Lecture02

Lecture02 - Physics 7A-2 (C/D) Professor Chertok Fall, 2008...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Physics 7A-2 (C/D) Professor Chertok Fall, 2008 Lecture 2 Original presentations copyright M. Chertok 2008. All rights reserved. Questions? M. Chertok, Physics 7A 2 Outline Quick recap of last lecture Reminder of mks units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 3 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 4 Last time Course website Updated! Added lecture slides, FNTs, more office hours information Expect regular updates for content from now on Course policy Won’t go through this again. Please read carefully and ask me questions if you have any. M. Chertok, Physics 7A 5 Last time, (ii) Models Useful description of nature with predictive powers. You use them all the time in science In Physics 7, we will be explicit about models We will start with (and keep returning to): Three-phase model of pure substances (why pure?) Energy interaction model M. Chertok, Physics 7A 6 Last time, (iii) Demos Bowling ball pendulum Would the motion look different if I put the bowling ball in a freezer for an hour or two? Would the motion look different if this were done on the Moon? Ice and liquid nitrogen Why do we tend to assume that ice is always at 0 degrees C?? What must happen initially to the water in the beaker once the ice is added? How do we know? What might happen if we put a large icecube in the LN for several minutes and then dropped it into the beaker of water? CLICKER QUESTION: It would be possible for ALL the water to freeze. a) T, b) F M. Chertok, Physics 7A 7 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 8 meter-kg-second units AKA “SI” Forget Fahrenheit and pounds! If ΔT = Tf - Ti is what you need, then you can use C and K interchangeably Otherwise, stick to Kelvin (K): 0K: no motion; thermal energy vanishes 273K: water melts 300K: room temperature 373K: water boils M. Chertok, Physics 7A 9 mks/SI units (see p.27) SI Unit Joule Watt Newton Pascal Construct energy power force pressure 1 kWh = 3.6 MJ Symbol J W N Pa Equiv. form kg•m2/s2 = N•m J/s kg•m/s2 J/m3 Battery: 2000 mAh@1.5V=3000mWh=3W x 3600s ~ 10 KJ 1 cal = 4.184 J 1 food cal = 1 Calorie = 1kcal = 4.184 kJ 1 eV = 1.602 x 10-19 J M. Chertok, Physics 7A 10 “Chemical” properties table M. Chertok, Physics 7A 11 Clicker question It takes more than 5 times as much heat to boil 1 kg of water than to raise the temperature of 1 kg of water from 0C to 100C T (a) or F (b) ?? M. Chertok, Physics 7A 12 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 13 3-phase model For “pure” substances we observe 3 phases: solid,liq,gas Energy transfer (Q or W) can change T or change phase Phase change at specific T’s (Tpc) Tmp, Tbp, Tsp; however, these are P dependent Energy required for phase change is ΔH: ΔHm -- Heat of melting ΔHvap -- Heat of vaporization ΔHsp -- Heat of sublimation Q = ±| Δm|•|ΔH|; ΔH :intensive or intensive? Δm is amount that changes phase M. Chertok, Physics 7A 14 3-phase model, (ii) Thermal equilibrium (TE): All parts of substance (or substances) at same T In TE, a phase change occurs at Tpc A mixed phase (two coexisting phases) can occur in TE only at Tpc M. Chertok, Physics 7A 15 3-phase model, (iii) If E is transferred to/from substance not at Tpc, a temp. change occurs instead: Q = C • ΔT C is Heat Capacity. Extensive or intensive? c is specific heat. Extensive or Intensive? Formula becomes Q = m•c•ΔT M. Chertok, Physics 7A 16 3-phase model, (iv) gas Vaporization Sublimation Melting Condensation liquid Freezing Deposition solid Q: how do the BOND ENERGIES of these phases compare? M. Chertok, Physics 7A 17 3-phase model diagram compartment of your refrigerator? How do the algebraic relationships relate to the graph? Which parts? What is the relationship? These are the kinds of questions you need to be asking yourself and getting confident about. You want to practice using this representation enough so that it really does become a useful tool to make sense of thermal phenomena and to be comfortable using it to construct explanations for particular phenomena. Energy Added or Removed (at constant pressure) M. Chertok, Physics 7A 18 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 19 Energy-Interaction Model The EIM is basically a statement of energy conservation along with a useful diagram. Constructs physical system(s) process -- takes some time energy-systems. e.g., thermal energy, KE, ... indicators. e.g., T is indicator for Eth energy transfers; can include Q or W (with ± !) Q and W are NOT energy-systems internal energy vs. mechanical energy: are we discussing bowling balls or ice melting? M. Chertok, Physics 7A 20 EIM, (ii) We have two versions: Open system: a physical system is interacting with environment Closed system: two objects interacting w/ each other insulated from environment two objects transferring heat in an insulated box ice melting on a table You decide which is appropriate for each situation M. Chertok, Physics 7A 21 EIM Algebra Closed system ∆Etotal = Σ∆Ej = ∆E1 + ∆E2 + ∆E3 + ... = 0 Is it clear why there’s no Q or W for a closed system? Open system ∆Etotal = Σ∆Ej = ∆E1 + ∆E2 + ∆E3 + ... = Q + W Q and W can be positive or negative; so can the ΔE’s Where, in the process: ∆E1 ≡ E1f − E1i , etc. M. Chertok, Physics 7A 22 E.I. Diagram -- open system Physical system: Physical thing 1 Physical thing 2 initial final Generic Example of an