Transcript Heat and Phase Changes - Richard Barrans’s web site

Whiteboard Work
1.
The 2004 Tour de France’s Alpe d’Huez time trial
was a climb with its finish 1200 m higher than the
start. The winner, Lance Armstrong, and his gear
had a combined mass of 84 kg.
The work Lance had to do was mgh =
(84 kg)(9.8 N/kg)(1200 m) = about 106 J.
a.
b.
Muscle is about 20% efficient, so Lance had
to deplete 5 times as much potential energy
as the work he produced. How much
energy did he consume?
How much of the energy he consumed was
not converted to work?
Heat and Phase Changes
Our interaction with the world of
the small
What’s the Point?
• What happens to all our energy?
• How does energy convert in freezing,
melting, evaporation, and condensation?
Objectives
• Define and differentiate heat and
temperature.
• Explain the role of heat in conservation of
energy.
• Describe the energy transfer of phase
changes.
Mechanical Equivalent of Heat
James Joule’s life-long obsession
Identical effects of
adding heat
and doing work
Source: Griffith, The Physics of Everyday Phenomena
Heat and work
• Doing work on an object changes its
energy.
• Heat transfer is another way to change an
object’s energy!
• Work  heat: easy
• Heat  work: more difficult
Temperature and Energy
• Average translational molecular kinetic
energy is
1/2 kBT
per mode of motion.
• kB = 1.3806610–23 J/K
(Boltzmann constant)
• Individual molecules can have higher or
lower kinetic energies than average.
Terms
• Temperature is proportional to average
molecular translational kinetic energy.
• Internal energy (U) is total molecular
kinetic + potential energy.
• Heat is molecular energy transferred from
high to low temperature.
Heat Units
• Joule
• Calorie (cal): heat needed to raise 1 gram
of water 1 degree C (or K) = 4.184 J.
• British Thermal Unit (BTU): heat needed
to raise 1 pound of water 1 degree F =
1054.35 J
Poll Question
When a red-hot piece of iron is dropped into
a bucket of water,
A.
B.
C.
D.
E.
the water becomes hotter.
the water’s temperature increases .
the water’s internal energy increases .
the water receives heat from the iron.
all of the above.
Specific Heat (Capacity)
• Heat needed to change the temperature of
a unit amount of a substance.
q
c=
mDT
– q = heat input
– m = mass of sample
– DT = temperature change
• Units: J/(kg K) or J/(mol K)
• Intensive
Using c
The temperature change DT of an object to
which an amount of heat q is added is
DT = q/(mc)
where m is the object’s mass and c is its
specific heat.
Whiteboard Work
2. The specific heat of Lance Armstrong is
about the same as the specific heat of
water: 4184 J/(kg °C).
If all the non-work energy he converted
in the Alpe d’Huez climb stayed in his
75-kg body, by how many degrees would
his body temperature have risen?
Another Heat Unit
• U.S. Food Calorie: Cal = 1000 cal
• Food energy values are often presented in
kJ in other countries
Quick Whiteboard Work
3. How many Calories (food calories) did
Lance burn in the Alpe d’Huez climb?
Phase Changes
Phase Changes
• Melting, boiling, freezing, condensing…
• Added or removed heat changes the
substance’s potential rather than kinetic
energy
• Water freezes at 0 °C, boils at 100 °C (well,
about 92 °C in Laramie)
• Not all heat transfer is expressed as a
temperature change.
Heating Curve for Water
Water temperature with heating
temperature (C)
200
steam
Water boils
150
100
50 Ice melts
Liquid water
0
ice
-50
-100
0.0E+00
1.0E+06
2.0E+06
heat input (J/kg)
3.0E+06
4.0E+06
Think Question
When are the molecules’ intermolecular
potential energies highest?
A. When they are together in the liquid.
B. When they are separated in the gas.
Phase Changes
• Potential energies:
Solid < Liquid < Gas
• During a phase change, potential energy,
not kinetic energy (temperature) changes.
• Heating or cooling a changing phase does
not change its temperature!
Latent heat
• Potential energy of phase change (energy
required to change the phase of 1 kg of
substance)
• Water’s latent heat of fusion (melting):
335,000 J/kg
• Water’s latent heat of vaporization:
2,255,000 J/kg
Whiteboard Work
4. During the Alpe d’Huez climb, how much
sweat would Lance have needed to
evaporate to keep his body temperature
constant?
The heat q needed to vaporize a mass m of
water is
q = m (2.255  106 J/kg).
Solve for mass m and substitute in the values.
Evaporation of a Liquid
• More energetic jostling = higher
temperature
• An especially fast molecule at the surface
may detach!
Evaporation of a Liquid
• More energetic jostling = higher
temperature
• An especially fast molecule at the surface
may detach!
Evaporation
• Evaporating molecules carry away energy
• KE
PE
• Remaining liquid cools (KE decreases)
Reading for Next Time
• Thermodynamics
• Big ideas:
– Why most processes are irreversible
– Entropy