Pressure
Pressure is the amount of force applied perpendicular to the surface of an object per unit area.The usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, and on writing style.The minus sign comes from the convention that the force is considered towards the surface element, while the normal vector points outward.[5] It is defined as a derivative of the internal energy of a system:[6] where: The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m2, or kg·m−1·s−2).This name for the unit was added in 1971;[7] before that, pressure in SI was expressed in newtons per square metre.But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is deprecated in SI.Pressure is related to energy density and may be expressed in units such as joules per cubic metre (J/m3, which is equal to Pa).Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar).Similar pressures are given in kilopascals (kPa) in most other fields, except aviation where the hecto- prefix is commonly used.The water-based units still depend on the density of water, a measured, rather than defined, quantity.Differential pressure is expressed in units with "d" appended; this type of measurement is useful when considering sealing performance or whether a valve will open or close.Presently or formerly popular pressure units include the following: As an example of varying pressures, a finger can be pressed against a wall without making any lasting impression; however, the same finger pushing a thumbtack can easily damage the wall.If the flat edge is used, force is distributed over a larger surface area resulting in less pressure, and it will not cut.Whereas using the sharp edge, which has less surface area, results in greater pressure, and so the knife cuts smoothly.This confinement can be achieved with either a physical container of some sort, or in a gravitational well such as a planet, otherwise known as atmospheric pressure.In the case of planetary atmospheres, the pressure-gradient force of the gas pushing outwards from higher pressure, lower altitudes to lower pressure, higher altitudes is balanced by the gravitational force, preventing the gas from diffusing into outer space and maintaining hydrostatic equilibrium.The walls of the container can be anywhere inside the gas, and the force per unit area (the pressure) is the same.[15] According to the theory of general relativity, pressure increases the strength of a gravitational field (see stress–energy tensor) and so adds to the mass-energy cause of gravity.This effect is unnoticeable at everyday pressures but is significant in neutron stars, although it has not been experimentally tested.The concepts of fluid pressure are predominantly attributed to the discoveries of Blaise Pascal and Daniel Bernoulli.Properties of surface chemicals can be investigated by measuring pressure/area isotherms, as the two-dimensional analog of Boyle's law, πA = k, at constant temperature.According to the ideal gas law, pressure varies linearly with temperature and quantity, and inversely with volume:Thus, except for small changes produced by temperature, the density of a particular liquid is practically the same at all depths.A person will feel the same pressure whether their head is dunked a metre beneath the surface of the water in a small pool or to the same depth in the middle of a large lake.If the fish swims to the bottom, the pressure will be greater, but it makes no difference which vase it is in.[24] Mathematically, it is described by Bernoulli's equation, where velocity head is zero and comparisons per unit volume in the vessel areAn experimentally determined fact about liquid pressure is that it is exerted equally in all directions.Pressure also acts upward, as demonstrated when someone tries to push a beach ball beneath the surface of the water.[25] This is why liquid particles' velocity only alters in a normal component after they are collided to the container's wall.[25] As predicted by Torricelli's law this is the same speed the water (or anything else) would have if freely falling the same vertical distance h.
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