Concept of pressure
Pressure is the perpendicular force per unit area, or the stress at a point within a confined fluid. The pressure exerted on a floor by a 42-pound box the bottom of which has an area of 84 square inches is equal to the force divided by the area over which it is exerted; i.e., it is one-half pound per square inch. The weight of the atmosphere pushing down on each unit area of Earth’s surface constitutes atmospheric pressure, which at sea level is about 15 pounds per square inch. In SI units, pressure is measured in pascals; one pascal equals one newton per square metre. Atmospheric pressure is close to 100,000 pascals.
The pressure exerted by a confined gas results from the average effect of the forces produced on the container walls by the rapid and continual bombardment of the huge number of gas molecules. Absolute pressure of a gas or liquid is the total pressure it exerts, including the effect of atmospheric pressure. An absolute pressure of zero corresponds to empty space or a complete vacuum.
Measurement of pressures by ordinary gauges on Earth, such as a tire-pressure gauge, expresses pressure in excess of atmospheric. Thus, a tire gauge may indicate a pressure of 30 pounds (per square inch), the gauge pressure. The absolute pressure exerted by the air within the tire, including atmospheric pressure, is 45 pounds per square inch. Pressures less than atmospheric are negative gauge pressures that correspond to partial vacuums.
Atmospheric pressure
Atmospheric pressure, also called barometric pressure, force per unit area exerted by an atmospheric column (that is, the entire body of air above the specified area). Atmospheric pressure can be measured with a mercury barometer (hence the commonly used synonym barometric pressure), which indicates the height of a column of mercury that exactly balances the weight of the column of atmosphere over the barometer. Atmospheric pressure is also measured using an aneroid barometer, in which the sensing element is one or more hollow, partially evacuated, corrugated metal disks supported against collapse by an inside or outside spring; the change in the shape of the disk with changing pressure can be recorded using a pen arm and a clock-driven revolving drum.
Atmospheric pressure is expressed in several different systems of units: millimetres (or inches) of mercury, pounds per square inch (psi), dynes per square centimetre, millibars (mb), standard atmospheres, or kilopascals. Standard sea-level pressure, by definition, equals 760 mm (29.92 inches) of mercury, 14.70 pounds per square inch, 1,013.25 × 103 dynes per square centimetre, 1,013.25 millibars, one standard atmosphere, or 101.325 kilopascals. Variations about these values are quite small; for example, the highest and lowest sea-level pressures ever recorded are 32.01 inches (in the middle of Siberia) and 25.90 inches (in a typhoon in the South Pacific). The small variations in pressure that do exist largely determine the wind and storm patterns of Earth.
Near Earth’s surface the pressure decreases with height at a rate of about 3.5 millibars for every 30 metres (100 feet). However, over cold air the decrease in pressure can be much steeper because its density is greater than warmer air. The pressure at 270,000 metres (10−6 mb) is comparable to that in the best man-made vacuum ever attained. At heights above 1,500 to 3,000 metres (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.
Applications of atmospheric pressure
Drinking Straw 
When drinking with a straw, one has to suck the straw. This causes the pressure in hte straw to decrease. The external atmospheric pressure, which is greater, will then act on the surface of the water in the glass, causing it to rise through the straw.
Rubber Sucker
When the rubber sucker is put onto a smooth surface, usually a glass or tiled surface, the air in the rubber sucker is forced out. This causes the space between the surface and the sucker to have low pressure. The contact between the rubber sucker and the smooth surface is airtight. The external atmospheric pressure, which is much higher, acts on the rubber sucker, pressing it securely against the wall.
Vacuum Cleaner
vacuum cleaner applies the principle of atmospheric pressure to remove dust particles. When it is switched on, the fan sucks out the air from space inside the vacuum. Space A then becomes a partial vacuum. The atmospheric pressure outside, which is greater, then forces air and dust particles into the filter bag. This traps the dust particles but allows the air to flow through an exit ath the back.
Lift Pump
A lift pump is used to pump water out of a well or to a higher level. The greatest height to which the water can be pumped is 10 m. This is equivalent to the atmospheric pressure. When the plunger is lifted, the upper valve closes and the lower valve opens. The atmospheric pressure, acting on the surface of the water, causes water to flow past valve B into the cylinder. When the plunger is pushed down, the lower valve closes and the upper valve opens. Water flows above the plunger.When the plunger is next lifted, the upper valve closes again and the lower valve opens once more. the atmospheric pressure, acting on the surface of the water, forces water past the lower valve into the cylinder. Simultaneously, the water above the plunger is lifted and flows out through the spout. This process is repeated until sufficient water is obtained.
Hydraulic pressure
Hydraulic pressure is the pressure of hydraulic fluid which it exerts in all direction of a vessel, hose or anything in which it is supposed to exert the force per unit area. This pressure is responsible to create flow in a hydraulic system as fluid flows from high pressure to low pressure. Thus energy is transferred in a hydraulic system through a fluid medium. This pressure is measured in kgf/cm sq , PSI, Bar etc.
Applications of hydraulic pressure
Hydraulic Lifts and Fluid Power
Blaise Pascal derived a law that explains how people can harness the power of fluids. When you apply pressure to liquid in a confined container, that pressure transmits equally to all other points in the container. According to the law, it’s also possible for a hydraulic system to multiply forces. For instance, a hydraulic arm uses these principles to help you hoist thousands of pounds using your hands. You press down to apply a small force to one part of the jack’s fluid, and the force multiples enough to lift a car.
Hydraulic Braking
You witness hydraulics in action every time you ride in a vehicle or see one pass; car braking systems are among the most common uses of hydraulic machines. A vehicle’s braking system has several critical components, and one of them comes in a bottle or can. Brake fluid, a hydraulic liquid, is so important that brake systems could fail without it. When you press your foot on a brake pedal, a piston and rod in a master cylinder move. This movement exerts force on hydraulic fluid constrained inside brake lines. Because of Pascal’s law, the pressure moves through the lines, presses against another cylinder and causes the vehicle’s brake shoes and pads to contact the disc or drum and slow the wheels down.
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