physics 2b: lecture 1 secs. 11.1 - 11mulligangroup.ucr.edu/2bw2017lecture1.pdf · clicker questions...

Physics 2B: Lecture 1 Secs. 11.1 - 11.5

course organization

basic thermodynamics

course outline and organization

ALL THIS STUFF CAN BE FOUND ON THE ILEARN WEBSITE


Physics 2B: thermodynamics, fluids, and electricity and magnetism

8:10-9:00 AM MWF - Michael Mulligan

12:10-1:00 PM (full) and 3:10-4:00 PM (open) MWF - Stephen Wimpenny

[email protected]

office hours: Monday 9:30-11:30 AM

MS&E 342

textbook: University Physics for the Life Sciences by Knight, Jones, and Field


Grading Policy:

discussion problems (once a week): 10%

midterm exam (February 6 from 8:10-9:00 AM): 20%

final exam (March 22 from 3:00-6:00 PM): 40%

homework (3 problem sets each week): 15%

in-class clicker problems (every class): 15%


Grading Policy:


midterm exam (February 6 from 8:10-9:00 AM): 20%

final exam (March 22 from 3:00-6:00 PM): 40%



in previous years, about 85-90% of people passed with a grade of C- or better, 15-25% have gotten A- or better

approximate lower grade cutoffs (absolute grading scale): A-: 75%, B-: 60%, C-: 45%

clicker questions

CLICKER PROBLEMS BEGIN NEXT LECTURE (WEDNESDAY)


Grading scheme

correct answer: 3 points

incorrect answer: 1 points

no answer: 0 points

clicker questions

CLICKER PROBLEMS BEGIN NEXT LECTURE (WEDNESDAY)


Grading scheme

correct answer: 3 points

incorrect answer: 1 points

no answer: 0 points

clicker problems: based on the reading assignments for the current lecture — see Class Calendar tab in iLearn — or previous lecture

Clicker Test

Who is this?

A. Isaac Newton

B. Albert Einstein

C. Ludwig Boltzmann

D. Gandalf the Grey

clickers

If you haven’t done so, please make sure you purchase and register a clicker by next class.

Note: Physics 2000 no longer supports IR clickers.

If you have one of these (yellow, orange, green stick-type clickers) you’ll need to replace it with the new grey model.

homework


accessible via Mastering Physics on iLearn

typical homework schedule:

Monday assignment is due Wednesday at 11:50PM

Wednesday assignment is due Friday at 11:50PM

Friday assignment is due the following Monday at 11:50PM

a guide to registering is in the Class Organization tab in iLearn

all assignments for a particular topic released on first lecture on that topic — 3 assignments released TODAY with the first due WEDNESDAY

homework

LATE HOMEWORK SUBMISSIONS WILL NOT BE ACCEPTED

if the graded homework assignments aren’t enough, we’re also providing additional problems that have some type of biological application for the concepts

these are NOT graded, even though, they have due dates

discussion sections


you should only attend the discussion section you’ve signed up for

discussion sections begin this Wednesday, January 11

these sessions will go over problem solving strategies while working problems and (hopefully) help you learn the material better

exams

Midterm: Monday, February 6 from 8:10 - 9:00 AM

Final: Wednesday, March 22 from 3:00 - 6:00 PM

midterm review session: Friday, February 3 sometime

final review session: Friday, March 17 sometime

exams

Midterm: Monday February 6 from 8:10 - 9:00 AM

Final: Wednesday March 22 from 3:00 - 6:00 PM

seating is un-assigned for the midterm and assigned for the final

things to know:

exams will begin on time and no additional time given if you arrive late

you need some type of picture ID for the exams

exams are closed book and notes; basic formulas and numerical constants will be provided

under normal circumstances: early or make-up exams will NOT be offered

Labs: Physics 2LB

the labs are run independently of Physics 2B

questions: please contact Prof. Hanson, who will be responsible for Physics 2LB this quarter

first lab sessions begin TODAY

course outline — see Course Calendar for details

thermodynamics: chapters 11 - 12 (2 weeks)

fluids: chapter 13 (1 week)

electricity and magnetism: chapters 20 - 25 (6 weeks)

we’ll go thru about one chapter each week

there are a lot of different concepts to learn and we’ll move fast

thermodynamics

goals/topics for this lecture

sections: 11.1-11.5

heat and the first law of thermodynamics

what’s thermodynamics?


thermodynamics is the study of what can be said about a system without knowledge of its microscopic details



in Physics 2A, you studied Newton’s law and its consequences

~F = m~a



in Physics 2A, you studied Newton’s law and its consequences

~F = m~a

in the thermodynamics part of Physics 2B, we’ll study systems composed of “a lot” of particles in a box

our goal: to say what we can about the many-particle system

what’s thermodynamics?by “a lot,” I mean something like Avogodro’s number of particles 1023

in classical mechanics, to specify the “state” of the system, we’d need to specify the positions and velocities of each particle

keeping all of this information is rather challenging; furthermore, doing something with it is even harder

instead of keeping track of each particle individually, we think of them collectively in terms of macroscopic properties like

total mass of the system (kg)

pressure (Pa = N/m^2)

volume occupied (m^3)

number of particles

average kinetic energy (J)


total mass of the system (kg)

pressure (Pa = N/m^2)

volume occupied (m^3)

number of particles

average kinetic energy (J)

these macroscopic properties describe the “state" the system

thermodynamics can sometimes be hard because not all of these macroscopic properties are independent (the minimal number of things you need to completely specific a system) or you can often say the same thing in many different ways

what this means is that you’ll have to familiarize yourself with this redundancy

macroscopic properties of a system

the average kinetic energy of a particle defines the system’s “temperature” T:

