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A-Level Course Notes: PHYSICS SECTION III: Matter SECTION III Matter CIE A-Level [AS and A2] ________________________ Course Notes DIPONT Educational Resource - Science 1 A-Level Course Notes: PHYSICS SECTION III: Matter Syllabus Details______________________ DIPONT Educational Resource - Science 2 A-Level Course Notes: PHYSICS SECTION III: Matter 9. Phases of Matter [AS]______________________ Content 9.1 Density 9.2 Solids, liquids, gases 9.3 Pressure in fluids 9.4 Change of phase Learning outcomes_____________________________________ Candidates should be able to: (a) define the term density Density (kgm-3) = mass (kg) / volume (m3) (b) relate the difference in the structures and densities of solids, liquids and gases to simple ideas of the spacing, ordering and motion of molecules (c) describe a simple kinetic model for solids, liquids and gases Liquid Solid Gas Increasing Kinetic Energy • Fixed volume • Fixed shape • Molecules held in position by strong bonds • Molecules vibrate about fixed position • Higher temp = higher vibrations DIPONT Educational Resource - Science • Fixed volume • Shape of container • Molecules can vibrate and move but are held close together by strong bonds • Expands to fill container • Molecules can vibrate and move around freely • Only very weak bonds exist between molecule 3 A-Level Course Notes: PHYSICS SECTION III: Matter (d) describe an experiment that demonstrates Brownian motion and appreciate the evidence for the movement of molecules provided by such an experiment Smoke (oil droplets) Brownian Motion Path of one droplet Smoke (oil droplets) are seen to move randomly This motion is evidence that the air particles are also moving randomly and colliding with the smoke droplets The air particles cannot be seen but their motion can be understood by the smoke droplets which can be seen (e) distinguish between the structure of crystalline and non-crystalline solids with particular reference to metals, polymers and amorphous materials Crystalline Non - crystalline • regular repeating pattern • no regular repeating pattern Polymers Metals • Amorphous Amorphous Materials • no regular repeating pattern (eg glass) • poly-crystalline • Semi-crystalline DIPONT Educational Resource - Science 4 A-Level Course Notes: PHYSICS SECTION III: Matter (f) define the term pressure and use the kinetic model to explain the pressure exerted by gases Pressure (Pa) = Force (N) / Area (m2) Pa = Nm-2 Kinetic model • Molecule collides with wall • Momentum changes • Force on molecule from wall • Equal and opposite force on wall from molecule • This averages over time to a constant force on the wall • The force per unit area of the wall is the pressure Force on molecule Force on wall (g) derive, from the definitions of pressure and density, the equation p = ρgh (h) use the equation p = ρgh Liquid column Area = A Density = r Height = h Weight =F Hydrostatic Pressure Density: r=m/V Pressure: p = F/A Volume of column: V = Ah Weight of column: F = mg F = rghA h Hydrostatic pressure p = rgh A F This formula is given at the start of the test paper DIPONT Educational Resource - Science 5 A-Level Course Notes: PHYSICS SECTION III: Matter PHASE CHANGE Temperature (i) distinguish between the processes of melting, boiling and evaporation. B oiling PHASE CHANGE Condensing Melting Freezing SOLID LIQUID GAS Time Number At the phase changes the random kinetic energy of the molecules does not change, but the potential energy does. Intermolecular bonds are being broken which requires energy. Enough energy to Evaporate Energy A. At all temperatures there will be a distribution of kinetic energy within the liquid. B. Molecules with high kinetic energy can ‘escape’ the liquid and become a gas: Evaporation. C. The average speed of the molecules in the liquid will decrease: Therefore, the temperature of the liquid will decrease. DIPONT Educational Resource - Science 6 A-Level Course Notes: PHYSICS SECTION III: Matter 10. Deformation of Solids [AS]_________________ Content 10.1 Stress, strain 10.2 Elastic and plastic behaviour Learning outcomes_____________________________________ Candidates should be able to: a) appreciate that deformation is caused by a force and that, in one dimension, the deformation can be tensile or compressive Deformations caused by forces F F Tensile Deformation F Compressive Deformation F (b) describe the behaviour of springs in terms of load, extension, elastic limit, Hooke’s law and the spring constant (i.e. force per unit extension) No Force 10N Force 20N Force String F= k x Hooke’s Law: Up to the elastic limit the extension of a spring is proportional to the tension force. The constant of proportionality is called the spring constant (k) Extension / m Elastic Limit force / N DIPONT Educational Resource - Science 7 A-Level Course Notes: PHYSICS SECTION III: Matter SEE PHET SIM (c) define and use the terms stress, strain and the Young modulus Area = A F F Length = l0 Dl F F Tensile stress = force / area = F/A Tensile strain = extension / original length = Dl / l0 Young’s modulus = Tensile stress / Tensile strain E= [Nm-2] [Nm-2] (d) describe an experiment to determine the Young modulus of a metal in the form of a wire wire pulley 0 10 20 30 40 masses Using the apparatus above measure the