct weight but the monarch suspected that some silver had been used instead of the gold.

Since he could not prove it, he brought the problem to Archimedes.

Eureka

One day while considering the question, "the wise one" entered his bathtub and recognized that the amount of water that overflowed the tub was proportional the amount of his body that was submerged.

This observation is now known as Archimedes' Principle and gave him the means to solve the problem.

He was so excited that he ran naked through the streets of Syracuse shouting "Eureka! eureka!" (I have found it!). The fraudulent goldsmith was brought to justice.

Buoyant Force

11.6 Archimedes' Principle

Any fluid applies a buoyant force to an object that is partially or completely immersed in it; the magnitude of the buoyant force equals the weight of the fluid that the object displaces:

Derivation

Consider a cylinder of height h is being held under the surface of a liquid.

Derivation

The fluid applies a downward force P1A to the top face of the submerged cylinder and an upward force P2A to the bottom face.

Consider a cylinder of height h is being held under the surface of a liquid.

Derivation

The fluid applies a downward force P1A to the top face of the submerged cylinder and an upward force P2A to the bottom face.

Consider a cylinder of height h is being held under the surface of a liquid.

Derivation

The fluid applies a downward force P1A to the top face of the submerged cylinder and an upward force P2A to the bottom face.

Consider a cylinder of height h is being held under the surface of a liquid.

gVF FB