Physis 202, CH. 12 Book Notes
Physis 202, CH. 12 Book Notes PHYS 202
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This 6 page Class Notes was uploaded by Melissa on Thursday January 7, 2016. The Class Notes belongs to PHYS 202 at University of Oregon taught by Jenkins T in Fall 2015. Since its upload, it has received 18 views. For similar materials see General Physics >4 in Physics 2 at University of Oregon.
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Date Created: 01/07/16
Molecules bumping into each other creates pressure Molecules at a faster speed increase the temperature An increase in temperature increases the pressure which causes expansion because they are hitting the walls more frequently and with more force • 12.1 The atomic model of matter Phases ‣ Gas: system in which each particle moves freely through space until it collides with another particle or wall of its container ‣ Liquid: weak bonds permit motion while keeping the particles close together ‣ Solid: deﬁnite shape and can be compressed or deformed only slightly Atomic Mass and Atomic Mass Number ‣ Atomic mass number: sum of the protons and neutrons in the nucleus ‣ Molecular Mass: sum of the atomic masses of the atoms that form the molecule Deﬁnition of the Mole ‣ • Called Avogadro's number ‣ Molar mass: ‣ • Ex. 100g of oxygen and has molar mass of 32g/mol. How many moles are there? Volume ‣ Characteristic of a macroscopic system • 12.2 The atomic model of an ideal gas Gases are compressible because their particles are so spread out whereas in liquids and solids, the particles are either fairly close to each other or are in direct contact Temperature of an ideal gas is directly proportional to the average kinetic energy per atom ‣ ‣ ‣ ‣ ‣ Thermal energy is proportional to change in temperature Molecular Speeds and Temperature ‣ RMS speed: • The speed of an atom with the average kinetic energy • Also related to the speed • RMS speed is proportional to the square root of the temperature • Remember that changing x by a factor of c changes y by a factor of. ‣ Pressure • Collisions exert a force on wall they hit and net force causes gas to have a pressure • Area is proportional to the force exerted twice as big a circle means twice as many particles and thus double the force Pressure in a gas ‣ Be aware that pressure is not a force but a force creates the pressure Measured in pascals ‣ ‣ • A net pressure force is exerted only where there is a pressure diﬀerence between the two sides of a surface Ex. Force that holds the lid on a vacuum sealed jar where the pressure inside is less than the pressure outside ‣ To remove the lid, you have to exert a force greater than the force due to the pressure diﬀerence • Decreasing number of moles decreases the pressure From Collisions to Pressure and the Ideal Gas Law ‣ • Gas Law Video If you have a container with a constant volume and you make it extremely cold, what happens to the pressure? ‣ Pressure decreasing because the molecules are not moving as fast At a constant pressure, as the temperature increases the volume will also increase As you keep the temperature constant at room temperature but are decreasing the pressure, your volume will increase ‣ Inverse relationship • 12.3 Ideal Gas Processes Gas is a ﬁxed quantity ‣ In sealed containers the number of moles does not change so: • Final and initial states related by: PV diagrams ‣ Points represent the state of the gas Constant volume processes ‣ Gas in closes container: warming gas increases pressure but keeps volume the same so • On a graph it would be represented as a vertical line • Constant Pressure ‣ Called isobaric ‣ Found when there is a piston that is free to slide up and down which allows for the compression or expansion of gas until reaching • The equilibrium position where • If gas is heated in a cylinder, the pressure does not change because pressure is controlled by unchanging external pressure Will cause the piston to move up because molecules are moving so fast Represented as a horizontal line Constant Temperature ‣ Isothermal ‣ Occurs like in a situation where a container of gas is submerged in container of liquid that is at a constant temp • Causes piston to be pushed slowly which transfers heat and energy through the walls Isothermal compression ‣ Reverse is isothermal expansion ‣ There is an inverse relationship between P and V so graph is a hyperbola Thermodynamics of ideal gas processes ‣ When gases expand they are doing work by pushing a piston ‣ Work done in a isobaric process • • For gas to do work, volume must change • Equation only works with constant pressure processes • Work is positive if gas expands • Work is negative if gas is compressed or