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lecture 11

lecture 11 - Temperature and Heat Announcements Discussion...

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Unformatted text preview: Temperature and Heat Announcements: Discussion sessions (7-9 PM in YKK220-001) Monday by email appointment only with TA Matt Phillips at [email protected] Tuesday come as you please (no appt) General Physics Help (8 midnight, Silliman D.H.) Midterm Wednesday, February 27, Chap's 1 9 Today: Homework #5 due Wednesday Pick up: graded reading quizzes (sol's on web) Grade sheet posted on web Today's lesson Temperature Heat Heat Capacity Phases of Water Thermodynamics Boyle's Law Heat Flow Temperature Chap. 10 Reading Quiz Video - pressure, temperature Temperature Temperature "quantity that tells how hot or cold something is "quantity that tells how hot or cold something is with respect to a standard" with respect to a standard" Physical properties change with temperature. How do we measure temperature? Mercury thermometer How does it work? thermal expansion materials expand as they get warmer! Thermal Equilibrium Thermal Equilibrium "two or more objects in contact adjust their "two or more objects in contact adjust their temperatures until they are at the same temperature" temperatures until they are at the same temperature" 0th Law of Thermodynamics - Two or more objects in thermal equilibrium are at the same temperature. Measuring Temperature Fahrenheit (1686-1736) Boiling pt. H20 = Freezing pt. H20 = 212F 32F Celsius (1701-1744) 100C 0C Fahrenheit degree = 5/9 Celsius degree Tc = 5/9 (Tf 32) [98.6F = 36C] Absolute Zero P Tf = 9/5 Tc + 32 -273.2 -300 -200 -100 0 100 200 300 T (C) Curves of different gases intersect P = 0 at T = -273.2C Absolute T scale is called the Kelvin Scale: Tk = Tc + 273.2 Cannot get any colder than Tk = 0 Heat and Temperature Heat Heat flows from one object to another when two objects of flows from one object to another when two objects of different T come into contact different T come into contact (or regions of different T within an object) (or regions of different T within an object) heat is the energy that is transferred heat transfer or heat flow always goes from warmer to cooler temperature - indicates which direction heat will flow if heat added, T increases if heat removed, T decreases note: matter does not contain heat matter contains molecular kinetic and potential energy, not heat Units of heat Joule (unit of energy) calorie - unit of heat needed to raise T of 1 gram water 1C 1 calorie = 4.184 J 1 food Calorie = 1 kilo-calorie = 1000 calories Thermodynamics Thermodynamics the study of heat and its effects on matter the study of heat and its effects on matter Specific Heat Capacity Ever notice how the filling of a hot apple pie can burn your tongue, while the crust will not? [Toast, soup, aluminum foil over casserole] Different substances have different capacities for storing internal energy! Specific Heat Capacity Specific Heat Capacity amount of heat needed to change the T of a material amount of heat needed to change the T of a material Specific heat capacity (c) is a property of the material c (water) = 1cal / gm C >> c (steel ) = .11 cal / gm C c = the heat required to change 1 gram of material 1C Total amount of heat required = Q = m c T Large c requires large Q (amount of heat) to change T example - effects of Long Island Sound on T of New Haven Concept Question: Does a substance that warms up quickly have a high or a low specific heat capacity? A. high B. low What about in cooling? 100 % 0% A . B . C . D . E . Concept Question: Which has higher heat capacity? A. water B. sand 83 % 18 % A . B . C . D . E . Phase Change Four phases (states) of matter (e.g. H2O)Solid (e.g. ice) Liquid (e.g. water) Gaseous (e.g. water vapor) Plasma (break-up of molecules into ions and electrons) Adding or removing heat e.g. melting ice, boiling water ice and water are different phases of the same substance At a phase change, heat can be added or removed without changing the temperature. Latent heat - changes the phase of water without changing T Latent heat of fusion (Lf) takes 80 calories of heat to melt 1 gm of ice at 0C phase change no T change Latent heat of vaporization (Lv) takes 540 calories of heat to change 1 gm of water to steam at 100C What Happens at a Phase Change? Example - Water Latent heat of vaporization (Lv) takes 540 calories of heat to change 1 gm of water to steam at 100C evaporation (liquid to gas, water to vapor) [perspiration, cooling in people, dogs] condensation (gas to liquid, vapor to water) [drying off in shower stall, warming process] steam burns evaporation vs condensation (competition) Phoenix vs New York City in July Latent heat of fusion (Lf) takes 80 calories of heat to melt 1 gm of ice at 0C freezing (liquid to solid, water to ice) melting (solid to liquid, ice to water) Thought Question A pot of water heats up rapidly on a stove, but it takes a while to boil away. Why does it take so long to turn water to steam? Thought Question Fruit growers often spray their crop with water to protect it from freezing in unusually cold weather. How does liquid water keep the fruit from freezing? First Law of Thermodynamics For a system: U = Q --W For a system: U = Q W U ==increase in internal energy U increase in internal energy W ==amount of work W amount of work done by system done by system Q ==amount of heat added to system Q amount of heat added to system KE ++PE of molecules & atoms of system KE PE of molecules & atoms of system Work done by system is positive - W takes energy away Heat-work interchangeable energy System has distinct boundaries, but can be anything units: 4.19 joules of work 1 calorie of heat 1000 cal =1 kilo cal = 1 Cal U=mL Boyle's Law For a system at fixed T: For a system at fixed T: the volume of a gas is inversely proportional to its the volume of a gas is inversely proportional to its pressure. pressure. constant P If pressure increases, volume decreases proportionally (& vice versa) V ~ Equation of State of an Ideal Gas For an ideal gas For an ideal gas (P.E. of atoms small compared to K.E.): (P.E. of atoms small compared to K.E.): PV = NkT PV = NkT relationship between P, V, and T (k = Boltzmann constant, N = Avogadro's number) Demo - "hot" can Compress a Gas F W = F d = P V (F = P A) V d Ad compress V < 0 expand V > 0 P What happens to internal energy? Push on piston, do work compress gas V decreases, U increases expand gas V increases, U decreases Adiabatic change - no heat flow (into or out of gas) Ideal gas - internal energy of gas = KE (PE of interactions between molecules = 0) U (internal energy) (T = absolute temperature) T (absolute) Heat Flow Heat flows from higher T to lower T (warmer to cooler) Heat rises (hot air balloon) Three types of heat transport: Conduction flow of heat/energy through materials in contact thermal conductivity - property of material (metals, woods, plastics, hand, house insulation) travels through a medium [T average K.E. of atoms/molecules] Convection flow of heat or heat transfer via motion of a fluid contains thermal energy (air, water, steam) carried by a medium (energy) Radiation flow of energy by electromagnetic waves (chapter 15) Heat infra-red wavelengths (shorter than radio waves longer than visible light) thermos bottle (vacuum with silvered walls reflects waves) needs no medium ...
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