Pressure variation by volume change – Simulator

Pressure variation by volume change - Simulator

By definition, pressure is a perpendicular force exerted on a unit area, one of the ways to change pressure is by force variation. Interact with the pressure variation simulator by volume variation to visualize this mechanics.

When you work on the piston of the simulator cylinder, reducing the volume of the chamber below it, the mass of gas confined within it remains the same and occupies a smaller volume which increases the pressure (drag and drop the cylinder piston and note that the compressed particles on the underside collide more often with the plunger as compared to the ones on the top).

The action of moving the plunger down increases the volume of the chamber above it, it is open and the surrounding gases (atmosphere) occupy the space that has formed, keeping the pressure constant (drag and release the plunger from the cylinder to the Side and note that the particles on the top continue to collide at the same frequency).

By definition:

Pressure is a perpendicular force exerted on a unit area.


The piston area remains constant (the diameter does not change). Increasing the pressure on the underside produces an increase in the upward force (drag and release the piston from the cylinder to the side and see the vector). The downward force remains constant.

By definition:

Force is the product of mass and acceleration.

F = m.a

Part of the major force, that of the lower part, is neutralized by canceling the minor force. The resulting force produces the acceleration that raises the plunger.By the principle of inertia the plunger would continue forever if no force stopped it.

It occurs that when the plunger traverses its initial position, the mass of gas within the cylinder becomes larger than the initial volume producing a partial vacuum and as a result the pressure and the force on the upper side of the piston gets higher than the bottom and pushes it back down to its starting position. Back to the balance of forces condition.

Drag and release the plunger

n the system above:

  • When the piston is stopped at its upper point the pressure inside the cylinder is the same as the pressure outside it;
  • The weight of the plunger is disregarded;
  • There is no friction between the plunger and the cylinder walls;
  • The compression is isothermal (constant temperature) this means that the kinetic energy of the piston is not ‘transferred’ converted into thermal energy;
  • There is no net exchange of thermal energy with the midst.

Terminology (vocabulary):

  • Piston: disc or cylinder that moves longitudinally (length) inside a cylinder
  • Perpendicular: two geometric entities (vector and plane) whose intersection forms four congruent angles, right angle (90 °)
  • Product: the result of a multiplication
  • Vector: it is the group of:

    1. Point of application,

    2. Intensity (or module),
    3. direction and

    4. way.

    • It is represented by an arrow-shaped signal.

Table of relation between units of pressure

pascal bar atmosfera técnica atmosfera torricelli libra por polegada quadrada
Pa bar at atm torr psi
1 Pa 1 N/m2 10-5 1,0197×10-5 9,8692×10-6 7,5006×10-3 145,04×10-6
1 bar 100.000 10dyn/cm2 1,0197 0,98692 750,06 14,5037744
1 at 98.066,5 0,980665 1 kgf/cm2 0,96784 735,56 14,223
1 atm 101.325 1,01325 1,0332 1 atm 760 14,696
1 torr 133,322 1,3332×10-3 1,3595×10-3 1,3158×10-3 1 mmHg 19,337×10-3
1 psi 6.894,76 68,948×10-3 70,307×10-3 68,046×10-3 51,715 1 lbf/in2
1 kPa 103 0,01 0,0102 0,0099 7,5006 0,145
1 MPa 106 10 10,1972 9,8692 7500,638 145,0377
1 mH2O 9806,3828 0,0981 0,1 0,0968 73,5541 1,4223
Eduardo Stefanelli

Engenheiro por profissão, professor por vocação