extension of the wire with added masses DIPONT Educational Resource - Science 8 A-Level Course Notes: PHYSICS SECTION III: Matter (e) distinguish between elastic and plastic deformation of a material Elastic deformation: Material returns to its original shape when forces are removed Plastic deformation: Material does NOT return to its original shape when forces are removed (f) deduce the strain energy in a deformed material from the area under the forceextension graph Force / N Area under graph = Strain Energy Extension / m (g) demonstrate knowledge of the force-extension graphs for typical ductile, brittle and polymeric materials, including an understanding of ultimate tensile stress. Ductile Material Force Force Fracture Fracture Extension Extension Force Brittle Material Ultimate Tensile Stress Polymer Material Extension DIPONT Educational Resource - Science 9 A-Level Course Notes: PHYSICS SECTION III: Matter 11. Ideal Gases [A2]__________________________ Content 11.1 Equation of state 11.2 Kinetic theory of gases 11.3 Pressure of a gas 11.4 Kinetic energy of a molecule Learning outcomes_____________________________________ Candidates should be able to: (a) recall and solve problems using the equation of state for an ideal gas expressed as pV = nRT (n = number of moles) The equation of state for an ideal gas is PV = nRT P = Pressure V = Volume T = Temperature R = universal gas constant n= number of moles (b) infer from a Brownian motion experiment the evidence for the movement of molecules Smoke (oil droplets) Brownian Motion Path of one droplet Smoke (oil droplets) are seen to move randomly This motion is evidence that the air particles are also moving randomly and colliding with the smoke droplets The air particles cannot be seen but their motion can be understood by the smoke droplets which can be seen DIPONT Educational Resource - Science 10 A-Level Course Notes: PHYSICS SECTION III: Matter (c) state the basic assumptions of the kinetic theory of gases Kinetic model of ideal gas • Molecule collides with wall • Mo mentum changes • Force on molecule fro m wall • Equal and opposite force on wall fro m molecule • This averages over time to a constant force on the wall • The force per unit area of the wall is the pressure Assumptions: • Newton’s law apply to molecules • No intermolecular forces • The molecules are perfect spheres (treated as points) • The molecules are in random motion • The collisions between the molecules are elastic (no energy lost) • There is no time spent in these collisions DIPONT Educational Resource - Science Force on mo lecule Force on wall 11 A-Level Course Notes: PHYSICS SECTION III: Matter (d) explain how molecular movement causes the pressure exerted by a gas and hence deduce the relationship p = 1/3Nm/V < c 2 > (N = number of molecules) [a rigorous derivation is not required] L Cx Consider a cube of space with length L Consider a particle moving in one dimension x with velocity cx When the particle collides with the wall its velocity is reversed so its change in momentum is equal to... o Dpx = 2mcx The time between collisions with each wall of the cube is equal to... o Time between collisions = 2L / cx The rate at which momentum is transferred to the wall is... o Rate of change of momentum = 2mcx / (2L/cx) = mcx 2 / L If there are N particles in the cube the total force is... o Total force = Nmcx 2 / L Pressure is force over area so pressure is... o Pressure on one wall is Nmcx 2 / L3 L3 is the volume so... o Pressure = Nmcx 2 / V The average of cx 2 can be written as < cx 2> As all directions, x, y and z can be considered equal o < cx 2> = 1/3< c 2> Hence o P = 1/3Nm<c 2> / V This formula is given at the start of the test paper DIPONT Educational Resource - Science 12 A-Level Course Notes: PHYSICS SECTION III: Matter (e) compare pV = 1/ 3 Nm < c 2 > with pV = NkT and hence deduce that the average translational kinetic energy of a molecule is proportional to T. The average translational Ek of the particles can be expressed as ... o <Ek> = 1/2m< c2> Combining with P = 1/3Nm<c 2> / V we get.... o pV = 2/3N(1/2m< c2>) = 2/3N<Ek> Combining this with pV = NkT we get... o pV = 2/3N<Ek> = NkT o <Ek> =3/2kT Therefore, Temperature is proportional to Average translational kinetic energy 12. Temperature [A2]________________________ Content 12.1 Thermal equilibrium 12.2 Temperature scales 12.3 Practical thermometers Learning outcomes_____________________________________ Candidates should be able to: (a) show an appreciation that thermal energy is transferred from a region of higher temperature to a region of lower temperature (b) show an understanding that regions of equal temperature are in thermal equilibrium Temperature is a property that determines the direction of thermal energy transfer between two bodies in thermal contact. Thermal energy is transferred from ‘hot’ object to ‘cold’ object until they have reached the same temperature, this is thermal equilibrium. DIPONT Educational Resource - Science 13 A-Level Course Notes: PHYSICS SECTION III: Matter Property X (c) show an understanding that a physical property that varies with temperature may be used for the measurement of temperature and state examples of such properties t? = (Xt – XL) N + tL (XH – XL) N = Number of divisions between XH and XL XH = value of property at higher