Overview
Hydraulic brake systems use a incompressible fluid, such as brake fluid or oil, to transmit forces from one location to another within the fluid. Most automobiles use hydraulics in the braking systems.Pascal's law states that when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.
Hydraulic pressure is transmitted through liquid. Since liquid is effectively incompressible, pressure applied to a liquid is transmitted without loss throughout the liquid. In a braking system, this allows a force applied to the brake pedal to act upon the brakes at the wheels.
Hydraulic pressure can transmit increased force. Since pressure is force per unit area, the same pressure applied over different areas can produce different forces - larger and smaller.
This system has cylinders of different sizes. When the brake pedal is pressed, the force against the piston in the master cylinder applies pressure to the fluid. This same pressure is transmitted throughout the fluid, but it has a different effect on each piston in the other cylinders. The top cylinder is smaller than the master cylinder, so the force it exerts will be less than the force applied to the master cylinder. The middle cylinder is the same size as the master cylinder so the force from it will be the same too. The bottom cylinder is larger than the master cylinder, and so is its force.
Pressure
Pressure is the application of force to a surface, and the concentration of that force in a given area. 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, even though the force applied is the same, because the point concentrates that force into a smaller area.
More formally, pressure (symbol: p or P) is the measure of the normal component of force that acts on a unit area, see also stress (physics):
p = F/ A
where:
Often 'F' is taken to be the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not necessarily being a that of a body, but for example the cross sectional area of a conduit.
The gradient of pressure is force density.
Pressure is sometimes measured not as an absolute pressure, but relative to atmospheric pressure; such measurements are sometimes called gauge pressure. An example of this is the air pressure in a tire of a car, which might be said to be "thirty PSI", but is actually thirty PSI above atmospheric pressure. In technical work, this is often written as "30 PSIG" or, more commonly, "30 psig", though other methods which avoid attaching this information to the unit of pressure are preferred. 1
In the human body, pressure is measured by baroreceptors.
"Pressure is a scalar quantity, but teachers and authors do not appear to believe this in their hearts." (McClelland, 1987)
Scalar quantity
Let us look at a static gas; one that does not appear to move or flow. While the gas as a whole does not appear to move, the individual molecules of the gas, which we cannot see, are in constant random motion. Because we are dealing with a nearly infinite 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 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 a magnitude but no direction 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 to the surface.
Hydrostatic pressure
Hydrostatic pressure is the pressure due to the weight of a fluid.
where:
Units
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·s-2·m-1). This special name for the unit was added in 1971; before that, pressures in SI were expressed in units such as N/m²
Non-SI measures (still in use in some parts of the world) include the pound-force per square inch (PSI) and the bar.
The cgs unit of pressure is barye (ba). It is equal to 1 dyn·cm-2.
Pressure is still sometimes expressed in kgf/cm² or g/cm² (often as kg/cm² and g/cm² without properly identifying the force units). The technical atmosphere (symbol: at) is 1 kgf/cm².
In the United States air pressure is still measured in inHg — inches of mercury (as in the mercury barometer). Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, because it gives the same numbers as the older millibar (mbar).
The standard atmosphere (atm) is an established constant. It is approximately equal to typical air pressures at sea level and defined to be standard atmosphere = 101 325 Pa = 101.325 kPa = 1013.25 hPa.
Non-SI units presently or formerly in use include the following:
Atmospheres
Manometric units:
Customary and foot-pound-second units:
Non-SI metric units:
Source: Wikipedia
