E-StoreSTILL DOUBTS IN PRESSURE.......CLEAR IT NOW!!!
Pressure
MANY GUYS HAVE DOUBTS REGARDING PRESSURE.BASICALLY WHAT IT IS :A SCALAR , VECTOR ,OR BOTH…SO I PRESENT YOU THIS ARTICLE……
This article is about pressure in the physical sciences. For the psychological meaning, see Peer pressure. For the Billy Joel song, see Pressure (Billy Joel song). For the Paramore song, see Pressure (Paramore song). For Under Pressure, see Under Pressure (disambiguation).
Pressure (symbol: p or P) is the force per unit area applied to an object in a direction perpendicular to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
Definition
Pressure is an effect which occurs when a force is applied on a surface. The symbol of pressure is p (lower case). The upper case P is reserved for power.
Formula
Mathematically:
where:
p is the pressure,
F is the normal force,
A is the area.
Pressure is a scalar quantity, and has SI units of pascals; 1 Pa = 1 N/m2, and has EES units of psi; 1 psi = 1 lb/in2.
Pressure is transmitted to solid boundaries or across arbitrary sections of fluid normal to these boundaries or sections at every point. It is a fundamental parameter in thermodynamics and it is conjugate to volume.
In a static gas, the gas as a whole does not appear to move. The individual molecules of the gas, however, are in constant random motion. Because we are dealing with an extremely large number of molecules and because the motion of the individual molecules is random in every direction, we do not detect any motion. If we enclose the gas within a container, we detect a pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per unit area (the pressure) is the same. We can shrink the size of our "container" down to an infinitely small point, and the pressure has a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has magnitude but no direction sense associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular (at right angle) to the surface.
A closely related quantity is the stress tensor σ, which relates the vector force F to the vector area A via
This tensor may be divided up into a scalar part (pressure) and a traceless tensor part shear. The shear tensor gives the force in directions parallel to the surface, usually due to viscous or frictional forces. The stress tensor is sometimes called the pressure tensor, but in the following, the term "pressure" will refer only to the scalar pressure.
Types
Explosion or deflagration pressures
Explosion or deflagration pressures are the result of the ignition of explosive gases, mists, dust/air suspensions, in unconfined and confined spaces.
Negative pressures
While pressures are generally positive, there are several situations in which negative pressures may be encountered:
- When dealing in relative (gauge) pressures. For instance, an absolute pressure of 80 kPa may be described as a gauge pressure of -21 kPa (i.e., 21 kPa below an atmospheric pressure of 101 kPa).
- When attractive forces (e.g., Van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces. Negative pressure exists in the transpiration pull of plants.
- The Casimir effect can create a small attractive force due to interactions with vacuum energy; this force is sometimes termed 'vacuum pressure' (not to be confused with the negative gauge pressure of a vacuum).
- Depending on how the orientation of a surface is chosen, the same distribution of forces may be described either as a positive pressure along one surface normal, or as a negative pressure acting along the opposite surface normal.
- In the cosmological constant.
Stagnation pressure
Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible flow).
The pressure of a moving fluid can be measured using a Pitot tube, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressure.
Surface pressure
There is a two-dimensional analog of pressure – the lateral force per unit length applied on a line perpendicular to the force.
Surface pressure is denoted by π and shares many similar properties with three-dimensional pressure. 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.
Pressure of an ideal gas
In an ideal gas, molecules have no volume and do not interact. Pressure varies linearly with temperature, volume, and quantity according to the ideal gas law,
P =nRT/V
where:
p is the absolute pressure of the gas
n is the amount of substance (in mole)
T is the temperature (in kelvin)
V is the volume
R is the ideal gas constant.
Real gases exhibit a more complex dependence on the variables of state
SO DEAR GUYS HOPE I COULD SOLVE SOME OF YOUR DOUBTS AT LEAST ….PLS COMMENT ANT RATE
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