Introduction, and 1 st Topic: Temperature!

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1 Introduction, and 1 st Topic: Temperature! Modern Physics August 27-29,

2 Syllabus and Website (NOT blackboard. No password) { Don t think that just because notes are posted or e- mailed* that you don t have to come to class { Problems worked out during lecture, and other important things, are omitted online on purpose, and attendance is compulsory + I can see! (see syllabus) One relatively new policy: TO CONTINUE AS A PHYSICS MAJOR YOU NEED TO GET A GRADE OF C OR HIGHER IN COURSE (ENFORCED). Up front: a GATEKEEPER class { Chem or other majors too? Ask your academic advisor Everything (HWs, lecture slides) will be provided*. Nevertheless, good notetaking is a valuable practice { Especially true for formulae, easier on board than PPTX { Equally / more important 2 to actively LISTEN TO ME

3 Bonus: Makes up for Difficulty Attendance at the weekly departmental seminars and colloquia -- note, free cookies & coffee J { Graduate student seminars: 2:45 Tuesdays in Room 129 (first floor lecture room, in this building) { Physics colloquia: 2:45pm Fridays, also in room 129 Once SPS meetings are announced they count too (free pizza, same room). Will include career advice I am sorry if you can t make any of these (work etc.) but remember this is extra credit, not required, so I am not obligated to make it possible for everybody Up to a maximum of 5% credit *on your final grade*, so that is a lot (1% per attendance: stay whole time) 3

Grading System >85% is an A (informal *A+* will be included) 70-85% is a B 55-70% is a C 40-55% is a D It is my fervent hope this year that a smaller class will lead to better learning <40%

4 Grading System >85% is an A (informal *A+* will be included) 70-85% is a B 55-70% is a C 40-55% is a D It is my fervent hope this year that a smaller class will lead to better learning <40% is E (trying to be nice - hard to fail!) Broken down into + s and s by thirds (5%) in each range Will not curve, as this is already ~included { Based on typical distributions of earlier years 4

5 Course Overview 1 In Physics 140/150, you learned about Newtonian Mechanics, and Electricity & Magnetism In this course, we will focus on the physics which came about in the late 19 th century and during the early 20 th century At some point in the 1800 s, a noted physicist had remarked => There is nothing new left to discover in Physics we just have to refine our measurements { Was he wrong or what?! (astro/cosmology, nuclear) Evidence was beginning to gather to imply that there were big problems { The behavior of electromagnetic waves when viewed from a moving frame { Lack of understanding about blackbody radiation 5 Investigations into these led to relativity and QM

6 Course Overview 2 Special Relativity { What happens when speeds become very large, v ~ c, the speed of light Quantum Mechanics (QM) { What happens when objects get very small, e.g., atoms, electrons, protons, et al. What we cannot cover { Relativistic quantum mechanics: gives a natural explanation for anti-particles (a.k.a. Quantum Field Theory graduate level) What we can only cover a little, given time { General relativity: Einstein s explanation of gravity (dedicated 6 course)

7 Thermodynamics In this unit, we will deal with cases where the temperature, or the state of a system (whether it is solid/liquid/gas), changes due to energy transfers into or out of it { This field deals with how everyday phenomena such as cooling in a refrigerator occurs, etc. In this unit, we will cover topics such as temperature, the (0 th /1 st /2 nd /3 rd ) laws of thermodynamics, entropy, the kinetic theory of gases, etc. I will be using notes and other resources for this unit { Both will be available online and/or by Thermodynamics essentially sheds light on macroscopic properties of systems, which arise from the internal structure such as atoms and molecules, without getting bogged down 7 in little details

Temperature 1 Not a reliable What is temperature, and what does it measure? If you stand with one foot on a wooden floor and the other on a metal floor, what do you feel?

8 Temperature 1 Not a reliable What is temperature, and what does it measure? If you stand with one foot on a wooden floor and the other on a metal floor, what do you feel? { Is one cooler than the other? Why? Are they not both at the same temperature? { Eventually, both of your feet will feel the same. What is going on here? Have you wondered? measurement is transferred into or out of your foot Essentially, what you sense is the rate at which energy { When objects are in thermal contact, energy flows between them (on a hot summer day, hold an icecube to your forehead) When energy is no longer being transferred between two objects, they are said to be in thermal EQUILIBRIUM (I ll highlight important jargon differently) 8

9 Temperature 2 If two objects in thermal contact have the same temperature, no energy will flow between them { They will be in thermal equilibrium So, one can think of temperature as the property of a system that determines whether it is in thermal equilibrium with others or not { Without work, energy flows from hot to cold A thermometer is an instrument that measures the temperature of a system { There are many ways to do this => we shall return to that later on in this course { Units: Fahrenheit, Celsius/Centigrade, Kelvin 9

10 Temperature Scales 0 degrees F is kind of useless! COLD. Temperature where a certain salt solution freezes. 100 degrees F meaningless, but originally intended to be human body temperature (was mismeasured) Degrees R(ankine) exist as well (like K) 0 Kelvin (the *absolute* scale, in SI system) { Is degrees C and degrees F 0 degrees Celsius (metric system nations) { Is K (no degrees) and 32 degrees F { Water freezes 100 degrees Celsius (reason Centigrade ) { Is K and 212 degrees F { Water boils, at 14.7 psi pressure (or, 1 atm) Let s work out two examples on the board 10