fixed point XL = value of property at lower fixed point tL tH XH Xt XL t? Temperature Example properties... Expansion of a liquid (mercury) Electrical resistance (d) compare the relative advantages and disadvantages of resistance and thermocouple thermometers as previously calibrated instruments Thermometer Resistance Thermocouple DIPONT Educational Resource - Science Advantage Wide range Over short ranges variation is linear Very sensitive Large sensitivity range Small device so rapid response Disadvantage Variation is not linear over long ranges, so needs calibration at a number of temps. Variation not linear 14 A-Level Course Notes: PHYSICS SECTION III: Matter (e) show an understanding that there is an absolute scale of temperature that does not depend on the property of any particular substance (i.e. the thermodynamic scale and the concept of absolute zero) Volume / m3 Constant pressure -273oC Temperature / oC Volume / m3 Absolute Zero 0K Temperature / K All gases have variations of volume with temperature that extrapolate back to –273oC at zero volume. This can be described as absolute zero as we can not imagine a –ve volume. This absolute zero forms the basis of the Kelvin scale where absolute zero is defined as 0 Kelvin (0K). (f) convert temperatures measured in kelvin to degrees Celsius and recall that T / K = T / °C + 273.15 T/K = t/oC + 273 DIPONT Educational Resource - Science 15 A-Level Course Notes: PHYSICS SECTION III: Matter 13. Thermal Properties of Materials [A2]________ Content 13.1 Specific heat capacity 13.2 Specific latent heat 13.3 Internal energy 13.4 First law of thermodynamics Learning outcomes_____________________________________ Candidates should be able to: (a) explain using a simple kinetic model for matter why • melting and boiling take place without a change in temperature Temperature is a measure of the average random EE of the particles At a phase transition supplied energy is used to break bonds No change in KE occurs so temperature does NOT change • the specific latent heat of vaporisation is higher than specific latent heat of fusion for the same substance Fusion is from a solid to a liquid and vaporisation from a liquid to a gas More bonds are broken in vaporisation so the specific latent heat is higher • a cooling effect accompanies evaporation As the particles which evaporate are those with a higher velocity and so KE the average KE of the substance decreases As temperature is a measure of average KE the temperature decreases DIPONT Educational Resource - Science 16 A-Level Course Notes: PHYSICS SECTION III: Matter (b) define and use the concept of specific heat capacity, and identify the main principles of its determination by electrical methods Thermal Capacity The energy required to raise the temperature of an object by 1K C= DQ DT DQ = change in energy DT = change in temperature (J K -1 ) Specific Heat Capacity The energy required to raise a unit mass of a substance 1K c= DQ m DT (J kg-1 K-1 ) Electrical Method Heater Object V A c= ItV Variable power supply m(T2-T1) DIPONT Educational Resource - Science 17 A-Level Course Notes: PHYSICS SECTION III: Matter (c) define and use the concept of specific latent heat, and identify the main principles of its determination by electrical methods Specific Latent Heat The amount of energy per unit mass absorbed or released during a change of phase l= DQ m (J kg-1) Vaporization A V l= ItV m1 – m2 Heater 00250.0g (d) relate a rise in temperature of a body to an increase in its internal energy (e) show an understanding that internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system Internal Energy = Total Potential Energy + Total Kinetic Energy Random Kinetic Energy = Translational Kinetic Energy + Rotational Kinetic Energy Translational energy is the energy associated with the whole molecule moving in a certain direction. Rotational energy is the energy associated with the molecule rotation around a certain point. Potential energy is the energy associated with intermolecular forces. DIPONT Educational Resource - Science 18 A-Level Course Notes: PHYSICS SECTION III: Matter (f) recall and use the first law of thermodynamics expressed in terms of the increase in internal energy, the heating of the system and the work done on the system. THERMODYNAMIC SYSTEM: For the study of ideal gases, the gas being considered is the system. THE SURROUNDINGS: Everything other than the gas is the surroundings. DQ = DU + D W DQ = Thermal energy given to system DU = Internal energy DW = Work done DQ DU DW +ve Thermal energy going into system -ve Thermal energy going out of system +ve Internal energy of system is increasing -ve Internal energy of system is decreasing +ve System is doing work -ve Surroundings doing work DIPONT Educational Resource - Science 19 A-Level Course Notes: PHYSICS SECTION III: Matter Background Reading_________________ PHYSICS, Giancoli 6th edition, Chapter 11 and 24 Useful Websites______________________ http://phet.colorado.edu/en/simulations/category/new http://www.s-cool.co.uk/alevel/physics.html http://www.physicsclassroom.com/mmedia/index.cfm http://www.phys.hawaii.edu/~teb/java/ntnujava/index.html http://www.colorado.edu/physics/2000/index.pl Constants___________________________ [These are given on each test paper] DIPONT Educational Resource - Science 20