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# Lec11 - Today Finish Ch 11(Atomic Structure of Nature Start...

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Today: Finish Ch 11 (Atomic Structure of Nature) Start Ch 13 (Liquids)

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Ch 13 (Liquids) First, concept of density (in Ch 12, everything else of which we are skipping) Density = mass volume Density measures how squished up the matter is, not how heavy it is Eg. A feather quilt may be heavier than a metal spoon, but the spoon is more dense than the quilt. Only if you have equal sizes (ie. volumes) of two materials, are their relative densities directly related to their weight. Weight density = weight volume Simply related by g = 9.8 N/kg (near earth)
A little more on density… Some densities: Osmium (bluish-white metal) is the densest substance on earth. It’s an element (atomic # 76), whose crystalline form has very closely packed atoms. Density 22.6 g/cm 3 . Used eg in fountain pen tips, electrical contacts, where extreme durability/hardness needed. Water has density 1 g/cm 3 Ice has less density, 0.92 g/cm 3 , because when water freezes, it expands. Seawater has greater density, 1.03 g/cm 3 .

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Clicker Question Question: Which has greater density, 1 liter of water or 1 liter of ice? And which weighs more? A)The water has greater density and weighs more B) The water has greater density but weighs less C) The water has less density but weighs more D) The water has less density and weighs less Answer: A Any amount of water has greater density than any amount of ice (it doesn’t depend on amount, since density is the ratio of mass to volume). Since water has greater density, it weighs more than an equal volume of ice.
Pressure Why does it hurt so much more when a thumbtack pin is pressed into your hand than when a marble is, pressing equally hard?? Even though the force in each case may be the same, one (the pin) acts over a much smaller area than the other (marble). So, define force area Pressure = Question: Why is a bed-ridden person less likely to develop bedsores on their bodies if they use a waterbed rather than an ordinary (i.e. more rigid) mattress? Because more of the body is in contact with the supporting surface on the waterbed than on the mattress, so there is less bodyweight pressure. (Larger area in eqn above, same force, so less pressure)

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Pressure in a liquid A liquid is composed of molecules that move constantly and bounce off the sides of the container it is in and or bounce off objects (like a swimmer) in the liquid. Bouncing creates a force (recall momentum-impulse) – hence a pressure. Liquid Pressure = weight density x depth Proof ”: Pressure = force area = weight area = weight density x volume area area x depth = weight density x depth due to liquid’s weight directly above
Pressure in a liquid: dependence on depth. Pressure at a point twice as deep, is twice as much. Consider swimming. Near top surface of the water, don’t feel much pressure (depth is near 0). (More precisely, need to add air pressure of the atmosphere but you always feel that).

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Lec11 - Today Finish Ch 11(Atomic Structure of Nature Start...

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