Download Thermodynamics lesson 1 Tempersture

Thermodynamics
Should take about 7 weeks
Outcomes
• Be able to explain thermal equilibrium
• Describe the absolute scale of temperature
(i.e. the thermodynamic scale) that does not
depend on property of any particular
substance and explain why the triple point is
used.
• To be able to use and convert temperature
measurements both in degrees Celsius (°C)
and in kelvin (K)
• To recall that T(K)≈θ(°C) + 273…
Energy
• A can of worms – particularly at KS3
– Transfer
– Transport
– Transform
– Stores
– Pathways
• A problem, but not for today
• ( Some stuff on wiki, Millar)
Temperature and Heat
Light touch today, more another time
• Temperature – a measure of hotness or
coldness of an object
• Heat
(in parent language “we’ll see”)
– Energy
– Depends upon
• Mass
• Temperature
• Nature of object (specific heat capacity)
Temperature
• We all know what temperature is.
• So discuss.
• Watch the demo
• Zeroth Law.
Temperature Scales
• F,R,C,K
• oF is for old people, like pounds and
ounces BUT conversion is a skill so lets
not dispose of it all together
• R just for some US engineers
• oC not C. Centigrade just means that,
we want Celsius, and degress at that.
• K is not oK as it is absolute.
small point but important
Lets look at heat moving
(thermal transfer or energy)
HEAT
Work and heat
Work: energy transferred to a system by the
application of a force (ΔW)
Heat: energy transferred not by a force and our
old friend ΔT is the driving force for this (ΔQ)
Now, we are nearly ready to jump into the world
of thermodynamics
We want to care about particles
Just not yet – stay macro.
Bulk properties = not particle
Lets look at reality – go Macro
When I heat things, they expand
Thermal Expansion
ΔL = k L ΔT
So?
Well the amount something expands when
heated depends on how long it was in the first
place (L), the amount it’s temperature
changes (ΔT) and something to do with the
material (k) called the coefficient of thermal
expansion.
Two important physics ideas
• The coefficient: A way of making an inequality
into an equals. BUT the key thing here is that
it is something for a material and NOT an
object. Work out k for Copper and you can do
the sums for any object made of copper
• The gradient: A driving force behind so much
of things happening in physics. ΔT here but
could be anything
– ΔK is equivalent to ΔoC but best go the K way
Let’s quantify ‘heating up’
E = m c ΔT
c is specific heat capacity of material
Units= Jkg-1K-1
What happens when state changes?
Possibly not what you might expect
Because of state change
m c ΔT isn’t enough
E=mL
L is specific latent heat of fusion/vaporisation
Units= Jkg-1
Change of state
AT CONSTANT
TEMPERATURE
QE = m c ΔT
QE == m
m LL
Q=mL
Q = m c ΔT
Q = m c ΔT
Q=mL
Q = m c ΔT
Both L and c are material and not object specific
quantities, much more useful.
Thermal transfer of energy
Conduction
– Transferred directly within a material
– ΔT across material is the driving force
Convection
– Transport by bulk movement
– Density, buoyancy, currents
– Free and forced, Newton, T or T5/4
Radiation
– By means of electromagnetic waves
– The black body
– Stephan
Conduction
• Good conductors (metals) it’s mainly electrons
• Poor conductors it’s mainly inter-atomic
collisions
We have idealised models
Because the truth is messy
Thermal conductivity
We can quantify an ideal situation
Q/t = k A ΔT/L
Q/t = Rate of heat flow
k = Thermal conductivity (Wm-1K-1)
A = Cross sectional area
ΔT/L = Temperature gradient
An experimental value
U takes into account the reality of the situation
including convection at surface and a slow
moving ‘trapped’ layer
Radiation
• The energy radiated per second
– Area
– Temperature
– Nature of object
• Why T and not ΔT?
– Well, we are all at it. It is just often Qin=Qout
• What comes out?
– A continuous span of wavelengths, dependent upon T
– At T < 1000K almost all IR
– At T > 1000 Visible and UV also (1700K is white hot)
Radiation
The energy radiated per second
– Area
– Temperature
– Nature of object
As an equation
Q/t = e σ A T4
Q/t = rate of energy emitted by radiation
e = emissivity (B=1 skin=0.7)
σ = SB constant 5.67 × 10-8 Js-1m-2K-4
A = Area
T4= Temperature K
Some more terms
Internal energy: Potential energy in bonds and
KE of particle motion (ΔU)
Adiabatic: No heat transfer (ΔQ=0)
Isothermal: You guessed it (ΔT=0)
Now, lets go...
0,1
Zeroth: If Q=0 then ΔT = 0
First: ΔQ = ΔU + ΔW
Signs really matter
• ΔQ = Heat entering
• ΔU = Change in internal energy
• ΔW = Work done BY body
ΔQ
ΔU
ΔW
JPJ
The mechanical equivalence of heat
2
What we normally want is ΔQ going to ΔW
This is sort of the point of most engines
But life isn’t like that and imperfect.
The second law quantifies the imperfection
η = W/Q
• η = efficiency of heat engine
• W = work done by engine
• Q = heat provided to engine