T / Kaverage

possible units for measuring temperature:

Kelvin (K)

Fahrenheit ( )�F

Celsius ( )�C

T (�C) =5

9

⇣T (�F )� 32�

⌘

T (K) = T (�C) + 273

temperature

T (K) � 0

temperature

the average kinetic energy of a particle defines the system’s “temperature” T:

T / Kaveragebecause we are thinking of temperature as the average kinetic energy — a non-negative quantity — we use Kelvin as our temperature unit

Lord Kelvin

temperature

T =2

3

Kaverage

kB

Natoms

= number of atoms

Ethermal

= Natoms

Kaverage

=3

2N

atoms

kBT

Boltzmann

0s constant = kB = 1.38⇥ 10

�23J/K

thermal energy

thermal energy is linearly proportional to temperature and number of particles

thermal energy

Ethermal

= Natoms

Kaverage

=3

2N

atoms

kBT

SI units for thermal energy: Joules (J)

another option: calories

1 calorie = 4.186 Joules

1 calorie is the energy needed to raise 1 gram of water 1 degree K

forms of energy (not entirely distinct)kinetic energy of motion: translations and rotations

potential energy: gravitational (later, we’ll study electromagnetic)



chemical energy: energy held in bonds between collections of atoms (again, we’ll study this more precisely later in the course)



chemical energy: energy held in bonds between collections of atoms (again, we’ll study this more precisely later in the course)

thermal energy: average kinetic energy

for the most part in the thermodynamics part of the course, we’ll only be interested in thermal energy and ignore these other forms

when there is friction, the plane and block heat up and gain thermal energy

thermal equilibriumtwo systems in thermal contact with one another are in thermal equilibrium if they have the same temperature: T1 = T2

suppose:T1 < T2

T1 < Teq < T2

then thermal energy will generally transfer between the two systems so that they equilibrate to the same temperature Teq

Teq Teq

T1 T2

Clicker Question (not for credit, only for fun)

In the previous example, where the two systems are each a collection of atoms in a box,

T1 < T2

T1 T2

are the (absolute) thermal energies of the two systems necessarily equal after they have equilibrated ?

A. YesB. No

T1 = T2


In the previous example, where the two systems are each a collection of atoms in a box,

T1 < T2

T1 T2

are the (absolute) thermal energies of the two systems necessarily equal after they have equilibrated ?

A. Yes

B. No

Why? soEthermal =3

2NkBT E1 6= E2 if N1 6= N2

T1 = T2


in this example, the change in the thermal energy of the two systems are of equal magnitude, but opposite sign

in this example, this change is called heat energy Q

Q1 = �Q2

T1 < T2

T1 T2

so �E1 = Q1 > 0

�E2 = Q2 < 0

Note: the change in energy of the total system composed of system 1 and system 2 does not change — energy conservation


T1 < T2

T1 T2

more precisely, we’ll typically consider two distinct ways that the thermal energy of a system can change:

1. transfer of heat to or from the system

2. performing work on or by the system

in this example, no work is done on system 1 (or system 2), only heat energy is transferred between the two systems

typically, if there is no work done on the system, but the thermal energy changes, there must have been some heat transfer


we’ll define work thru the typical example that we’ll consider

increase the pressure on a box of gas and don’t allow heat transfer

Ti 7! Tf

Ti < Tf

thermal energy change is equal to the work done if no heat is transferred

�E =3

2NkB(Tf � Ti) = W

heat and the first law of thermodynamicsin general, the change of thermal energy of a system is due to heat loss or gained and work given or received

first law of thermodynamics

�E = Q+W

system environment

heat and the first law of thermodynamics�E = Q+W

Qs > 0 :system absorbs heat

Qs < 0 :system loses heat

Ws > 0 :work is done on the system

Ws < 0 :system does work

�Es > 0 : system gains thermal energy

�Es < 0 : system loses thermal energy


What is true about the heat ?

A.B.

Qs

C.

Suppose the environment does positive work on the system, but the temperature of the system does not change . Ti = Tf

0 < Ws = 35J

Qs = 35J

Qs = �35J

Qs = 0J


What is true about the heat ?

A.B.

Qs

C.0 =

3

2NkB(Tf � Ti) = Qs +Ws

Suppose the environment does positive work on the system, but the temperature of the system does not change . Ti = Tf

0 < Ws = 35J

Qs = 35J

Qs = �35J

Qs = 0J

summary

Ethermal =3

2NkBT

�Ethermal = Qs +Ws

next lecture

Secs. 11.5-11.7

heat engines and heat pumps

physics 2b: lecture 1 secs. 11.1 - 11mulligangroup.ucr.edu/2bw2017lecture1.pdf · clicker questions...

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