Why does fluids exert pressure

The difference in height of the fluid between the input and the output ends contributes to the total force exerted by the fluid. For a hydraulic press, the force multiplication factor is the ratio of the output to the input contact areas.

Pressure is often measured as gauge pressure, which is defined as the absolute pressure minus the atmospheric pressure.

An important distinction must be made as to the type of pressure quantity being used when dealing with pressure measurements and calculations. Atmospheric pressure is the magnitude of pressure in a system due to the atmosphere, such as the pressure exerted by air molecules a static fluid on the surface of the earth at a given elevation.

In most measurements and calculations, the atmospheric pressure is considered to be constant at 1 atm or , Pa, which is the atmospheric pressure under standard conditions at sea level. Atmospheric pressure is due to the force of the molecules in the atmosphere and is a case of hydrostatic pressure.

Depending on the altitude relative to sea level, the actual atmospheric pressure will be less at higher altitudes and more at lower altitudes as the weight of air molecules in the immediate atmosphere changes, thus changing the effective atmospheric pressure.

Atmospheric pressure is a measure of absolute pressure and can be affected by the temperature and air composition of the atmosphere but can generally be accurately approximated to be around standard atmospheric pressure of , Pa.

In this equation p 0 is the pressure at sea level , Pa , g is the acceleration due to gravity, M is the mass of a single molecule of air, R is the universal gas constant, T 0 is the standard temperature at sea level, and h is the height relative to sea level. Pressure and Height : Atmospheric pressure depends on altitude or height. For most applications, particularly those involving pressure measurements, it is more practical to use gauge pressure than absolute pressure as a unit of measurement.

Gauge pressure is a relative pressure measurement which measures pressure relative to atmospheric pressure and is defined as the absolute pressure minus the atmospheric pressure. Most pressure measuring equipment give the pressure of a system in terms of gauge pressure as opposed to absolute pressure.

For example, tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. For most working fluids where a fluid exists in a closed system, gauge pressure measurement prevails. Pressure instruments connected to the system will indicate pressures relative to the current atmospheric pressure.

The situation changes when extreme vacuum pressures are measured; absolute pressures are typically used instead. To find the absolute pressure of a system, the atmospheric pressure must then be added to the gauge pressure. While gauge pressure is very useful in practical pressure measurements, most calculations involving pressure, such as the ideal gas law, require pressure values in terms of absolute pressures and thus require gauge pressures to be converted to absolute pressures.

Barometers are devices used for measuring atmospheric and gauge pressure indirectly through the use of hydrostatic fluids. In practice, pressure is most often measured in terms of gauge pressure. Gauge pressure is the pressure of a system above atmospheric pressure. Since atmospheric pressure is mostly constant with little variation near sea level, where most practical pressure measurements are taken, it is assumed to be approximately , Pa.

Modern pressure measuring devices sometimes have incorporated mechanisms to account for changes in atmospheric pressure due to elevation changes. Gauge pressure is much more convenient than absolute pressure for practical measurements and is widely used as an established measure of pressure. However, it is important to determine whether it is necessary to use absolute gauge plus atmospheric pressure for calculations, as is often the case for most calculations, such as those involving the ideal gas law.

Pressure measurements have been accurately taken since the mids with the invention of the traditional barometer. Barometers are devices used to measure pressure and were initially used to measure atmospheric pressure. Early barometers were used to measure atmospheric pressure through the use of hydrostatic fluids.

Hydrostatic based barometers consist of columnar devices usually made from glass and filled with a static liquid of consistent density. The columnar section is sealed, holds a vacuum, and is partially filled with the liquid while the base section is open to the atmosphere and makes an interface with the surrounding environment.

As the atmospheric pressure changes, the pressure exerted by the atmosphere on the fluid reservoir exposed to the atmosphere at the base changes, increasing as the atmospheric pressure increases and decreasing as the atmospheric pressure decreases.

This change in pressure causes the height of the fluid in the columnar structure to change, increasing in height as the atmosphere exerts greater pressure on the liquid in the reservoir base and decreasing as the atmosphere exerts lower pressure on the liquid in the reservoir base.

