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Thermal Phys

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by

Emma van Leeuwen

on 20 May 2016

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Transcript of Thermal Phys

Insulators
Insulators are
poor conductors of heat.

See this saucepan handle?
It's made of
insulating materials
Many animals have a
layer of fat
and
feathers or fur
that
fluff up to trap more air
Conduction
,
Convection
, and
Radiation
When
radiated energy
hits a surface
three
things can happen:

Temperature
is a measure of the
average kinetic energy
of the particles in an object
Time how long the wax takes to melt
Draw a graph of your results
Go to different surfaces around the school:
Take off a shoe, rank from 0-10 how cold/hot it feels on your foot.
Measure the temperature with the laser gun
Comment on your results
concrete, grass, carpet, plastic (chair), linoleum, plaster (wall), wood, tree, stone, tile
Air
trapped by woolen blankets and jumpers help
insulate our bodies from losing heat
Gases are also

insulators
Objects can transfer heat in the form of
electromagnetic waves
which

can travel though a
vacuum
some of which we can see....
We
SEE
visible light as colour
We
FEEL
infrared as heat
Blue - white - yellow - red
Hottest
Coolest
The colour changes too:
Which stars are the hottest?
See this one?
It's not. It will burn you.
Radiation
Convection & conduction
Radiation
...and some we can't
When particles are hot they vibrate and collide with neighboring particles spreading the vibration through or between
touching solids
until
thermal equilibrium
is reached
Conduction & Convection
Convection
As air warms up, it becomes less dense and is pushed up by cooler air that is sinking. These convection currents spread heat in
fluids

