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AP Physics

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A measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale ~American Heritage Dictionary 3 Scales ScaleBoiling Point of H 2 OFreezing Point of H 2 OAbsolute Zero Fahrenheit Celsius Kelvin

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Two systems are in thermal equilibrium if and only if they have the same temperature. Two systems that are each in thermal equilibrium with a third system are in thermal equilibrium with each other.

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Most materials expand when their temperature increases. Linear Expansion Expansion in one dimension (usually the length) change in length is proportional to the change in temperature Materialα (1/ o C) Aluminum24 x Brass & Bronze19 x Copper17 x Steel11 x Concrete12 x 10 -6

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A steel railroad track has a length of m when the temperature is 0 o C. What is its length on a hot day when the temperature is 40 o C? α = 11 x ( o C) -1

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James Joule discovered by various means that he could heat a body of water by purely mechanical means: a) by lowering a weight and letting a paddle wheel stir the water b) by passing electric current through a resistor c) by compressing a piston immersed in the water d) by friction from rubbing blocks together. He found that about 800 foot-pounds ( 1 kilo joule) of work could raise the temperature of one pound (.45 kilograms) of water one Fahrenheit degree (0.55 o C).

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Energy that is transferred from one object or system to another object or system as a result of a temperature difference. (The concept of heat only has meaning as energy in transit.) Heat flow is energy transfer. Units: Joule (J), calorie (cal), Calorie (Cal), British thermal unit (Btu) Unit Relations JoulescaloriesCaloriesBtu x x x The calorie is defined as the amount of heat required to raise the temperature of 1 gram of water from 14.5 o C to 15.5 o C. One Btu is defined as the quantity of heat required to raise the temperature of 1 pound of water from 63 o F to 64 o F.

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Quantity of heat needed to change the temperature of a system is dependent on three things: 1. The quantity of the temperature change. 2. The mass of the system. 3. The nature of the material. The amount of heat Q needed for a certain temperature change Δ T is proportional to the temperature change and to the mass m of substance being heated;

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Materialc [J/(kg · K)] Water4.19 x 10 3 Ice (-25 o C to 0 o C)2.01 x 10 3 Aluminum0.91 x 10 3 Salt0.88 x 10 3 Iron0.47 x 10 3 Silver0.23 x 10 3 During a bout with the flu, an 80 kg man ran a fever of 2.0 C o. His body temperature was 39.0 o C (102.2 o F) instead of the normal 37 o C. Assuming that the human body is mostly water, how much heat was required to raise his temperature by that amount?

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You are an electric engineer designing an electronic circuit element made of 23 mg of silicon. The electric current through it adds energy at the rate of 7.4 mW (7.4 x J/s). If your design doesn’t allow for any heat transfer out of the element, at what rate does its temperature increase? The specific heat capacity of silicon is 705 J/(kg · K).

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Ingot - metal that is cast in the shape of a block for convenient handling A kg ingot of metal is heated to o C and then dropped into a beaker containing kg of water that is initially at 20.0 o C. If the final equilibrium temperature of the mixed system is 22.4 o C, find the specific heat of the metal.

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Three familiar phases of matter include: 1. Gases 2. Liquids 3. Solids …but there are more than those three including… plasmas, superfluids, supersolids, Bose-Einstein condensates, fermionic condensates, liquid crystals, strange matter and quark-gluon plasmas

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Transition from one phase of matter to another What goes on during a phase change? At a given pressure, phase changes: 1. usually occur at a definite temperature* 2. heat is absorbed or released 3. there is a change in volume of the matter 4. there is a change in density of the matter *At this temperature and pressure both phases of matter can coexist, this is called phase equilibrium.

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Phase ChangeProcessExampleHeat Required per unit mass Solid Liquid Metal gallium melts in your hand Latent Heat of Fusion (L f ) Liquid SolidWater freezes Liquid Gas Liquid Nitrogen boils at room temp. Latent Heat of Vaporization (L v ) Gas LiquidWater vapor condenses Solid Gas Dry Ice (CO 2 ) sublimes to its gaseous phase at room temp Latent Heat of Sublimation (L s ) Gas SolidFrost forms on cold objects To melt a mass m of material that has a heat of fusion L f requires a heat Q given by: To freeze a mass m of material that has a heat of fusion L f requires a heat Q given by:

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To cause a phase change energy must be absorbed or released by the object/system. During the phase change the heat energy does not change the temperature of the object.

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A physics student wants to cool 0.25 kg of Mountain Dew (mostly water and high fructose corn syrup ) initially at 20 o C by adding ice initially at -20 o C. How much ice should she add so that the final temperature will be 0 o C with all the ice melted? Assume that the heat capacity of the container may be neglected.

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There are three mechanisms of heat transfer. 1. Radiation 2. Convection 3. Conduction

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Energy transfer through electromagnetic radiation (fancy words for light) Ex.

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Energy transfer of heat by the motion of a mass of fluid Ex.

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heat transfer through a material from regions of higher temperatures to regions of lower temperatures Good thermal conductors are metals because they have many free electrons Ex. Rate of Heat Transfer…Heat Current (H)

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The rate of heat transfer through a material with regions of varying temperatures is: proportional to the cross sectional area A. proportional to the temperature difference Δ T. inversely proportional to the distance between the regions L. The constant of proportionality is the thermal conductivity of the material k.

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The Styrofoam TM box seen below is used to keep drinks cold at a picnic (or wild beach party whatever suits your taste). The total area of the sides, top, and bottom is 0.80 m 2, and the wall thickness is 2.0 cm. The box is filled with ice and Root Beer, keeping the inner surface at 0 o C. What is the rate of heat flow into the box if the temperature of the outside surface is 30 o C? How much ice melts in one day (24 hr.)? Materialk (W/(m · K) Silver406 Copper385 Styrofoam TM 0.01 Wood0.12 – 0.04 Air0.024

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A steel bar 10.0 cm long is welded end to end to a copper bar 20.0 cm long. Each bar has a square cross section, 2.00 cm on a side. The free end of the steel bar is in contact with steam at 100 o C, and the free end of the copper bar is in contact with ice at 0 o C. Find the temperature at the junction of the two bars and the total rate of heat flow. Materialk (W/(m · K) Steel50.2 Copper385

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