chapter 13 states of matter
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Chapter 13 States of Matter. Quiz 13. Chapter 13 Objectives. Describe how fluids create pressure and relate Pascal's principle to some everyday occurrences Apply Archimedes' and Bernoulli's principles - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 13 States of Matter
Quiz 13
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Chapter 13 Objectives
• Describe how fluids create pressure and relate Pascal's principle to some everyday occurrences
• Apply Archimedes' and Bernoulli's principles • Explain how forces within liquids cause
surface tension and capillary action, and relate the kinetic model to evaporation and condensation
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Chapter 13 Objectives
• Compare solids, liquids, gases, and plasmas at a microscopic level, and relate their properties to their structures
• Explain why solids expand and contract when the temperature changes
• Calculate the expansion of solids and discuss the problems caused by expansion
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Pressure
• Pressure is a result of gas molecules crashing into things.
• As the molecules collide with objects (such as walls of a container) they impart a force onto the object.
• Momentum is conserved as the objects bounce off and the force is directly related to the time of contact and change in momentum.
Ftvmvm 2211
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Pressure: Temperature Influence
• Temperature is an indicator of the amount of random kinetic energy or molecules.
• As temperature goes up, so does the velocity of the molecules as they crash into the walls.
• Temperature up: Increase in pressure
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Pressure: Volume Influence
• Pressure is also related to density (which is a combination of volume and mols).
• As density decreases, the molecules are more spread out and have less chance of colliding with the walls, which results in lower pressure.
• Density up: Pressure up
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Pressure
• Pressure is measured in many things, but the SI unit is Pascals: Newtons per meters2
• Question: Estimate (to an order of magnitude) the pressure on your feet to the ground.
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PV = NkT (Don’t Write))
This is the way physicists’ describe gas pressure and volume.
• This describes the Microscopic form of gas, where as PV=nRT describes the macroscopic form of gas
• N = number of molecules • k = Boltzmann’s constant = 1.38 E-23 J/K• V = Meters Cubed (not Liters)• P = Pascals
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Other Gas Laws (know these)
• The PV = NkT is actually the combined gas law, and often not necessary to use, but always gets the right answer regardless
• Charles Law = • Gay-Lussac’s Law = • Boyle’s Law
• Avogadro’s =
TV TP
VP 1
NP
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Question
• A certain jar is closed. The volume of the jar is 22.4 L and the temperature inside the jar is 273 K.
• What happens to the pressure if– Temp is raised?– More gas is added?– Volume decreases?
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Question
• Before going on a trip, you check the tire pressure of your car wheels. The pressure reads 31.0 lb/in2 (214 kPa) at a temperature of 15.0 C. After driving for a few hours, you stop and take the pressure of the tires again. The pressure reads 35.0 lb/in2 (241 kPa). What is the temperature of the air in the tires now (assume no loss of air or change in volume)?
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Follow up Question
• Suppose you then released some air, to keep the pressure between 28-32 psi. If you brought the psi down to 31 again, what will the gauge pressure read when the tires cool back down to 15.0 C?
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Kinetic Energy and Temperature
• Temperature again, is average random kinetic energy. Rolling a ball down a bowling lane does increase the total amount of kinetic energy, but it does not increase the temperature (other than resistance).
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Kinetic Energy (don’t write equation)
• The average kinetic energy of gas molecules is proportional to the absolute temperature of the gas.
• Total kinetic energy equals Boltzmann constant times Kelvin
kTKTr 23
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Maxwell Boltzmann Graph
• A probability graph of what the average kinetic energy of each molecule is.
• If all molecules are the same, then the X-axis can become velocity.
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Maxwell Boltzmann Graph
• Some water evaporates before 100 C because temp is an average with a huge range of motion
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Maxwell Boltzmann Graph
• The area under the graph is a fraction of how many molecules are moving this fast at the given temperatures. Low Temperatures have a very high peak, but low average velocity.
• High temperatures are much more rounded but extend further on the X-axis.