Energy System Diagram involving two physical systems, three energy systems, and with Heat Input Identification of beginning and end of interval: indicator b for thing 1 is X indicator a for thing 2 is Y Physical thing 1 Ea ! Indicatora ! Indicatora, initial = Indicatora, final = Physical thing 1 Eb " Indicatorb " Indicatorb, initial = X Indicatorb, final = Physical thing 2 Ea ! Indicatora ! Indicatora, initial = Indicatora, final = Y heat Comments: #E1,a + #E1,b + #E2,a = Q On 1) What is shown in the diagram above is the minimum that must always be written down. quizzes and hard DL we will have been done to get to this point. Often, many Most of the in thinking expect complete diagrams like this M. Chertok, Physics 7A 23 explanations of physical phenomena can be constructed using this diagram without going E.I. Diagram -- closed system Generic Example of an Energy System Diagram involving two physical systems, three energy systems, and with Heat Input Identification of beginning and end of interval: initial final Physical system: Physical thing 1 Physical thing 2 energy-systems (to indicate the open physical system boundary) and drawing arrows terminating on the boundary to show a Q or a W transfer. Physical thing 1 1 Physical Thing Ea ! Ea Indicatora ! Physical thing 1 Eb " Indicatorb " Physical thing 2 Physical Thing 2 heat Ea Ea ! Indicatora ! Indicatora, initial = Indicatora, final = Indicator a Indicator a Indicatora, initial = Indicatora, final = Indicatorb, initial = Indicatorb, final = indicator a, initial = indicator a, initial = indicator a, final = #E1,b + #E2,a = Q indicator a, final = #E1,a + Comments: example: a cold piece of copper absorbing heat from a warm 2) In an open-system diagram, the arrow iron energy transfer into the system is drawn pieceand labeled with the words “heat” or “work,” and not “Q” of showing in the direction of energy flow or “W.” This is to preserve the standard convention of calling Q and W positive when M. Chertok, Physics 7A 1) What is shown in the diagram above is the minimum that must always be written down. Most of the hard thinking will have been done to get to this point. Often, many a,1 a,2 explanations of physical phenomena can be constructed using this diagram without going further and substituting in explicit expressions for the individual change in energy terms and numerical values for various parameters. Even if you are required to continue the process through to a numerical result, you must do the mental work of constructing the energy-system diagram to this point prior to doing any numerical calculations. ΔE + ΔE =0 24 Total energy If a physical system’s TOTAL energy is known at some point, we can use alternative form of energy conservation: Etot,i = E1i + E2i + E3i + ... = Etot,f = E1f + E2f +E3f + ... This is particularly useful for mechanical systems: ball thrown in air mass-spring etc. Never confuse ΔE’s with E’s The sum of the ΔE’s is the net heat (or work) transferred into or out of the system. M. Chertok, Physics 7A 25 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 26 DLM01 1.1.1 Trigger and observe heatpack Heat water and record T versus time. Plot T versus energy added 1.1.2 Try to understand heatpack using 3-phase model. Take heatpack through full cycle. Determine melting point. Which phase change (s→liq, or liq→s) seems to follow the model? 1.1.3 Plot T vs. energy added and EID for several processes M. Chertok, Physics 7A 27 DLM02 1.1.4 FNTs from DLM01 1.1.5 Break down heatpack cycle into 5 subprocesses and diagram them Big question: when you triggered the heatpack, where did all that thermal energy come from? Draw 3-phase diagram with and without supercooling “A quantity of ice at 00C must contain less total energy than the same quantity of water at 00C” Can an energy system contain an amount of heat? M. Chertok, Physics 7A 28 DLM02, (ii) 1.2.1 Heat capacity meaning Example heat capacity measurement Connection with thermal energy Specific heat: “mass” and “molar” Heat of (for example) melting Example of a ΔH measurement Connection with bond energy Extensive versus intensive quantities M. Chertok, Physics 7A 29 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 30 What’s up in DLM03 and 04 DLM03 Lots of recap of the FNTs you did, with lots of quantitative examples Plaster of Paris FNT DLM04 FNTs from DLM03 More quantitative examples with Eth and Eb Mechanical systems introduction: KE PE-gravity PE-mass-spring Allow me to demonstrate... M. Chertok, Physics 7A 31 Mechanical energy-systems Translational Kinetic Energy Indicator: speed (speed vs velocity?) KE = ½ mv2 ΔKE = ½mΔ(v2) Gravitational Potential Energy PEg = mgEy ΔPEg = mgE(Δy) Mass-spring oscillator ΔKE = ½mΔ(v2) ΔPEm-s = ½kΔ(x2) M. Chertok, Physics 7A 32 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 33 Quiz 2 on Friday At beginning of lecture Covers the first 3 DLMs plus the FNTs from DLM03 (discussed and followed up in DLM04) M. Chertok, Physics 7A 34 Outline Quick recap of last lecture Reminder of m-k-s units Three-phase model Energy-interaction model Recap of DLM01 and DLM02 What’s next (DLM03 & 04) Quiz info Reading assignment M. Chertok, Physics 7A 35 Reading assignment Read chapter 2 You can skim section on forces M. Chertok, Physics 7A 36 ...
View Full Document

This note was uploaded on 05/30/2009 for the course PHY 7A taught by Professor Pardini during the Fall '08 term at UC Davis.

Ask a homework question - tutors are online