Chapter 2 Energy:
Look at fig 2.16
Energy is balanced, so temperature of earth and atmosphere is not changing
(equilibrium).
Energy from sun is radiation (electromagnetic waves).
Heat
A
B
Hot
Cold
Air or copper bar
Conduction : heat transferred from molecule to molecule
Convection : mass movement of fluid, like stirring – this will not happen in copper
Radiation : Electromagnetic waves emitted by A and absorbed by B
Note : No molecules of A leave and move to B or vice versa in conduction,
convection or radiation.
In figure 2.16, evaporation is different from the 3 above because mass leaves the earth.
Experiment : 2 burners, glass rod, Aluminum rod
Have student feel ends of rods opposite flame.
Aluminum is better conductor!
Which rod has hottest end in flame?
Glass rod glows ⇒ hotter (~500°C)
Now : We guess glass would be hotter because it is poorer conductor.
But, what if glass and Aluminum have different heat capacity?
Q
C =
= heat capacity
Tf - Ti
Where Q is the heat absorbed by an object, and Tf – Ti is its temperature increase.
Cglass << Caluminum
Let’s say, for example, that
Then ∆Tglass =
Q
C glass
and
∆Tal =
Q
C al
(assume Q is same for both)
⇒ ∆Tglass >> ∆Tal
So, this is a possible reason the glass got hotter, but it turns out that the specific heats are
nearly the same :
cglass = 0.20
caluminum = 0.22
[cal/g ⋅ K]
Specific heat is heat capacity per unit mass
So, C = heat capacity , refers to a particular object
c = specific heat , refers to a type of material
What else could affect which rod gets hotter?
- ability to absorb heat
Which gets hotter on a sunny day – a black dog or a white dog?
(both have same thermal conductivity and heat capacity!)
A perfect blackbody absorbs all radiation incident on it
- other things transmit or reflect some incident radiation
probably aluminum is better than glass – why?
ANS: Glass transmits a lot, and both aluminum and glass reflect some. Since aluminum
doesn’t transmit any, probably it is the better blackbody.
But small effect because glass is hotter!
Back to fig 2.16 – what is latent heat?
( Energy required to change the state of a material )
1
2
Each cup has same
number of H2O
molecules.
Ice water at
T1 = 0°C
Water at
T2 = 0°C
Add ¼ cup of water at 30°C to each cup - what happens?
T1 stays same – less ice (added heat goes towards converting ice to water {melting})
T2 goes up – added heat raises temp of water (sensible heat) – you can sense it
In figure 2.16, radiant energy from sun enters ocean, some of that energy converts liquid
water to vapor (latent heat of vaporization is absorbed).
- mass leaves earth and goes to atmosphere
Later, vapor condenses into water droplets in clouds, releasing latent heat to atmosphere.
Revisit Radiation:
Look at Figure 2.7 – all of these are electromagnetic (EM) waves
What is not an EM wave ?
Sound, water waves, particle waves (deBroglie)
What is an EM wave?
Oscillating electric and magnetic fields at right angles
Amazing facts:
1) Don’t need any matter to carry an EM wave
2) EM waves always travel at speed of c in vacuum, even in different reference
frames (special relativity)
What is a photon? Name of an EM wave that behaves as a particle!
How can EM radiation be both wave and particle (photon)?
Wave packet
Predominant property depends on scale of environment:
particle
Antenna to receive wave has dimension of ~ λ .
How can EM waves be emitted and absorbed by an object?
eEi
An atom in a gas:
Ef
+
Nucleus
In a solid, many atoms
bind together and discrete
energy levels become
bands
Any energy photon can
be absorbed or emitted
Note: A moving charge causes an EM wave to propagate away (antenna)
hc
= Ei - Ef
λ
Ephoton = photon energy =
hc
λ
λ is wavelength of wave
h is a constant
c is a constant (speed of light)
Example:
The energy of photons having λ = 1.0µm is 1.24 eV. What is the energy of photons
having λ =0.2µm ?
E1 =
hc
λ1
E2 =
E1 λ1 = E2 λ2
Therefore,
E2
=
E1
λ1
λ2
hc
λ2
=
1.24 eV
1.0µm
= 6.2 eV
0.2µm