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Lecture 1 particle view of matter

  1. The Particle View of Matter
  2. Everything is made of atoms • Only about 100 varieties (about 90 naturally occurring). • Everything is made from a combination of these 100 elements. • Think of them as the ingredients that go into a recipe where each substance we know has it’s own recipe. Image from wikipedia
  3. What are the properties of a gas? The atmosphere is a gas. Does it: • Have weight? How do you know? What do you notice when you move quickly? When the wind blows? What’s going on when a sailboat moves? • What can we say about the particles that make up the atmosphere? Is air solid? Can air be compressed?
  4. In a SCUBA tank, 100 cubic feet of air can be compressed into ½ cubic foot (the air is compressed by a factor of 200). What do the particles in the air look like? In the tank? What occupies the space between the particles?
  5. Consider a balloon full of air: what happens when it is heated? • Why does this happen? • What is happening with the particles that make up the air? • How does increasing the temperature affect the particles that make up the air? HOT PLATE
  6. How do we know that the molecules in air are moving? • What happens when you open a perfume? The smell spreads until eventually the whole room picks up the scent. How can we model this?
  7. • Consider four adjacent boxes • Place 10 air particles in each box – Assume that on any given turn 50% of the particles in a box will move one box to the left, and 50% will move one box to the right. – Particles that would hit a side wall stay in their box. etc.
  8. Consider the same 4 boxes with 32 ‘perfume’ particles in box 1 at the start. • As time passes, perfume particles spread. • Over time, they become more uniform throughout the space.
  9. Boyle’s Law Stated mathematically: P1 * V1 = P2 * V2 Practically, what does this mean? • If you have a sealed container of gas and you decrease the volume the observed pressure increases. An example...balloon in a vacuum
  10. Balloon Demo • What will happen to the balloon when we reduce the pressure around it? • Why will this happen?
  11. Demo: Marshmallow in a vacuum • A marshmallow consists of many little bubbles of air within a sugary, stretchy matrix. • At normal pressure the bubbles have their normal volume. This P times this starting V equals a constant number which, no matter how P or V may be changed later, must remain constant.
  12. Liquids • Are liquid compressible? Are there spaces between the particles that make up a liquid? • Do liquids expand when heated? • If you put a drop of dye into water, what eventually happens? – Hot water? – Cold water? – How do the results compare?
  13. Dip your fingertip into a glass of water, then lift it out. • What do you notice? • What is holding the bottom of the water drop to the rest of the drop? Look at a small drop of mercury on a glass surface. • What do you notice? What does the behavior of the drop tell you about liquids?
  14. What happens when you heat a liquid? (Demonstration) • Marked spot on tube. What will happen as we heat the water? • What is our view in particle terms? • Did what we expected to happen actually happen?
  15. Demo – surface tension • Particles in a liquid feel a ‘sticky’ force which tends to hold them together • For substances like water, where this force is unusually strong, it can be exploited...
  16. Solids • Are they compressible? • Do solids expand when heated? (DEMO--ball and ring) – So are the particles moving? • If you leave a book on a table will it evaporate? • Can you poke your finger into a rock? • What does this tell you is an appropriate model for solids?
  17. Structure of a Solid • Molecules held to well-defined positions. • Molecules close together. • Significant energy must be expended to push a molecule aside.
  18. Demo: Thermal Expansion • Materials tend to expand when heated. • Ball will not pass through ring until the ring has been heated.
  19. Demo – bimetallic strips • All materials undergo thermal expansion. • This expansion is different for different materials. (Dental filling materials are chosen based on how close their expansion is to human teeth – why?) Image authored by Joh3.16 released under GNU Free Document license The upper material expands more than lower under heating. When materials are fixed to one another this results in bending.
  20. Recap: phases and the particle view of matter • Gases are spread out, little force between particles • Liquids have a cohesive force that holds them together, more dense than gases • Solids are more dense than gases but have a rigid structure. gas liquid solid
  21. Evaporation When you put a glass of water on the counter the water will slowly evaporate. • Which evaporates faster, a pint of water in a wide pan or a pint of water in a tall glass? What does this tell us? • In terms of the particle model, what is happening during evaporation?