The height of the liquid within the glass column then gives a measure of the atmospheric pressure. Pressure, as determined by hydrostatic barometers, is often measured by determining the height of the liquid in the barometer column, thus the torr as a unit of pressure, but can be used to determine pressure in SI units.

Hydrostatic based barometers most commonly use water or mercury as the static liquid. While the use of water is much less hazardous than mercury, mercury is often a better choice for fabricating accurate hydrostatic barometers. The density of mercury is much higher than that of water, thus allowing for higher accuracy of measurements and the ability to fabricate more compact hydrostatic barometers.

In theory, a hydrostatic barometer can be placed in a closed system to measure the absolute pressure and the gauge pressure of the system by subtracting the atmospheric pressure.

Another type of barometer is the aneroid barometer, which consists of a small, flexible sealed metal box called an aneroid cell. The aneroid cell is made from beryllium-copper alloy and is partially evacuated. A stiff spring prevents the aneroid cell from collapsing. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction is amplified by mechanical mechanisms to give a pressure reading.

Such pressure measuring devices are more practical than hydrostatic barometers for measuring system pressures. Many modern pressure measuring devices are pre-engineered to output gauge pressure measurements. While the aneroid barometer is the underlying mechanism behind many modern pressure measuring devices, pressure can also be measured using more advanced measuring mechanisms.

Hydrostatic Column Barometer : The concept of determining pressure using the fluid height in a hydrostatic column barometer. A liquid doesn't have this long-range order: pushing down on a block of water just makes the water shear to the side.

On a microscopic level, you can't push the atoms together only in one direction because directional correlation decays fast; all you can do is push them together in general. Then the liquid responds by pushing out in all directions too. Your example with a bin of baseballs is in between the two cases, but I think it's closer to a solid. Most of the balls are locked in place by the weight of the balls above, making a lattice.

If you pop a small hole in the side of the bin, the balls won't flow out, they're jammed. Even in zero gravity fluids will exert an equal pressure on all walls. In zero gravity the balls would continue to remain stationary without exerting any force on the surroundings. So just thinking in terms of basketballs isn't enough. The best answer I can give is extends on what knzhou said, liquids do not have a preferred direction. Sign up to join this community.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Now fill half of plastic bottle with water. On filling water, the rubber sheet tied to the mouth of glass tube gets stretched and bulges out. The bulging out of rubber sheet tied to the glass tube fixed in the wall of plastic bottle demonstrates that water present in plastic bottle exerts pressure on the walls of the bottle.

It is the sideways pressure exerted by water which inflates the thin rubber sheet forming a buldge. If we pour more water in the plastic bottle to increase depth, we will see that the bulge in the rubber sheet increases. This indicates that the pressure exerted by water increases with increasing depth. Activity: A liquid exerts equal pressure at the same depth. Make 2 small holes of equal size on the two opposite sides of the plastic bottle some distance above the bottom of the bottle.

The holes should be at exactly the same height from the bottom of the plastic bottle. Now fill the bottle with water. The two jets of water coming out of the two holes fall at the same distance away from the base on its either side. The two jets of water can fall at equal distance on the two sides of the bottle only if the pressure of water at the depth of 2 holes in the bottle is equal.

The formation of fountains of water from the leaking pipes of water supply pipeline tells us that water exerts pressure on the walls of its container. University and has many years of experience in teaching.

She has started this educational website with the mindset of spreading Free Education to everyone. All the information given by u are very helpful and beneficial, helped me in my hhw. Thankyou very much.

I have added few diagrams in the above post. Hope it will help u to understand the topic more clearly. Liquids and gases are fluids. A fluid is able to change shape and flow from place to place. The atmosphere exerts a pressure on you, and everything around you. You may have seen a demonstration of the effects of this atmospheric pressure. The Magdeburg hemispheres are two metal cups that fit together. If most of the air is removed from inside them using a vacuum pump, it is almost impossible to pull them apart again.