Changes of Phase

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27 Οκτ 2013 (πριν από 4 χρόνια και 11 μέρες)

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Changes of Phase


List the four phases of
matter in order of increasing
internal energy

Changes of Phase


List the four phases of
matter in order of increasing
internal energy

solid => liquid => gas =>
plasma


List the processes that take
you from a solid to a plasma


Changes of Phase


List the four phases of
matter in order of increasing
internal energy

solid => liquid => gas =>
plasma


List the processes that take
you from a solid to a plasma

melting => evaporation =>
ionization


List the processes that take
you from a plasma to a solid

Changes of Phase


List the four phases of
matter in order of increasing
internal energy

solid => liquid => gas =>
plasma


List the processes that take
you from a solid to a plasma

melting => evaporation =>
ionization


List the processes that take
you from a plasma to a solid

electron capture =>
condensation => freezing

Evaporation and Boiling


Evaporation


A phase change from liquid to
gas at the surface of a liquid


Which molecules are most likely
to escape surface?


Boiling


Evaporation beneath the surface
of a liquid


Vapor pressure must balance
surrounding pressure


boiling
point


How does boiling point change
with altitude?

Condensation


A phase change from a gas to
a liquid


Opposite of evaporation


Since evaporation (and
boiling) are cooling processes,
is condensation a warming
process?

Condensation


A phase change from a gas to
a liquid


Opposite of evaporation


Since evaporation (and
boiling) are cooling processes,
is condensation a warming
process?


Yes. Gas molecules near
surface of a liquid are attracted
to the liquid => collide with
greater K.E. => temperature of
liquid rises

Water condenses on the outside

of a cold soda can and warms the

soda in the process.

Melting and Freezing


Melting


Random thermal motion at high
temp. breaks molecular bonds


Freezing (reverse process)


Decreased motion allows
binding due to interparticle
attraction


Regelation


Continued melting and refreezing
of water under pressure


Ex. Ice skating


What special property of water
makes this possible?

Regelation


the wire will slowly

pass through the ice without cutting

it in half.

Melting and Freezing


Melting


Random thermal motion at high
temp. breaks molecular bonds


Freezing (reverse process)


Decreased motion allows
binding due to interparticle
attraction


Regelation


Continued melting and refreezing
of water under pressure


Ex. Ice skating


What special property of water
makes this possible?


Density of ice less than that of
water => melts under pressure!

Regelation


the wire will slowly

pass through the ice without cutting

it in half.

Temperature and Phase Changes


Temperature is constant
during a change of phase


Absorbed heat goes into
breaking molecular bonds
=> increased potential
energy. Translational
K.E. remains unchanged.
(Left to Right)


Extracted heat allows
molecular bonds to form
(decreased potential
energy) at fixed temp.
(Right to Left)

A material absorbs energy when melting

or vaporizing and expels energy when

condensing or freezing.


During a phase change, the energy

goes into the change of state rather than

into changing the object's temperature.

Thermodynamics


The study of heat and its
transformation to mechanical
energy and work


1
st

law of thermodynamics


When heat flows into (or out of) a
system, the system gains (or loses)
an amount of energy equal to the
amount of heat transferred.

ΔHeat = ΔInternal Energy + Work


Adding heat to a system can:


increase the internal energy of the
system


enable the system to do external
work
(or both)

Device demonstrating the

conversion of mechanical

energy to heat energy

1
st

Law (cont.)


What fundamental principle in physics does the 1
st

law
express?

1
st

Law (cont.)


What fundamental principle in physics does the 1
st

law
express?


Conservation of energy
(
ΔHeat = ΔInternal Energy + Work)


Holds for
all

systems, regardless of the specifics of their inner
workings


Adding heat


to fixed volume (sealed container of air)


How does the temperature and pressure of the air change?


How much work is done? Where does the energy go?


to changeable volume (e.g. Piston)


What happens to the piston?

1
st

Law (cont.)


If we do mechanical work on a system, we can also
increase its internal energy


Your hands get warmer if you rub them together


What happens to the air in a bicycle pump as the handle
is pushed down?

1
st

Law (cont.)


If we do mechanical work on a system, we can also
increase its internal energy


Your hands get warmer if you rub them together


What happens to the air in a bicycle pump as the handle
is pushed down?


Air is compressed and temperature (measure of internal
energy) rises


You can always transform mechanical energy
completely into heat, but you can never transform
heat completely into mechanical energy!


Directionality to nature of heat flow and energy


Imagine two bricks at different temperatures in
thermal contact


If the hot brick were able to extract heat from the cold
brick, would this violate the 1
st

law of
thermodynamics?

2
nd

Law of Thermodynamics


Imagine two bricks at different temperatures in
thermal contact


If the hot brick were able to extract heat from the cold
brick, would this violate the 1
st

law of
thermodynamics?


No. Not if the cold brick becomes even colder so that the
total amount of energy is conserved.


This sort of behavior is prohibited by the
2
nd

law of
thermodynamics:


Heat never spontaneously flows from a cold object to a
hotter object.


Heat can be made to flow in the opposite direction, but only by
doing work on the system or by adding energy from another
source.

Heat Engines


Heat = disordered energy


random thermal motion


Alternative statement of 2
nd

law:


No device is possible whose
sole

effect is to transform heat
completely

into work.


There is a maximum efficiency
(
η < 1)

for
any

heat engine
-

3
rd

law


Depends only on operating temps.


It is easy to convert work entirely into
heat, e.g., friction


Reverse is not possible

Some heat must always be

“wasted” (exhausted to a

low temperature reservoir)

η =

T
hot

-

T
cold

T
hot


If you open a bottle of perfume, what happens to
the perfume molecules?

Entropy and Disorder


2
nd

Law


Systems left to themselves
evolve towards states of increasing
disorder


Entropy: Measure of disorder


Organized energy (e.g. gasoline) degenerates
to disorganized and less useful energy (heat)


Perfume diffuses in a room


goes from
organized state (all molecules in bottle) to
more disordered state (all throughout room)


Defines an “arrow” of time


Microscopic laws of physics are time
-
reversible

Will the perfume

molecules scattered

throughout the room

ever all spontaneously

return to the bottle?

Entropy and Disorder (cont.)


Disordered energy can be changed to ordered energy
only with organizational effort or work input


Work put into refrigeration cycle => water freezes (more
ordered state)


Gas compressed into a smaller volume requires outside
work to be done on the gas


Living organisms concentrate and organize energy from
food sources


In each case, the entropy of the system decreases. Do
these examples violate the 2
nd

law of Thermo?

Entropy and Disorder (cont.)


Disordered energy can be changed to ordered energy
only with organizational effort or work input


Work put into refrigeration cycle => water freezes (more
ordered state)


Gas compressed into a smaller volume requires outside
work to be done on the gas


Living organisms concentrate and organize energy from
food sources


In each case the entropy of the system decreases. Do
these examples violate the 2
nd

law of Thermo?


No. In each entropy of the environment increases, which more
than compensates for the decrease in entropy. The total entropy
of the universe always increases (or stays the same) in any given
process.