SC PHYS 201 - Pressure (2 pages)

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Pressure



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Phys 201 1nd Edition Lecture 23 Outline of Last Lecture I Review for Exam 3 Outline of Current Lecture II Properties of Materials A Solids B Gas C Liquids III Pressure A ATM B PSI C Gauge Pressure Current Lecture Properties of materials Solids Molecules in a solid are frozen into place They are tightly packed into a given form However these molecules within a solid can wiggle around which means that they do have kinetic energy However this energy is not measured the same way that we normally measure kinetic energy Gas Molecules in a gas are widely separated and move about freely Gas molecules can fill any container and their kinetic energy is still measured the standard way KE 5mV 2 When gas molecules hit the walls of the container they re in it is considered a collision The momentum of the molecules changes causing an impulse Remember that a change in momentum p is calculated as force times a change in time Unfortunately the mass of these molecules is very very small meaning that the momentum is very small as well Liquids Molecules in a liquid exhibit properties of both solid and gas molecules They are compacted like solid molecules but are not attached and therefore free to move around similar to gas molecules The kinetic energy and momentum of liquid molecules is very similar to that of gas molecules These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Pressure Pressure describes the force that liquid or gas molecules exert on the surfaces of their container Pressure is simply the force of the molecule divided by the area of the container This means that the unit for pressure is N m2 which is referred to as a Pascal Pressure does not have directionality which means that a uniform container such as a cube in equilibrium will have the same pressure on every surface Other ways to describe pressure Atmospheric pressure atm describes the pressure of the air above a certain point One unit of atmospheric pressure is equal to 1 01x10 5 Pascals Atms are best used when comparing large amounts of pressure The English unit for measuring pressure is based on the primary units for measuring weight and area in the United States which is pounds per square inch psi One Atm is equal to 14 7 psi Liquid in a container The mass of liquids in a container is equal to the density of the liquid times the volume of the container The volume of the container is equal to the area of the container times the length Gauge pressure For a liquid in a container the force of water due to pressure minus the weight of the water Pressure area mg Because the mass of a liquid is equal to density x area x height the area on both sides of the equation cancel out and we are left with the equation Pressure density x height x gravity This is referred to the gauge pressure which is due to the depth of the liquid


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