pressure

Physical terms

open 9 entries with the same name

Collection

zero Useful+1

zero

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Physical pressure refers to the force acting on the contact surface of two objects, or Gas about solid and liquid The vertical force on the surface, or the vertical force of the liquid on the solid surface. (The force between objects acting vertically on the surface of an object due to mutual extrusion is called pressure.) For example, the force of a football on the ground, the force of an object on a slope, and the force of a hand on a wall.

The direction of pressure is Perpendicular to the contact surface and pointing to the pressed object (Note: the meaning of "vertical" is different from that of "vertical") Contact And occurs Mutual extrusion 。^[1]

Chinese name: pressure
Foreign name: pressure
expression: F=pS
Applicable fields: Dynamics
Applied discipline: Physics

Action point: On the surface of the pressed object
Direction: Perpendicular to the contact surface and pointing to the pressed object
Conditions: Objects contact and squeeze each other
Force applying object: Supported objects
Stressed object: Compressed object
Nature: elastic force

catalog

seven Pressure and gravity
eight Relevant explanations

Generation mechanism

Announce

edit

U-shaped pressure gauge (3 sheets)

The pressure on the solid surface is usually Elastic deformation The result of is generally Contact force 。 The pressure on the surface of liquid and gas is usually gravity and Molecular motion Results. The direction of action of the pressure is usually perpendicular to the contact surface of the object. If it is observed that the acting direction of the pressure is not perpendicular to the contact surface, it is usually due to the pressure and friction The result of joint action.

characteristic

Announce

edit

1. The action direction is perpendicular to the action area and opposite to the outer normal direction of the action area;

2. When the pressure is constant, the smaller the force area is, the more significant the pressure effect is;

3. When the force area is fixed, the greater the pressure, the more significant the pressure effect.

Calculation formula

Announce

edit

Pressure=pressure/stress area (p=F/S);

Pressure=pressure × stress area (F=pS);

Stress area=pressure/pressure (S=F/p).

Main units

Announce

edit

pressure

International unit: "Newton", referred to as "cow", symbol“ N ”。

pressure

International unit: "pascal", short for "pascal", symbol "Pa";

Conversion: 1 Pa=1N/㎡;

one Megapascal (MPa)=145 pounds/ Square inch (psi)=10.2 kgf/ square centimetre (kgf/c ㎡)=10 bar=9.8 atmospheric pressure (at m)

1 pound/inch2 (psi)=0.006895 MPa=0.0703 kg/cm2=0.0689 bar=0.068 atm;

1 bar=0.1 MPa=14.503 lb/ Square inch (psi)=1.0197 kg/cm2 (kg/c ㎡)=0.987 atmospheric pressure (atm);

1 Atmospheric pressure (at m)=0.101325 MPa=14.696 psi=1.0333 kg/cm2=1.0133 bar;

1 mm mercury (mmHg)=133.33 Pa;

The conversion relations of several common pressure units in the project are as follows:

1MPa=10 six Pa≈145psi≈10.2kgf/cm²

1kgf/cm²=98.067kPa≈98kPa

1psi（1bf/in²）=6.8948kPa≈6.9kPa

1mmH two O=9.8067Pa≈9.8Pa

Fluid pressure

Announce

edit

The stress state of fluid at any point can be expressed by stress tensor. The stress tensor has 9 components in total, expressed as:

weight

among σ x , σ y , σ z The volume elements of the rest of the fluid x，y，z Normal stress in direction;

Is 6 shear stresses.

Static fluid and ideal fluid

If the fluid is at rest or in motion but the fluid is ideal (that is, its viscosity can be ignored), then the six shear stress components are equal to zero, only σ x ,σ y, σ z Not equal to zero. σ x ,σ y, σ z The positive direction is the external normal of the force bearing surface, and the positive direction of the pressure is exactly the opposite, which can prove that: σ x = σ y = σ z =－ p That is, the pressure of static fluid or ideal fluid is equal to the negative value of positive stress in any direction.

Viscous fluid

For a moving viscous fluid, the six shear stress components will not be equal to zero at the same time. At this time, under the condition that the Stokes hypothesis (see Viscosity) holds, the pressure is defined as:

That is, the pressure is equal to the average value of three normal stresses at the investigated point. It can be proved that it is also the average value of normal stress in all directions at the inspection point.

The unit force (1 gram force) acting on 1 square centimeter is usually taken as the unit of pressure. However, it is too small in practical application. The pressure unit used in industry is kgf/cm. The atmospheric pressure at sea level is p A =1 033 g/cm two , which is close to the pressure unit used in industry. International System of Units The pressure unit in (SI) is Pascal, 1 Pascal=1 N/m2.

