Transcript Thermodynamics

Thermodynamics
Chapter 11
Heat, work and internal energy
• Heat can be used to do work
– Work can transfer energy to a substance,
which increases the internal energy of a
substance.
Fig 11-1
• Work increases
the nail’s internal
energy at the
nail’s surface.
This energy is
transferred away
from the nail’s
surface as heat.
Internal Energy
• There are two ways to change the internal
energy:
with work, and everything else.
Everything else is defined as heat.
Heat is the defined as the transfer of
energy to a body that does not involve
work or those transfers of energy that
occur only because of a difference in
temperature
Recall…
• Balloon over heated flask…
– Energy transferred as heat turns water into
steam.
– Energy from the steam does work against the
force exerted by air outside the balloon.
Heat and Work Energy
• Both transferred to or from a system
• SYSTEM – a collection of matter within a
clearly defined boundary across which no
matter passes
• All parts of a system are in thermal
equilibrium with each other before and after
a process adds or removes more energy.
SYSTEMS
• Example: Flask, water, balloon, and
steam.
• As the hot plate transferred energy as heat
to the system, that system’s internal
energy increased
• When the expanding steam (a part of the
system) did work on the balloon, the
system’s internal energy decreased
• WHY??
Heat  Work
• The decrease occurs because some of the
energy transferred to the system as heat
was transferred out of the system as work
done on the balloon!
ENVIRONMENT
• Systems are often treated as if they are
isolated, but in most cases it will interact
with its surrounds
• The surroundings with which the system
interacts are referred to as its
environment.
WORK in terms of
changing volume
• Remember that… W=F x d
and P = F/A
Work = pressure (volume change)
• W = F x d A = F (Ad) = P(V2 – V1)
A
A
• Therefore, W = P(V2 – V1)
work = Pressure x Volume
WORK = P(V)
• If the gas volume remains constant, there
is no displacement and NO WORK is done
on or by the system.
• Work is done ONLY If the
volume changes.
If pressure increases and Volume
remains constant – this is
comparable to a force that does
not displace mass even if the
force is increased. Thus work is
not done in either situation.
Thermodynamic Processes
Suppose the car’s windows are closed and
parked inside a hot garage.
Internal energy of system (inside the car)
increases as energy is transferred as heat
into the car from the hot air in the garage.
Car’s heave steel and sealed windows keep
the system’s volume constant
Thus, no work is done by the system.
All changes in the system’s internal
energy are due to the transfer of energy
as heat.
Isovolumetric Processes
• Last example, car system, was an
illustration of an isovolumetric (or constant
volume) process.
• Isovolumetric Process = a thermodynamic
process that takes place at constant volume so
that no work is done on or by the system.
• Another example takes place inside a
bomb calorimeter.
• A small container in which a small quantity of a
substance undergoes a combustion reaction
• Energy released by the reaction increases the pressure and
temperature of the gaseous reaction products.
• Walls are thick, thus NO CHANGE in volume of the
gas; energy transferred only as HEAT
Internal Energy
• Internal energy remains constant in a
constant-temperature process.
• When you are indoors in a controlled
temperature—any temperature change
outside the building, will not take place
indoors.
• However, buildings are not perfectly
sealed so changes in the pressure outside
will also take place inside the building
• Think about a balloon that has been
inflated and sealed off.
• As the atmospheric pressure inside the
building slowly decreases, the balloon
expands and slowly does work on the air
outside the balloon.
• At the same time, energy is slowly
transferred into the balloon as heat.
• Net result of these two processes
is that the air inside the balloon is
at the same temperature as the
air outside the balloon
• Thus, internal energy of the
balloon’s air does not change
• The energy transferred out of the
balloon as work is matched by the
energy transferred into the
balloon as heat.
ISOTHERMAL PROCESS
• This process  isothermal process
• ISOTHERMAL PROCESS- a thermodynamic
process that takes place at constant temperature
and in which the internal energy of a system
remains unchanged.
FIG 11-6 in textbook
• Transfer of energy as heat can occur in an
isothermal process if it is assumed that the process
takes place as a large number of very gradual, very
small changes as shown above.
• When air inside balloon expands, its internal energy
and temperature slightly decrease
• As soon as they decrease, the energy is transferred
as heat from higher temp outside air to the air inside
the balloon
FIG 11-6 in textbook
• The temperature and internal energy of the air
inside the balloon end up rising to their
original values
• Thus, the internal energy of the balloon’s air
effectively remains CONSTANT!
Adiabatic Process
• A thermodynamic process during
which work is done on or by the
system but NO energy is transferred to
or from the system as heat.
Adiabatic Process
• In an adiabatic process, the
decrease in internal energy must
be equal to the energy
transferred from the gas as work.
• Ex: filling up a balloon with air
from a compressed air tank
• This work is done by the gas
pushing against the inner wall
of the balloon and overcoming
pressure exerted by the air
outside the balloon.
* As a result the balloon inflates
• Adiabatic expansion and compression of
gases is found in many applications…
• Both refrigerators and internal combustion
engines require that gases be compressed
or expanded rapidly.
HOMEWORK
• Page 405 #1
• Page 408 # 1 and 2
Thermodynamics
Chapter 11
Section 2: Thermodynamic Processes
1st law of thermodynamics
• Considers both a system’s internal energy
as well as work and heat.
Change in a system’s internal energy =
energy transferred to/from –
system as heat
energy transferred
to/from system as work
OR
U = Q-W