Transcript Heat Transfer - SFSU Physics

Evaporation and condensation
• Individual molecules can
change phase any time
• Evaporation:
– Energy required to
overcome phase cohesion
– Higher energy molecules
near the surface can then
escape
• Condensation:
– Gas molecules near the
surface lose KE to liquid
molecules and merge
Ways to Increase Evaporation Rate
• Increase temperature
– Kinetic energy increases which increases the number of
high-energy molecules that can escape from liquid state
• Increase surface area of liquid
– Increases the likelihood of molecules escaping to air
• Remove water vapor from surface of the liquid
– Prevents return of vapor molecules to liquid state
• Reduce pressure on liquid
– Reduces one of the forces holding molecules in liquid
state
Relative Humidity
• Ratio of how much water vapor is in the air
to how much water vapor could be in the air
at a certain temperature
• Expressed as a percentage
Relative Humidity =
Water vapor in air
Capacity at present temperature
X
100 %
Heat Transfer
Heat flow
Three mechanisms for heat transfer due to a
temperature difference
1. Conduction
2. Convection
3. Radiation
Natural flow is always from higher temperature
regions to cooler ones
Conduction
• Heat flowing through
matter
• Mechanism
– Hotter atoms collide with
cooler ones, transferring
some of their energy
– Direct physical contact
required; cannot occur in
a vacuum
• Poor conductors =
insulators (Styrofoam,
wool, air…)
Conduction is the flow of heat directly
through a physical material.
Experimentally, it is found that the amount of
heat Q that flows through a rod:
• increases proportionally to the crosssectional area A
• increases proportionally to the temperature
difference from one end to the other
• increases steadily with time
• decreases with the length of the rod
16-6 Conduction, Convection, and Radiation
Combining, we find:
The constant k is called the thermal
conductivity of the rod.
Some typical thermal
conductivities:
Substances with high
thermal conductivities
are good conductors of
heat; those with low
thermal conductivities
are good insulators.
Convection
• Energy transfer
through the bulk
motion of hot material
• Examples
– Space heater
– Gas furnace (forced)
• Natural convection
mechanism - “hot air
rises”
Convection is the flow of
fluid due to a difference
in temperatures, such as
warm air rising. The fluid
“carries” the heat with it
as it moves.
Radiation
• Radiant energy - energy associated with electromagnetic
waves
• Can operate through a vacuum
• All objects emit and absorb radiation
• Temperature determines
– Emission rate
– Intensity of emitted light
– Type of radiation given off
• Temperature determined by balance between rates of
emission and absorption
– Example: Global warming
Electromagnetic Spectrum
• Transverse waves
• Regenerating co-oscillation of
electric and magnetic fields
• Electric, magnetic and velocity
vectors mutually
perpendicular
• Form when electric charge is
accelerated by external force
• Frequency depends on
acceleration of charge
– Greater the acceleration, higher
the frequency
Blackbody radiation
Blackbody
– Ideal absorber/emitter of
light
– Radiation originates from
oscillation of nearsurface charges
Increasing temperature
– Amount of radiation
increases
– Peak in emission
spectrum moves to
higher frequency
Spectrum of the Sun
All objects give off energy in the form of
radiation, as electromagnetic waves – infrared,
visible light, ultraviolet – which, unlike
conduction and convection, can transport heat
through a vacuum.
Objects that are hot enough will glow – first
red, then yellow, white, and blue. Objects at
body temperature radiate in the infrared, and
can be seen with night vision binoculars.
The amount of energy radiated by an object due
to its temperature is proportional to its surface
area and also to the fourth (!) power of its
temperature.
It also depends on the emissivity, which is a
number between 0 and 1 that indicates how
effective a radiator the object is; a perfect
radiator would have an emissivity of 1.
Thermodynamics
Thermodynamics
• The study of heat and
its relationship to
mechanical and other
forms of energy
• Thermodynamic
analysis includes
– System
– Surroundings (everything
else)
– Internal energy (the total
internal potential and
kinetic energy of the
object in question)
Heat engines - devices
converting heat into
mechanical energy
The Zeroth Law of Thermodynamics
If object A is in thermal
equilibrium with object
C, and object B is
separately in thermal
equilibrium with object
C, then objects A and B
will be in thermal
equilibrium if they are
placed in thermal
contact.
The First Law of Thermodynamics
The first law of thermodynamics is a statement of
the conservation of energy.
If a system’s volume is constant, and heat is
added, its internal energy increases.
The First Law of Thermodynamics
If a system does work on the external world, and
no heat is added, its internal energy decreases.
The First Law of Thermodynamics
Combining these gives the first law of
thermodynamics. The change in a system’s
internal energy is related to the heat Q and the
work W as follows:
It is vital to keep track of the signs of Q and W.
The First Law of Thermodynamics
The internal energy of the system depends only
on its temperature. The work done and the heat
added, however, depend on the details of the
process involved.