Relation with temperature and volume

The pressure of fluid is closely related to temperature and volume. For example, ideal gas obeys the following equation of state:

pv=nRT，

In the formula p Is the pressure; υ Is the volume of gas per unit mass; T by Thermodynamic temperature ； R Is the gas constant, equal to 287.14 m/(s two ·On).

Fallible understanding

Announce

edit

one
Pressure is a kind of elastic force associated with the deformation of objects.
two
Pressure occurs between two objects that touch and squeeze against each other. Pressure is a kind of contact force. It is impossible to generate pressure between any two objects that are separated from each other, and there is no squeezing effect between the objects that are in contact with each other.
three
The direction of pressure is always perpendicular to the contact surface of the object and points to the object being pressed.
four
Gravity can make objects produce pressure, but pressure is not gravity, and pressure is sometimes independent of gravity.

Pressure and gravity

Announce

edit

(1) Pressure occurs when two objects in contact squeeze each other deformation According to the nature of force, pressure is elastic force; Gravity is caused by the attraction of the earth to objects near the ground, which belongs to gravity.

(2) The direction of pressure is not fixed, but it is always perpendicular to the contact surface of the force bearing object. (Because the contact surface may be horizontal, vertical or inclined) Gravity has a fixed direction, always vertically downward.

(3) Pressure can be generated by or independent of gravity. When the object is placed on a horizontal plane and there is no other external force, the pressure is equal to the gravity. When an object is placed on a slope, the pressure is less than gravity. When an object is pressed on a vertical surface, the pressure is sometimes independent of gravity.^[1] When an object is lifted and pressed against the ceiling, gravity weakens the effect of pressure.

(4) The point of action of pressure is on the force surface of the object, the point of action of gravity is on the center of gravity of the object, and the center of gravity of regular and uniform geometry is on the geometric center of the object.

Comparison of pressure and gravity
project	pressure	gravity
Force applying object	Supported objects	earth
Stressed object	Compressed object	Object itself
Direction of force	Perpendicular to the contact surface and pointing to the pressed object	Vertical Down
Force	It is related to the degree of extrusion
Nature of force	elastic force	gravitation

A force can deform an object. For example, use a stick to squeeze the dough from each corner. It can be seen that when the stick leaves, there are pits left on the dough. The force that causes the dough to sag is pressure.

Many disciplines often call pressure pressure and pressure total pressure. At this time, the pressure does not represent the force, but the force acting vertically on the unit area of the object. In engineering, the terms pressure and pressure are sometimes not strictly distinguished, but they are definitely not a physical quantity in specific calculation. This term is wrong or at least non-standard. They are scalar and vector, and are not the same. Just when you read engineering books or documents, you should know that there is such a name. How to distinguish it depends on the unit it uses.

Relevant explanations

Announce

edit

1. Atmospheric pressure: Air column Pressure due to gravity. It and the Altitude , latitude and meteorological conditions.

2. Differential pressure (pressure difference): the relative difference between two pressures.

three Absolute pressure : All pressures in the space where the medium (liquid, gas or steam) is located.

Absolute pressure is relative Zero pressure Pressure.

four Gauge pressure (Relative pressure): If the difference between absolute pressure and atmospheric pressure is a positive value, then this positive value is the gauge pressure, that is, gauge pressure=absolute pressure - atmospheric pressure>0.

5. Negative pressure（ vacuum meter Pressure): corresponds to "gauge pressure". If the difference between absolute pressure and atmospheric pressure is a negative value, then this negative value is negative pressure, that is, negative pressure=absolute pressure - atmospheric pressure<0.

6. Static pressure: generally understood as "the pressure that does not change with time, or the pressure that changes slowly with time, that is, the gauge pressure value measured in the fluid is not affected by the flow rate".

7. Dynamic pressure: corresponding to "static pressure", "pressure that changes rapidly with time, namely dynamic pressure It refers to the kinetic energy of fluid per unit volume. " Usually 1/2 ρν two calculation. Where ρ - Fluid density ； V - fluid Movement speed 。”

8. Gas pressure: commonly mentioned gas pressure perhaps atmospheric pressure In fact, the value of pressure per unit area is taken. For the application of this concept in physics and thermodynamics, see pressure 。

Novice on the road

Growth task Getting Started with Editing Edit Rule Edit by myself

I have questions

Content query Online Service Official post bar Feedback

Complaints and suggestions

Report bad information Failed to appeal through entries Complaint of infringement information Blocking query and unblocking

Jinggong Network Anbei No. 1100000200001