Download Introduction, and 1st Topic: Temperature!

st
1
Introduction, and
Topic: Temperature!
Modern Physics
August 29, 2016
1
Syllabus and Website
|  http://www.albany.edu/physics/phy240.shtml
|  One more item that I was only reminded of this
morning: TO CONTINUE AS A PHYSICS MAJOR
YOU NEED TO GET A GRADE OF C OR HIGHER IN
THIS COURSE (NEW POLICY. STRICTLY ENFORCED)
|  Everything will be posted OR e-mailed. So, for
note-taking, you only take notes on the “missing
bits.” Most of the time just sit and LISTEN TO ME
2
Extra Credit: Forgot
|  Attendance at the weekly departmental
seminars and colloquia (note, free food :)
|  Graduate student seminars: 3pm Tuesdays in
Room 129 (first floor lecture room)
|  Physics colloquia: 3pm Fridays also in 129
|  I am sorry if you can’t make either of these,
but remember this is bonus not required so I
am not obligated to make it possible for all
|  Up to a maximum of 5% credit *on your final
grade* so that is a lot (1% per attendance)
3
Grading System
|  >90% is an A (informal A+ included)
|  80-90% is a B
|  70-80% is a C
|  60-70% is a D
|  <60% is an E
|  Broken down into +’s and –’s by thirds in
each range
|  Will curve only if needed
{  I will not allow: many E’s, few A’s
4
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 19th century and during the
early 20th 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??!!
|  Evidence was beginning to gather to imply that there
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
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 (dedicated6course)
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 (0th/1st/2nd/3rd) 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 email
|  Thermodynamics essentially sheds light on
macroscopic properties of systems, which arise from
the internal structure such as atoms and molecules,
7
without getting bogged down
in little details
Temperature 1
|  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?
{ 
When objects are in thermal contact, energy flows
between them (on a hot summer day, hold an ice
cube to your forehead)
Not a
reliable
|  Essentially, what you sense is the rate at which energy
measureis transferred into or out of your foot
ment
|  When energy is no longer being transferred between
two objects, they are said to be in thermal
EQUILIBRIUM (I’ll highlight8important jargon in CAPS)
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
{  Energy always flows from hotter to cooler
|  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
Temperature Scales
|  0 Kelvin (the *absolute* scale, in SI system)
{  Is -273.15 degrees C and -459.67 degrees F
|  0 degrees Celsius (metric system nations)
{  Is +273.15 K (no degrees) and 32 degrees F
{  Water freezes
|  100 degrees Celsius (reason “Centigrade”)
{  Is +373.15 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
Problems 1 and 2
|  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)
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 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 = ( delta-V / Vi ) / deltaT (one
can relate alpha and
beta). (Area too)
12
Thermal Expansion 2
Aluminum
Lead
Glass
Steel
Brass
alpha(°C^-1)
24x10^-6
29x10^-6
9x10^-6
11x10^-6
19x10^-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
Problems 1 and 2
|  A steel railroad track has L = 30.0 m @ 0°C.
What is its length @ 30°C?
|  At which temperature will the two example
steel and brass bolts touch?
Steel
L_B = 0.030 m
L_S = 0.010 m
Brass
5 um
@27°C
14
Bimetallic Strips
An example of how the property of thermal expansion/contraction is exploited in real life.
Thermostat with bimetal coil at (2)
15
Solid 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 water,
there is usually cold water at
the bottom, which allows
plants/animals to survive!!
Ideal Gas
|  For solids or liquids, beta = ( deltaV / Vi ) /
deltaT
|  However, for a gas what is the initial volume?
{  It depends on the container
Imagine
pumping the
{ 
same amount
of air into a
bicycle tire
versus a car
tire
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 3 quantities
|  In general, relationship can be very
complicated but, as usual, we start with
simple assumptions 17
Equation of State
|  Assumptions
Such a gas does not exist in real
life, but this is a useful starting point
{  T is neither too high, nor too low
{  P is low
{  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
For H -> M mass and M is molar mass, i.e., atomic mass
= 1 g / mol
{  So, 1 mole of a gas has a mass equal to the
He -> ~4 g
atomic mass and contains 6.022 x 1023 atoms
O -> ~16 g
(Avogadro’s number,
or, NA)
18
Ar -> ~40...
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 = 8.314 J / ( mol * K)
|  Joules = N * m
L is liters and atm is unit of Pressure
|  Alternate: 0.082 L * atm / ( mol * K )
|  P V = ( N / NA ) * R * T = N * kB * T
19
{  Boltzmann’s constant = R/NA = 1.38x10-23 J/K
Problems 1 and 2
|  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.)
P0 = atmospheric pressure
mass = m
h
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
20
piston will be in equilibrium
(Hint: equalize F)
Homework
|  Read PDF attached to course webpage
21