Transcript The internal energy of a substance can be changed in different ways

The internal energy of a
substance can be
changed in different ways.
Work can transfer energy
to a substance and
increase its internal
energy.
Heat can be lost by
the substance, which
results in a decrease
of internal energy.
Energy can also be
transferred to the
substance as heat
and from the
substance as work.
Energy is added to substances
or groups of substances. It is
also removed from these
substances. Such a substance
or combination of substances is
called a system.
The surroundings
with which the
system interacts is
called the
environment.
Work done on or by
a gas is the
pressure multiplied
by the change in
volume.
W = P∆V
An engine cylinder has a
cross-sectional area of
2
0.010 m . How much work
can be done if a gas exerts
a constant pressure of
5
7.5 x 10 Pa and moves
the piston 0.040 m?
We have been discussing
three quantities and how they
relate to each other: internal
energy (U), heat (Q), and
work (W). The study of how
these relate is called
thermodynamics.
We can simplify our
discussion by
considering situations
where one of these
properties (U, Q, or W)
does not change.
No work is done in a
constant volume
process.
W = P∆V
Such processes are
isovolumetric.
From PV/T, a change
in P without a change
in V requires a
change in T.
When the temp of a gas
changes without a change
in volume, no work is
done on or by the system.
(Energy must be added to
or taken from the system
by some other means.)
Internal energy is constant in
a constant temperature
process (isothermal).
This remains constant even
as energy is transferred to or
from the system as heat or
work.
When heat energy is
not transferred in a
process it is called
adiabatic. Any change
in internal energy is
due to work done on or
by the system.
The first law of thermodynamics
states that any change in
internal energy is equal to the
energy transferred to or from
the system as heat and the
energy transferred to or from
the system as work.
∆U = Q + W
This is another statement
of the law of conservation
of energy. In this case, it
is not just mechanical
energy that is conserved,
but all the energy of a
system.
∆U = Q + W
Q is positive if heat is
added to a system.
Q is negative is heat
is removed from a
system.
∆U = Q + W
W is positive if work is
done on a system
(gas compression).
W is negative if work is
done by a system
(gas expansion).
Important Note:
The signs for “work done
on the system (+)” and
“work done by the system
(-)” are the opposite listed
in your book to reflect
changes in the AP test.
A gas in a cylinder with a movable
piston is submerged in ice water.
The initial temperature of the gas is
0°C. 1200 J of work is done by a
force that slowly pushes the piston
inward.
A) Is this process isothermal,
adiabatic, or isovolumetric?
B) How much energy is transferred
as heat?
A total of 135 J of work is done
on a gas through compression.
If the internal energy of the gas
increases by 114 J, what is the
total amount of energy
transferred as heat? Has energy
been added to or removed from
the gas as heat?
A refrigerator does work
to create a difference in
temperature between its closed
interior and its environment.
This is a cyclic process and the
change in internal energy
of a system is zero in a cyclic
process.
In a cyclic process:
∆Unet = 0 and Qnet = Wnet
The difference in the
transfer of heat from the
system ,Qh, and the
transfer of heat to the
system ,Qc, is equal to
Wnet: Qh - Qc = Wnet.
A refrigerator uses
work to remove heat
from the system.
A heat engine does the
opposite, it uses heat to
do mechanical work.
It is still a cyclic process,
and the formulas and
relationships are still the
same:
∆Unet = 0 and Qnet = Wnet
Qh - Qc = Wnet
The second law of
thermodynamics
states that no cyclic
process that converts
heat entirely into
work is possible.
W can never be equal
to Qh, there must
always be a value
Qc > 0 indicating a loss
of heat to the
environment.
The efficiency of a
thermodynamic system is
calculated using these
equations:
eff = Wnet/Qh
eff = (Qh - Qc)/Qh
eff = 1 - Qc/Qh
A heat engine can only
be 100% efficient if no
energy is removed as
heat Qc = 0.
Engines are most
efficient if Qh is high
and/or Qc is low.
Find the efficiency of a
gasoline engine that,
during one cycle, receives
204 J of energy from
combustion and loses 153
J as heat to the exhaust.
In thermodynamics, a
system tends to change
from a very ordered set of
energies to one where
there is less order.
The measure of a
system’s disorder
is called entropy.
Systems with maximum
disorder are favored.
Greater disorder
means there is less
energy available to do
work.
The second law of
thermodynamics, (No cyclic
process that converts heat
entirely into work is possible.),
can be stated in terms of
entropy: The entropy of the
universe increases in all natural
processes.
Entropy can increase or
decrease within a system.
Entropy can decrease for
parts of a system as long as
this decrease is offset by a
greater increase in entropy
elsewhere in the universe.