Matter and the Universe Ancient Views Early atomists believed that matter had a smallest indivisible bit, an atom. Aristotle, the most famous of the early Greek philosophers, didn't agree with the idea of atoms. In the fourth century BC he taught that all matter is composed of different combinations of four elements earth, air, fire, and water. This view seemed reasonable, for in the world around us matter is seen in only four forms: solids (earth), gases (air), liquids(water), and the state of flames (fire).. Aristotle's ideas lasted for more than 2000 years. Modern Views Periodic Table of Elements To date(2001) we know of 118 distinct atoms, called the chemical elements. Only 88 elements are found naturally; the others are formed in laboratories with high-energy nuclear accelerators and reactors. These heaviest elements are too unstable (radioactive) to occur naturally inappreciable amounts. Ernest Rutherford New Zealand born British physicist Ernest Rutherford, who in 1909 oversaw the now-famous gold-foil experiment 1
Atomic Model Size Analogy: If on Atom were the size of this building, then the nucleus would be the size of? The Carbon Atom Neutrons and Protons have approximately the same mass but mass of electrons is negilbible by comparison. The mass of a proton or neutron is 1 amu and the atomic mass of any atom is just the mass of the sum of protons + neutrons Charcoal consists mainly of carbon atoms, atomic #6. To the left are the subatomic particles (protons, neutrons, and electrons) that make up a typical carbon atom, plus an outline of the atom. Masses Isotopes To identify isotopes, we use the mass number, which is equal to the number of protons plus the number of neutrons in the nucleus. The three isotopes of carbon (with mass numbers 12, 13, and 14) are called carbon-12, carbon-13, and carbon-14. These names can also be written 12 C, 13 C, and 14 C. 2
Check yourself 1. Which contributes more to an atom's mass, electrons or protons? To an atom's volume ( size)? Models of simple molecules. The atoms in a molecule are not just mixed together but are joined in a well-defined way. What is heavier? A pound of feathers or a pound of iron? Density Density is how much a unit cube weighs. Units are g/cm 3, kg/l, lbs/ft 3 Wood or iron? Aluminum or copper? ft 3, cm 3, m 3 If you have 20 cubes, and they weigh 40 g all together, how much does one cube weigh? (0.5g?, 2g?) " = m V Density of water is 1 gm/cm 3 or 1 kg/l. The mass of a given volume can be found from Mass=Density x Volume and W=Mg=DgV Densities of different materials Liter of Water Solids Aluminum Copper Iron lead ice stone wood Density (gm/cm 3 ) 2.7 8.96 7.87 11.4 0.92 2.4-3.1 0.6-0.9 Liquids alcohol mercury oils Salt water Water at 4 Density (gm/cm 3 ) 0.8 13.6 0.9 1.025 1.0 A liter of water occupies a volume of 1000 cm 3, has a mass of 1 kg, and weighs 10 N. Its density may therefore be expressed as 1 kg/l and its weight density as 10 N/L. (Seawater is slightly denser, about 10.0 N/L). 3
Liquids A liquid contained in a vessel exerts forces against the walls of the vessel. Bed of Nails e.g. Pressure in a pool full of water increases with depth. Liquid pressure=weight density x depth Buoyancy Anyone who has ever lifted a submerged object out of water is familiar with buoyancy, the apparent loss of weight experienced by objects submerged in a liquid The increase in water level is the same as that which would occur if, instead of putting the stone in the container, we had poured in a volume of water equal to the stone's volume. 4
B Displacement A completely submerged object always displaces a volume of liquid equal to its own volume. Archimedes Principal An immersed body is buoyed up by a force equal to the weight of the fluid it displaces. (not weight of object. B could be bigger, smaller or equal to W) W Totally Submerged Object Demo: Archimedes principal The object is pushed up by a force called the buoyant (B) force. This force (B) is equal to weight of water displaced. B may be bigger than w B may be smaller than w. B may be equal then w 5 N Weight in Water Archimedes Principal: An object in a fluid experiences an upward force equal to the weight of the fluid it displaces. 4 N Check yourself 1. Does Archimedes' principle tell us that if an immersed object displaces liquid weighing 10 N, the buoyant force on the object is 10 N? 2. A 1-liter container completely filled with lead has a mass of 11.3 kg and is submerged in water. What is the buoyant force acting on it? 3. A boulder is thrown into a deep lake. As it sinks deeper and deeper into the water, does the buoyant force on it increase? Decrease? 5
Different densities Float and Sink: is B><=W? 1. If an object is denser than the fluid in which it is immersed, it will sink. 2. If an object is less dense than the fluid in which it is immersed, it will float. 3. If an object has a density equal to the density of the fluid in which it is immersed, it will neither sink nor float. Check yourself 1.Two solid blocks of identical size are submerged in water. One block is lead and the other is aluminum. Upon which is the buoyant force greater? 2. Two blocks of identical size, one of lead the other of wood. On which one is the BF greater? 3. What is BF on a ten ton ship floating in water? Salt water? Mercury? As an object gets heavier it sinks deeper increasing B For any floating object B=W Why is it easier for you to float in salt water than in fresh water? It's easier because when displacing your weight a lesser amount of your body is immersed you don't sink as far. You'd float even higher in mercury (density 13.6 g/cm3), and you'd sink completely in alcohol (density 0.8 g/cm3). Iron is much denser than water. A solid chunk of iron sinks, as you would expect, but an iron ship floats. Why? 6
Floating Mountains Changing Density Example: Empty soda can vs full Mixed Densities Metal balls inside jar sink What happens if you now put in styrofoam? Volume is fixed but Mass varies What happens if we add mass to the outside of the container? Heat and Temperature Heat and Expansion Water is an exception 7
Heat and Expansion Water is the exception Increase the temperature of any common liquid and it will expand. But not water at temperatures near the freezing point: Ice-cold water does just the opposite! As water cools, it sinks until the entire pond is 4 C. Then, as water at the surface is cooled further, it floats on top and can freeze. Once ice is formed, temperatures lower than 4 C can extend down into the pond. Check yourself 1. What was the precise temperature at the bottom of Lake Michigan, where the water is deep and winters long, on New Year's Eve in 1901? Air is Matter Air Pressure We live at the bottom of of a pool of air which has no definite beginning. The pressure on us from the atmosphere is 14.7 lbs per in 2 or 10 5 N/m 2 8
Air Pressure Buoyancy of Air We live at the bottom of of a pool of air which has no definite beginning. The pressure on us from the atmosphere is 14.7 lbs per in 2 or 10 5 N/m 2 Weight and Pressure of air At sea level air has a mass of 1.25 kg per cubic meter or a weigth of 12.5 N. At 40 km it would weigh only 4 N. A 1 cm 2, 30 km column of air would weigh about 10 N. 1m 3 =0.4 kg Atmospheric Pressure Density of air is 1.25 kg per meter 3. What is mass of air in this room? Why doesn t this force break windows? 1 m 3 =1.25 kg How much force would air exert on 1m 2? 1cm 2? 9