Pressure

(noun)

the amount of force that is applied over a given area divided by the size of that area

Related Terms

Examples of Pressure in the following topics:

  • Gauge Pressure and Atmospheric Pressure

    • Pressure is often measured as gauge pressure, which is defined as the absolute pressure minus the atmospheric pressure.
    • 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.
    • Explain the relationship among absolute pressure, gauge pressure, and atmospheric pressure

  • Measurements: Gauge Pressure and the Barometer

    • In practice, pressure is most often measured in terms of gauge pressure.
    • Gauge pressure is the pressure of a system above atmospheric pressure.
    • Gauge pressure is much more convenient than absolute pressure for practical measurements and is widely used as an established measure of pressure.
    • Barometers are devices used to measure pressure and were initially used to measure atmospheric pressure.
    • Many modern pressure measuring devices are pre-engineered to output gauge pressure measurements.

  • Variation of Pressure With Depth

    • Pressure is defined in simplest terms as force per unit area.
    • However, when dealing with pressures exerted by gases and liquids, it is most convenient to approach pressure as a measure of energy per unit volume by means of the definition of work (W = F·d).
    • For liquids and gases at rest, the pressure of the liquid or gas at any point within the medium is called the hydrostatic pressure.
    • As a result, pressure within a liquid is therefore a function of depth only, with the pressure increasing at a linear rate with respect to increasing depth.
    • Equation 2 by itself gives the pressure exerted by a liquid relative to atmospheric pressure, yet if the absolute pressure is desired, the atmospheric pressure must then be added to the pressure exerted by the liquid alone.

  • Pressure in the Body

    • One such critical bodily system which relies on pressure for functionality is the circulatory system, which is an example of a closed fluid system under pressure.
    • The mean arterial pressure (MAP) is the average pressure over a cardiac cycle and is determined by , where CO is the cardiac outputs, SVR is the systemic vascular resistance, and CVP is the central venous pressure (CVP).
    • In practice, the mean arterial pressure (MAP) can be approximated from easily obtainable blood pressure measurements in , where Psys is the measured systolic pressure and Pdias is the measured diastolic pressure.
    • The mean arterial pressure (MAP) is the average pressure over a cardiac cycle and is determined this equation, where CO is the cardiac outputs, SVR is the systemic vascular resistance, and CVP is the central venous pressure (CVP).
    • In practice, the mean arterial pressure (MAP) can be approximated from easily obtainable blood pressure measurements.

  • Pressure

    • Pressure can be expressed in a number of units depending on the context of use .
    • The elementary mathematical expression for pressure is given by:
    • Just as a solid exerts a pressure on a surface upon which it is in contact, liquids and gases likewise exert pressures on surfaces and objects upon which they are in contact with.
    • Another common type of pressure is that exerted by a static liquid or hydrostatic pressure.
    • We will further discuss hydrostatic pressure in other sections.

  • Constant Pressure and Volume

    • An isobaric process occurs at constant pressure.
    • Since the pressure is constant, the force exerted is constant and the work done is given as PΔV.
    • An example would be to have a movable piston in a cylinder, so that the pressure inside the cylinder is always at atmospheric pressure, although it is isolated from the atmosphere.
    • If a gas is to expand at a constant pressure, heat should be transferred into the system at a certain rate.
    • Since pressure is constant, the work done is PΔV.

  • Pascal's Principle

    • Pascal's Principle states that pressure is transmitted and undiminished in a closed static fluid.
    • Qualitatively, Pascal's Principle states that pressure is transmitted undiminished in an enclosed static liquid.
    • As, by Pascal's Law, a change in pressure is linearly proportional to a change in height within an incompressible, static liquid of constant density, doubling the height between the two points of reference will double the change of pressure, while halving the height between the two points will half the change in pressure.
    • As a result of Pascal's Law, the pressure change (pressure applied to the static liquid) is transmitted undiminished in the static liquid so that the applied pressure is 2 N/m2 at the bottom of the bottle as well.
    • Furthermore, the hydrostatic pressure due to the difference in height of the liquid is given by Equation 1 and yields the total pressure at the bottom surface of the bottle.

  • Application of Bernoulli's Equation: Pressure and Speed

    • The kinetic energy of the fluid is stored in static pressure, $p_s$, and dynamic pressure, $\frac{1}{2}\rho V^2$, where \rho is the fluid density in (SI unit: kg/m3) and V is the fluid velocity (SI unit: m/s).
    • The SI unit of static pressure and dynamic pressure is the pascal.
    • Static pressure is simply the pressure at a given point in the fluid, dynamic pressure is the kinetic energy per unit volume of a fluid particle.
    • Thus, a fluid will not have dynamic pressure unless it is moving.
    • A streamline can be drawn from the top of the reservoir, where the total energy is known, to the exit point where the static pressure and potential energy are known but the dynamic pressure (flow velocity out) is not.

  • Origin of Pressure

    • In Newtonian mechanics, if pressure is the force divided by the area on which the force is exerted, then what is the origin of pressure in a gas?
    • What forces create the pressure?
    • Therefore the pressure of the gas is:
    • Pressure arises from the force exerted by molecules or atoms impacting on the walls of a container.
    • Real gases do not always behave according to the ideal model under certain conditions, such as high pressure.

  • Constant Pressure

    • Isobaric processis a thermodynamic process in which the pressure stays constant (at constant pressure, work done by a gas is $P \Delta V$).
    • An isobaric process is a thermodynamic process in which pressure stays constant: ΔP = 0.
    • Since the pressure is constant, the force exerted is constant and the work done is given as W=Fd, where F (=PA) is the force on the piston applied by the pressure and d is the displacement of the piston.
    • Specific heat at constant pressure is defined by the following equation:
    • A graph of pressure versus volume for a constant-pressure, or isobaric process.