11 Problems 1 and 2: TEAMS! Why teams? (77.8% of companies rank LEADERSHIP & TEAM-BUILDING the highest) To convert from degrees Celsius into Kelvin, we just add/subtract what? For degrees Celsius into degrees Farenheit, we use the formula... (derive if don t know) Teams of 3. Roles: Leader, recorder, monitor (rotate!) Leader speaks or delegates 11

Thermal Expansion 1 As temperature increases, solids and liquids expand { This is why (most) train tracks leave a little gap between adjacent units We will see later what this really means { But for

12 Thermal Expansion 1 As temperature increases, solids and liquids expand { This is why (most) train tracks leave a little gap between adjacent units We will see later what this really means { But for now, you can simply think of this as the space between atoms increasing Solids: coefficients of expansion { Linear: alpha = ( deltal / Li ) / deltat, i.e., relative change in length divided by change in temperature { Volume: beta = ( deltav / Vi ) / deltat (one can relate alpha and beta). (Area too) 12

13 Thermal Expansion 2 alpha ( C^-1) Aluminum 24 x 10^-6 Lead 29 x 10^-6 Glass 9 x 10^-6 Steel 11 x 10^-6 Brass 19 x 10^-6 Similarly, we have values for beta (mainly for fluids) beta_mercury = 1.8 x 10^-4 ( C^-1) beta_air (@ 0 C) = 3.7 x 10^-3 (Expansion of mercury was used in old thermometers to see if you have a fever!!) In general we use the Kelvin scale, though when we have deltat in the calculations, then that is equivalent to using the Celsius scale. (Makes sense?) Time for another in-class example 13

14 Problems 3 and 4 Same groups every time! But equal amount of all speakers A steel railroad track has L = C. What is its 30 C? At which temperature will the two example steel and brass bolts touch? Steel Brass 5 C 14 L_B = 0.03 m L_S = 0.01 m

15 Bimetallic Strips An example of how the property of thermal expansion/contraction is exploited in real life. Thermostat with bimetal coil at (2) 15

16 Solids and Liquids Generally expand when heated. (Cold!) water is a significant exception, however! rho (density) = m / V, where m is mass and V is volume { So, if m is constant (conserved quantity) and volume increases, then rho will decrease The density of water is at a maximum at 4 degrees C 16 This phenomenon explains why even when a pond surface freezes in the winter, there is usually cold water at the bottom, which allows plants/animals to survive!!

in a small container or a large one, and their volumes will be different, but so will be the pressure & the temperature For gases then, we

17 Ideal Gases For solids or liquids, beta = ( deltav / Vi ) / deltat However, for a gas what is the initial volume? Imagine pumping the same amount of air into a bicycle tire versus a car tire { It depends on the container { You can put the same mass of gas in a small container or a large one, and their volumes will be different, but so will be the pressure & the temperature For gases then, we must deal with P, V, and T { An EQUATION OF STATE relates these quantities In general, relationship can be very complicated but, as usual, we start with 17 simple assumptions

The Equation of State Assumptions { T is neither too high, nor too low { P is low Such a gas does not exist in real life, but this is a useful starting point { Gas atoms do not interact with each

18 The Equation of State Assumptions { T is neither too high, nor too low { P is low Such a gas does not exist in real life, but this is a useful starting point { Gas atoms do not interact with each other except via collisions { Molecular volume is << size of container We use moles to represent the amount of gas with which we are dealing The number of moles n = m / M, where m is mass and M is molar mass, i.e., atomic mass For H -> M = 1 g / mol He -> ~4 g O -> ~16 g Ar -> ~ { So, 1 mole of a gas has a mass equal to the atomic mass and contains x atoms (Avogadro s number, or, N A ) << means MUCH MUCH (also >>)

$Ideal Gas Law For an ideal gas, we have (from experiment) P V = n R T { Where P is the pressure (SI unit Pascal = 1 N / m^2) { V is volume (m^3) { n is #moles { T is temperature (in Kelvin) { R is a$

19 Ideal Gas Law For an ideal gas, we have (from experiment) P V = n R T { Where P is the pressure (SI unit Pascal = 1 N / m^2) { V is volume (m^3) { n is #moles { T is temperature (in Kelvin) { R is a constant = J / ( mol * K) Joules = N * m L is liters and atm is unit of Pressure Alternate: L * atm / ( mol * K ) P V = ( N / N A ) * R * T = N * kb * T 19 { Boltzmann s constant = R/N A = 1.38x10-23 J/K

(We will return to this for the kinetic theory of gases.

20 Problems 5 and 6 Emphasis: practice, trial & error. Graded on trying, not succeeding for these tasks! Think about why soda spews out of a bottle, if you shake it, and then open it up? (We will return to this for the kinetic theory of gases.) P 0 = atmospheric pressure h mass = m The gas is at pressure P and temperature T and contains n moles area = A We want to know the height h at which the piston will be in equilibrium 20 (Hint: equalize F)

Homework See PDFs attached on course webpage { Reading AND writing (problem-solving) { Latter: trying to make a hard class more fun Due 3 weeks from now because of various intervening

21 Homework See PDFs attached on course webpage { Reading AND writing (problem-solving) { Latter: trying to make a hard class more fun Due 3 weeks from now because of various intervening holidays and covering of 2+ topics together, but assigning today to give you a huge head start on it { One inaccurate point in syllabus: homework won t always be assigned/due on Wed. s 21

Physics 207 Lecture 23

Physics 207 Lecture 23 Thermodynamics A practical science initially concerned with economics, industry, real life problems. DYNAMICS -- Concerned with the concepts of energy transfers between a system and its environment and