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Average* Speed (don’t write eq)
• The “average speed” of the gas molecules can be described by the following equation.
• m is mass of molecule (kg)• Vrms = not quite the same as average speed.
RMS refers to square root of the mean of the speed squared, and is generally higher than average
mkTvrms3
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Average speed
• What has a larger average speed? Argon (molar mass 40) or Nitrogen (molar mass 28)?
• Which molecule would have a larger Maxwell Boltzmann range, Argon or Nitrogen?
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Archimedes principle
• Mass of water displaced equals the buoyant force of the water– If 10 grams of water are displaced by a boat, the
boat is lifted with a bouyant force of 10 grams– If your density is less than 1, you can think of the
density being a percentage of what is under the water.• Density of 0.9 = 90% of volume under the water
– Mercury 13x more dense, 13 times the lift
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Thermal Expansion of Liquids/Solids
• When molecules have more energy, they take up more room.
• The range of jiggling is much larger. As temperature goes up, objects increase in size due to the increase in temperature.
• Very noticeable for gases, less noticeable for solids and liquids, but still important for some projects.
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Thermal Expansion
• Coefficient of Thermal Expansion – Alpha: A value which states: How much does the
material change with temperature? • Change in T is in Kelvin, Change in Length over
initial Length
TLL
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Relevance to Engineers
• Thermostats have two different metals connected to one another. (Bimetallic)
• As temperature changes, one expands faster than the other, causing it to bend (turn on and off switch)
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Relevance to Engineers
• Important for Roads, as joints are left on bridges to expand during warm weather– But not so far apart
that when it gets cold they lose contact…
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Relevance to those scared of the dark…
• House creaks during the night as thermal contraction takes place
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Relevance to Engineers
• St. Louis Arch
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Some Alpha Values (all E-6)
• Brick 1.0• Glass (Pyrex) 3.25• Granite 8.0• Glass (most types) 9.4• Iron/Steel/Cement 12.0• Copper 16.0• Brass 19• Lead 29
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Questions
• A brass rod of length 10.0cm is at an intial temperature of 20 C. If the temperature increases to 50 C, by what length will the brass rod change?
• Two steel rods of length 70.0 m each are separated by 1.0cm at an initial temperature of 273 K. At what temperature do the two steel rods touch?
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Question
• Two steel beams, one of length 10.0 m and the other of length 20.0 m are at the same temperature. Which beam, if either, will have the larger change in length if temperature changes?
• Molecularly, what are a few factors which influence the alpha value? How do they affect the expansion?
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Area and Volume Expansion
• If length expands and contracts under temperature changes, then since area and volume are affected by length, they will also expand.
• Area Expansion
• Volume Expansion
TAA
2
TVV
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Area and Volume Expansion
• Liquids and Gases do not have alpha values (for linear expansion)
• For Solids, their beta values are 3 times the size of their alpha values (due to 3 dimensional expansion)
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Questions
• A 30cm by 30cm aluminum foil sheet is at an initial temperature of 300 K. What is the new area of the aluminum foil if the temperature decreases by 20 K?
• When is the best time to fill up your car with gasoline? Why?
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Gasoline Question
• By what percentage does a change in 20 C (roughly 40 F) make in the volume of 1.0 gallons of fuel? Is filling up in the morning worth it? Beta value of Gasoline = 950 E-6
– (also think about temperature difference below surface of the earth)
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Question
• Water has two beta values: – 0 Degrees C = - 68– 20 Degrees C = 207
• Explain the difference and what is going on.
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Capillary Action
• The ability of a molecule to pull itself up (through electromagnetic interaction)
• Cohesion: Attractive force to oneself• Adhesion: Attractive force to others
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Solid/Liquid/Gas
• Solid Liquid Gas– Strong IMF Middle Weak IMF– Strong Bond Weak Bond No Bond– Fixed position Mobile Position No set – Equal amounts of kinetic energy at same temp– Condensed state of matter Expanded– Little adjustable volume Largely adjustable