  22. Demonstration: Evaporation • Feel the bottle of rubbing alcohol. Notice that it doesn’t feel particularly warm or cool. • Now put a drop of alcohol on your finger. What do you notice as the alcohol evaporates?
  23. • What does this graph tell you about? • What is the most probable age of a Mt. SAC Student? • What would happen to the average age of a Mt. SAC student if the oldest students (those 50+ years old) left? AGE 19 or less 20-24 25-29 30-34 35-39 40-44 45-49 50+ PERCENT 23 32.4 11.2 7.1 5.2 4.6 4 12.5 35 30 25 20 15 10 5 0 19 or less 20-24 25-29 30-34 35-39 40-44 45-49 50+ Age distribution of Mt. SAC Students, Spring 2003 Percent of total student population Age
  24. • The highest energy particles escape. • What happens to the average energy of the liquid? Energy required to escape liquid surface.
  25. Examples of Evaporative Cooling • Sweating • A Dog’s panting • Evaporative coolers
  26. Condensation • The lowest energy molecules in the gas get stuck to the liquid. • Low energy gas >= highest energy liquid :: mean energy of liquid increases. Fractionof moleculesinthe sample Energy of the molecules Lowest energy gas molecules may stick to liquid surface.
  27. Melting and Freezing • What happens to our picture of a solid during melting? • What happens to our view of a liquid during freezing?
  28. Recap: Thermal Expansion, Evaporation, Condensation • Higher energy and higher temperature go together. • Higher temperature “stuff” vibrates more. • The result of increased vibration is thermal expansion unless: – The vibration is strong enough to break the rigid bonds in a solid, resulting in melting. – The vibration is strong enough to break the surface tension resulting in evaporation.
  29. • Temperature is a way of measuring mean energy of particles. Fractionof moleculesinthe sample Energy of the molecules for a particular Temp.
  30. How the distribution of molecular energies changes with temperature from plasma/node7.html Fractionofatomsinthe sample Energy of the atoms
  31. But above a certain energy particles escape • What happens to the temperature of the remaining liquid? (Recall Evaporation) • So what will the temperature of the liquid do as it boils off? Energy required to escape liquid surface.
  32. Temperature
  33. Temperature • How do we measure temperature? = How do we assign numbers to a measurements of hotness? – Thermocouples – Liquid crystals – Thermometers--Conventional Temperature Scales • Fahrenheit • Celsius (or Centigrade)
  34. Shortcomings of conventional Temperature Scales • Suppose that it is 10ºC (50ºF) outside. What temperature is twice as hot? – Double the Celsius temperature?: 20ºC (68ºF); – Double the Fahrenheit temperature?: 100ºF (38ºC) • Can “twice as hot” depend on which temperature scale that you are using? • If one car is twice as expensive as another car, does this depend upon whether you are buying the car with dollars, pesos, or euros?
  35. Shortcomings of conventional Temperature Scales (continued) • Consider that you and Shaquille O’Neill are standing behind a counter. – The top of Shaquille’s head is 3 ft above the top of the counter. – The top of your head is 1.5 ft above the top of the counter. – Is it correct to say that Shaquille is twice as tall as you are? Why or why not? 1.5 ft 3 ft
  36. Shortcomings of conventional Temperature Scales (continued). • Consider that you and Shaquille O’Neill are standing behind a different counter. – The top of Shaquille’s head is 4 1/2 ft above the top of the counter. – The top of your head is 3 ft above the top of the counter. – Is it correct to say that Shaquille is 50% taller than you are? Why or why not?
  37. • Can how many times taller someone is compared to someone else depend upon which counter they are standing behind? • What is the appropriate, reasonable way to compare how tall two people are? • How does this relate to measuring temperature? Comparing with how we usually measure height, what point is it reasonable to measure temperature from?
  38. A new temperature scale • We’d like a temperature scale that: – starts with “as cold as it can get” and goes up from there. – correlates with molecular activity; that is, twice the temperature means that the molecules are twice as energetic – has the size of one degree on the new scale be the same as 1 degree on the common (Celsius) scale. • Folks came up with the KELVIN temperature scale, which meets all of these requirements.
  39. The Kelvin Temperature scale • Starts at absolute zero (0 K or –273ºC); as cold as it can get. • Each degree is the same size as a Celsius degree • Ice melts at 273 K • Water boils at 373 K What does absolute zero mean?