(
gases and liquids).
~~~Convection Currents~~~
Conduction
3 Modes of Heat Transfer:
The amount of energy needed to raise the temperature of 1 kg of a substance by 1 K (or 1 C)
o
Specific heat capacity (c)
Q = m c T
EX: How much energy is needed to heat 0.5 kg of Copper by 10 K?
The amount of energy needed to
change the state
of 1 kg of a substance
Specific latent heat (l)
EX: How much energy is needed to melt 3 kg of Pb?
How much energy is released to liquify 2 kg of He gas?
*Draw state changes on board
E in
to break bonds (endothermic)
Latent heat of fusion (l )
f
NB: There is no temperature change during a phase change
Q = m l
The amount of energy needed to
change the state
of 1 kg of a substance
Specific latent heat (l)
Land has a lower specific heat than water so it heats and cools faster leading to monsoons
Climate
So how to clothes dry?
Even in summer it does not get hot enough for water to boil ...
Evaporation
Heat Transfer
Heat always flows from hot to cold
"Cold" does not flow.
Heat flows out of sth; cold does not flow into it.
Convection currents in Earths mantle...
And atmosphere; small scale weather...
And large scale...
Also why bombs make mushroom clouds
Temperature Scales
Zeroth Law of Thermodynamics
the telescopes currently operating at each wavelength
How much light?
Light energy from stars
B = T
4
L = 4 R T
2 4
energy radiated per second per unit area
Actual brightness (B)
Luminosity (L)
total energy radiated per second
=
Stefan-Boltzmann
constant
= 5.67x10 Wm k
=
-2 -4
SA =
4 R
2
depends on
temperature
size
-8
http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html
Wien's Displacement Law:
peak
=
2.9x10
T
-3
_______
http://www.mhhe.com/physsci/astronomy/applets/Blackbody/frame.html

"continuous spectrum"
ex:
gif: http://voyager.egglescliffe.org.uk/physics/astronomy/blackbody/animation.gif
Blackbodies
all
incident light is absorbed
all
light coming from it is due to its temperature (blackbody radiation)
hole in a cavity
Star
Idealised objects:
(not perfect but as close as it gets irl)
(theoretically perfect)
Planck's Law (graph)
Classification System:
Oh Be A Fine Girl/Guy Kiss Me
(according to your preference)
https://astrojourney.wordpress.com/tag/star-colors/
Black body radiation is
continuous
(no missing wavelengths)
about room/you/me temp
Mostly IR
Rods used in dim light
can't detect colour well
temperature of photosphere (outermost layer of star)
All matter with a temperature greater than absolute zero emits electromagnetic radiation!
The spectrum it emits is called
blackbody radiation
and it depends
only
on its temperature
Examples:
We need machines to detect the rest...
In reality the night sky is very colourful:
Our night vision sucks so most stars look white.
Rods used in dim light
can't detect colour well
So where are these coloured stars then?
Our Star
Dark colours absorb thermal radiation
Light colours reflect thermal radiation
Clear materials transmit thermal radiation
* tin can prac??
"Objects". What objects?
Blackbody radiation
Earths climate
Goldilocks Zone
not too hot, not too cold
Blackbody radiation from sun
mostly visible light
Some reflected rest
absorbed
increasing Earths temperature
Earth also produces blackbody radation but
mostly Infrared
The hotter Earth gets the more IR it emits until a balance is reached = stable climate
IR absorbed and re-emitted
atmosphere adds 33 degrees
Traps heat
Atmosphere traps heat in
First law of Thermodynamics
The change in
internal energy of a system (U)
is equal to the sum of energy entering the system
through heating (Q)
and energy entering a system
through work done on it (W)
2 ways you can heat something:
Heating:
apply a temperature gradient (put it next to sth hot)
Doing work:
apply a force over a distance (agitate, compress, rub)
Pressure
Temperature
Force
x
I
I
I
U = Q + W
I
I
I
I
I
I
Systems can also
do work
(apply a force over a distance) and loose internal energy
I
I
I
U = Q - W
I
I
I
I
I
I
Heat in (Q)
Force
Work done (W)
States of matter
Alcohol freezing point:
-115 C
o
Measuring Temperature
The more energetic particles can escape and form a vapor, lowering the overall average kinetic energy of the remaining liquid (i.e. lowering the temperature)
Entropy
The disorder of a system
Natural processes increase entropy
Need to do work to decrease entropy
until
thermal equilibrium
is reached
If A is in thermal equilibrium with B and B is in thermal equilibrium with C then A and C are also in thermal equilibrium
Exception: Water expands when frozen
Matter expands when heated
Gas:
-
- diatomic molecules move faster but also spin and stretch (rotational EK and vibrational EK as well as translational)
- multi atom (e.g. H2O) can spin, stretch and twist as well
Liquid
-
Solid
- also has electrostatic potential energy


Monatomic
gas molecules (e.g. Ne) move faster when heated

(translational KE)
increasing its temperature
There are
3

translational

degrees of freedom
Diatomic
molecules move faster (translational KE) but also spin
(rotational KE)
and stretch
(vibrational KE)
These rotational and vibrational degrees of freedom do not increase the temperature
Polyatomic
molecules have even more vibrational degrees of freedom (twisting and bending)
Only translational motion is related to temperature
The more degrees of freedom the higher the specific heat
Degrees of Freedom
The number of ways a molecule can move
How many “air molecules” are there in this class room?
If these are all moving in random directions, what is the probability they will all be found in the left half of the room?
The hotter Earth gets the more IR it emits until a balance is reached -> stable climate
Earth should be -18 degrees!
Energy input
= energy reflected
+ energy absorbed
& re-radiated
Total energy must be conserved
percentage of light reflected
The Albedo
A=31%
A=35%
A=50%
A=99%
Blackbody radiation
340 W in = 100 W reflected + ? absorbed &re-radiated
The average temperature is actually +15 degrees
Clearly we are missing something....
240 W
1.7 ppm
about 1%
Methane
Only tri-atomic molecules or greater absorb and re-emit IR
Gases in the Atmosphere
*
* increasing
Oceans (human changes insignificant)
(0.03%)
(0.06%)
The bad news: we are on a path to self-destruction
The good news: some people are doing something
Tesla Motors received > 325,000 reservations for the Model 3
Mostly visible light
mostly IR
Are such small amounts really noticeable?
temperature means atoms are accelerating and accelerating charges produce electromagnetic radiation
Blackbody Radiation
(tricky concept - just accept it for now)
Not us though. We kinda suck ....
... quite a lot actually.
But it's the poorest and least polluting countries who are going to pay the highest price.
wtf Australia :(
Full transcript