The Second Law of Thermodynamics
We observe that heat always flows
spontaneously from a warmer object to a
cooler one, although the opposite would not
violate the conservation of energy. This
direction of heat flow is one of the ways of
expressing the second law of
thermodynamics:
When objects of different temperatures are brought
into thermal contact, the spontaneous flow of heat
that results is always from the high temperature
object to the low temperature object. Spontaneous
heat flow never proceeds in the reverse direction.
Refrigerators, Air Conditioners, and Heat
Pumps
While heat will flow spontaneously only from a
higher temperature to a lower one, it can be
made to flow the other way if work is done on
the system. Refrigerators, air conditioners,
and heat pumps all use work to transfer heat
from a cold object to a hot object.
Refrigerators, Air Conditioners, and Heat
Pumps
If we compare the
heat engine and the
refrigerator, we see
that the refrigerator
is basically a heat
engine running
backwards – it uses
work to extract heat
from the cold
reservoir (the inside of the refrigerator) and
exhausts to the kitchen.
Refrigerators, Air Conditioners, and Heat
Pumps
An air conditioner is
essentially identical to a
refrigerator; the cold reservoir
is the interior of the house or
other space being cooled, and
the hot reservoir is outdoors.
Exhausting an air conditioner
within the house will result in
the house becoming warmer,
just as keeping the refrigerator
door open will result in the
kitchen becoming warmer.
Refrigerators, Air Conditioners, and Heat
Pumps
Finally, a heat pump is the
same as an air conditioner,
except with the reservoirs
reversed. Heat is removed
from the cold reservoir
outside, and exhausted
into the house, keeping it
warm. Note that the work
the pump does actually
contributes to the desired
result (a warmer house) in
this case.
Entropy
For this definition to be valid, the heat transfer
must be reversible.
In a reversible heat engine, it can be shown
that the entropy does not change.
Second law: Entropy
• Real process = irreversible
process
• Measure of disorder =
entropy
Second law, in these terms:
• The total entropy of the
Universe continually
increases
• Natural processes degrade
coherent, useful energy
– Available energy of the
Universe diminishing
– Eventually: “heat death”
of the Universe
• Direction of natural
processes
– Toward more disorder
– Spilled milk will never
“unspill” back into the
glass!
18-8 Entropy
A real engine will operate at a lower efficiency
than a reversible engine; this means that less
heat is converted to work. Therefore,
Any irreversible process results in an
increase of entropy.
Entropy
To generalize:
• The total entropy of the universe increases whenever
an irreversible process occurs.
• The total entropy of the universe is unchanged
whenever a reversible process occurs.
Since all real processes are irreversible, the
entropy of the universe continually increases. If
entropy decreases in a system due to work
being done on it, a greater increase in entropy
occurs outside the system.
18-8 Entropy
As the total entropy of the universe
increases, its ability to do work decreases.
The excess heat exhausted during an
irreversible process cannot be recovered;
doing that would require a decrease in
entropy, which is not possible.
18-9 Order, Disorder, and Entropy
Entropy can be thought of as the increase in
disorder in the universe. In this diagram, the
end state is less ordered than the initial state –
the separation between low and high
temperature areas has been lost.
18-9 Order, Disorder, and Entropy
If we look at the ultimate fate of the universe in
light of the continual increase in entropy, we
might envision a future in which the entire
universe would have come to the same
temperature. At this point, it would no longer be
possible to do any work, nor would any type of
life be possible. This is referred to as the “heat
death” of the universe.
18-9 Order, Disorder, and Entropy
So if entropy is continually increasing, how is
life possible? How is it that species can evolve
into ever more complex forms? Doesn’t this
violate the second law of thermodynamics?
No – life and increasing complexity can exist
because they use energy to drive their
functioning. The overall entropy of the universe
is still increasing. When a living entity stops
using energy, it dies, and its entropy can
increase rather quickly.
The Third Law of Thermodynamics
Absolute zero is a temperature that an object
can get arbitrarily close to, but never attain.
Temperatures as low as 2.0 x 10-8 K have been
achieved in the laboratory, but absolute zero will
remain ever elusive – there is simply nowhere to
“put” that last little bit of energy.
This is the third law of thermodynamics:
It is impossible to lower the temperature of an object
to absolute zero in a finite number of steps.
Q1.
Substance A has a higher specific heat than
substance B. Which requires the most energy to
heat equal masses of A and B to the same
temperature?
A) Substance A
B) Substance B
C) Both require the same amount of heat.
D) Answer depends on the density of each substance.
Q2.
Anytime a temperature difference occurs, you can
expect
A) cold to move to where it is warmer.
B) energy movement from higher temperature regions.
C) no energy movement unless it is warm enough, at
least above the freezing temperature.
D) energy movement flowing slowly from cold to
warmer regions.
Q3.
As a solid goes through a phase
change to a liquid, heat is absorbed
and the temperature
A) increases.
B) decreases.
C) remains the same.
D) fluctuates.
Q4.
The transfer of energy from molecule to
molecule is called
A) convection.
B) radiation.
C) conduction.
D) equilibrium.