if energy is transferred out of the system • First law of thermodynamics can be written as • Thermal energy of ideal gas can be written as ‣ Adiabatic Processes • Remember Doing work on a gas increases the thermal energy • When Q= 0 for expansion or compression then it is adiabatic Temperature will decrease in adiabatic expansion Temp increases in adiabatic compression • Allows you to use work rather than heat to change temperature of a gas • 12.4 Thermal Expansion ‣ Equation for volume thermal expansion ‣ Beta is the coeﬃcient of volume expansion • Value depends on the material the object is made of Equation for linear thermal expansion : ‣ Alpha is the coeﬃcient of linear expansion Special Properties of Water and Ice ‣ As water approaches the freezing point, molecules begin to form clusters that are more strongly bound and thus get farther apart to form clusters so volume increases ‣ Water expands signiﬁcantly when making the transition from liquid to solid ‣ When becoming solid it becomes more dense • 12.5 Speciﬁc Heat and Heat of Transformation Amount of heat that raises the temp of 1kg of a substance by 1 K Heat needed to produce a temperature change with speciﬁc heat c: ‣ ‣ Q can be positive(temp goes up) or negative( temp goes down) ‣ Takes a large amount of heat energy to change the temperature of a substance with a large speciﬁc heat Phase changes ‣ Temperature is constant and does not increase when it is changing phases ‣ Only increases when something is being warmed or cooled ‣ Thermal Energy of a solid is the kinetic energy of the vibrating atoms plus the PE of stretched and compressed molecular bonds • When being heated, thermal energy gets large and this bonds break and move around: solid is melting ‣ When thermal energy is reduced or when a solid becomes liquid: melting pt • At the melting point, system is in phase equilibrium ‣ Temperature where liquid becomes a gas is the boiling point ‣ Gas to liquid: condensation Heat transformation ‣ During the phase changes, thermal energy continues to be added but only to break bonds, not to increase the temperature ‣ Heat transformation use the symbol : L • The amount of energy that causes a kg of a substance to undergo. Phase change • Heat of fusion( solid and liquid) and heat of vaporization ( liquid and gas) Evaporation ‣ Water evaporates as sweat below the boiling point of water (100*C) ‣ In gases: diﬀerent molecules have diﬀerent speeds and some will go into The gas phase and thus they take thermal energy with them • They have the highest KE so evaporation reduces the average KE and the temperature of liquid left behind • Sweating is also a sign of body exhausting excess heat • 12.6 Calorimetry The quantitative measurement of the heat transferred between systems or evolved in reactions Insulation prevents an heat energy from being transferred to or from the environment ‣ If energy enters the system, Q is positive ‣ Negative if it leaves the system ‣ • 12.7 Speciﬁc Heat of Gases Quantity of heat needed to change the temperature of n moles of gas by the change in temperature for constant volume processes: ‣ For constant pressure: ‣ Is the molar speciﬁc heat at constant volume while The molar speciﬁc heat for constant pressure ‣ If you heat a gas in a sealed container so that there is no change in volume, then no work is done ‣ If you heat gas with a piston to keep it at constant pressure, then the gas will cause it to expand and work is done ‣ Molar speciﬁc heats ar Higher for monatomic gases because they only have the translational KE and thus allows it to move faster when heated • While diatomic has rotational KE as well ‣ Regualar speciﬁc heat is greater for diatomic molecules • 12.8 Heat Transfer Conduction ‣ Transfer of thermal energy directly theough a physical material ‣ The faster moving molecules on the hotter end transfer energy to the slower molecules ‣ Q increases if temperature diﬀerence between hot and cold is increased ‣ Q increases if cross section is increased ‣ Q decreases if length of the rod is increases ‣ Rate of conduction • K is the thermal conductivity of the material Convection ‣ When the heat expands and becomes less dense than wate above it, it rises to the surface while cooler denser water sinks ‣ Transfer of thermal energy by the motion of a ﬂuid Radiation ‣ Where heat energy is transferred to your body ‣ Consists of electromagnetic waves ‣ Transfer energy from object that emits radiation to object that absorbs it ‣ Body absorbs heat from outside and radiates heat outward to get rid of it ‣ Rate of transfer by radiation ‣ • E is the emissivity or a ,easier of the eﬀectiveness of radiation; between 0 and 1 •
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